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Project Summary: Nitrogen Retention and Ecosystem Succession: Theory Meets Data
Intellectual Merit
This proposal addresses a widely accepted conceptual model in ecosystem science which states that the
retention of nitrogen (or other limiting nutrients) in a terrestrial ecosystem is controlled by the
accumulation of biomass in the ecosystem, and that losses of those limiting nutrients will be small or
nonexistent when the rate biomass accumulation is high, such as in a rapidly growing forest. Conversely,
when the rate of biomass accumulation is low, losses of critical nutrients should balance inputs. Longterm monitoring of vegetation and streams at the Hubbard Brook Experimental Forest (HBEF) in central
NH provides data that contradict this model. Even though the vegetation in the reference watershed at
HBEF is no longer accumulating biomass, losses of nitrogen in the streamwater draining the watershed
have been declining for decades and are presently are very low. This proposal hypothesizes that a critical
aspect is often overlooked in the conceptual model: the role of soil organic N. Early in the development
of a forest stand, the rate of accumulation of nitrogen into plant biomass often exceeds the supply from
atmospheric deposition, and the vegetation extracts nitrogen from deeper soils to meet its demands. Later
in the development of the forest, as net biomass accumulation ceases, nitrogen should re-accumulate in
these deep soils to replace the amount “mined” earlier by the vegetation. Over the course of forest
development, borrowing from and repaying this nitrogen “bank” in the soil allows the pattern of nitrogen
output to be offset from the pattern of plant biomass accumulation.
To evaluate this hypothesis, this project proposes an integrated set of field experiments, forest
chronosequence studies, and computer model simulations. The field experiments use soil solution
chemistry and foliar litter labeled with 15N and 13C to trace the movement and retention of C and N
though soils in small plots in forests at different stages of development. The chronosequence studies
examine the total amount of N in soil and vegetation, and its isotopic composition, in two different series
of northern hardwood forests that range in age from ~15 years to old-growth. The temporal patterns of N
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N will be examined for consistency with the hypothesis that successional forests “mine” N
from the soil that is “repaid” after stand maturation. The modeling study expands a new forest ecosystem
model to include the soil N “bank” concept, and to determine how these changes affect model predictions
regarding forest responses to atmospheric N pollution, climate change, and forest harvesting.
Broader Impacts
This work is important to the fields of ecology and ecosystem science because it could result in the
revision of a long-standing, widely-accepted conceptual model in these fields. If the hypothesis is
supported, the result will change how linkages between terrestrial and aquatic ecosystems are managed,
modeled, and discussed in classrooms. The project offers educational opportunities, and the proposed
work includes training of high school teachers, science undergraduates, graduate students, and
postdoctoral scholars. A simplified forest model will be developed to be used as a teaching tool in high
school science classrooms. The work also has management implications, and project personnel will hold
a workshop to better inform forest managers in the White Mountain and Green Mountain National Forests
(in NH, ME and VT) about the latest research on the linkages between forest management and water
quality. This workshop, and the report that results from it, will be timed to inform the National Forest
Management Plans for both of these National Forests, which are expected to be revised within the term of
this project.
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Introduction
Ecosystem ecology is rarely guided by formal theory, but it does have important elements of
theory, including useful conceptual models that help organize observations and provide a foundation for
new research. One of the most enduring conceptual models in terrestrial ecosystem ecology, presented as
a hypothesis by Peter Vitousek and William Reiners in 1975, states that the net retention (inputs –
outputs) of critical limiting nutrients, such as nitrogen (N) in temperate and boreal forests, should be
related to the net biomass accumulation of the system (Vitousek and Reiners 1975). They reasoned that if
the rate of net biomass accumulation (Fig. 1a) was high, the rate of N accumulation (Fig. 1b) would also
be high, and outputs of N (e.g., leaching into streamwater, Fig. 1c) would be low. They hypothesized that
young forests with high rates of live biomass accumulation should exhibit high retention of N, while more
mature forests should have rates of biomass accumulation and net N retention approaching zero. They
supported their hypothesis with measurements of higher nitrate losses in streamwater from old-growth
forests than from nearby young forests at sites located on Mt. Moosilauke in the White Mountains of New
Hampshire (Vitousek and Reiners 1975).
Despite its widespread acceptance, a growing body of evidence suggests that the VitousekReiners model is either incorrect or incomplete. At the Hubbard Brook Experimental Forest (HBEF),
which is located just 11 km to the east of Mt. Moosilauke, precipitation inputs and streamwater outputs of
N have been measured for over 40 years (Likens and Bormann 1995). During this time, the forest in the
biogeochemical reference watershed (W6) has aged from a vigorously growing successional forest in the
1960s and 1970s to a ~95 y old mature forest. Its net rate of aboveground biomass accumulation has been
near zero for almost 20 years (Fahey et al. 2005). Based on the Vitousek-Reiners hypothesis, the N
export in streamwater should have increased as the rate of biomass accumulation declined, especially
given chronically elevated inputs of N from atmospheric deposition. However, the observations from the
HBEF show exactly the opposite
pattern— after a peak in the 1970s,
Vitousek and Reiners
N export in streamwater has steadily
a.
Hypothesis
declined and is presently just a small
Net Biomass
fraction of atmospheric N input.
Accumulation
Present-day N losses are as low as
they have been at any time in the 40year record (Fig. 2). Thus, the data
do not match the theory, and the
b.
obvious question is “Given the lack
of N retention in the biomass, why is
Biomass
Nitrogen
N export in streamwater so low?”
Accumulation
Within the framework of the
N mass balance, several possibilities
exist to explain the low N export.
One possibility is a decline in N
c.
inputs, but recent declines in N
Nitrogen
deposition at HBEF are much
Export
smaller in magnitude, and occurred
0
well after, the decline in N export.
A second possibility is that
Time
denitrification has increased, either
in streams (as proposed by Bernhardt Figure 1 Vitousek and Reiners (1975) hypothesis concerning rates
et al. 2005) or in upland soils.
of (a) net biomass accumulation, and (c) N leaching in forests over
However, recent studies have failed
the course of secondary succession. Panel (b) is added to illustrate
to find evidence for increased
corresponding patterns of biomass N accumulation.
denitrification at HBEF (Warren et
al. 2007, Bernal et al. 2012).
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A third possibility is that N may be accumulating somewhere within the ecosystem. Given the
near steady-state condition of the plant biomass, coarse woody debris, and forest floor pools (Fahey et al.
2005), the most obvious potential N sink is in the mineral soil. In this proposal we suggest that the
Vitousek-Reiners model is incomplete and requires explicit consideration of the soil as a source of N
during stand development and as a sink for N after plant N demands are met. Further, we suggest
that an extended period of N accumulation in soils of mature forests is both expected and may be
large in magnitude. In a simple analogy, we propose that the mineral soil constitutes an N “bank,” from
which the vegetation can borrow early in succession, but which must be repaid later on.
Stream Export NO3-N
Accum. in Live + St. Dead Biomass
Deposition - Export
20
15
kg N/ha/y
10
5
0
-5
-10
-15
1960
1970
1980
1990
2000
Figure 2 Temporal trends of
stream export of NO3--N,
accumulation of N in
live+standing dead
vegetation, and atmospheric
deposition minus stream
export of N at Hubbard
Brook W6. Deposition and
stream data provided by G.
Likens et al., biomass data
provided by T. Siccama, J.
Battles et al.
2010
In this proposal we describe a range of experimental and modeling approaches to better understand the
dynamics of the soil N pool and its influence on the observed patterns of N export.
Background
N Budgets at Hubbard Brook
The HBEF in central NH is the site of the longest record of precipitation inputs and streamwater
outputs for a watershed ecosystem in the U.S. The input and output record for N in the reference
watershed (W6) has been the subject of considerable analysis. Early in the study, Bormann et al. (1977)
noted that the N budget was out of balance and required an extra source of N to balance the accumulation
in the vegetation and forest floor and loss via streamwater, assuming that the mineral soil N pool was not
changing. Bormann et al. suggested the missing source was N fixation, but later investigations did not
find significant N fixation in the ecosystem (Roskoski 1980), which lacks symbiotic N fixers. Likewise,
dry deposition of N, which was an unmeasured input in the Bormann et al. analysis, was found to be
small at this site (Lovett et al. 1997). We suggest that the unmeasured “source” of N in the Bormann et
al. budget was in fact extraction of N from the mineral soil by the rapidly-growing plants (see
“Interchange Between Soil and Vegetation N Pools” below.)
Over time, the nature of the N budget at HBEF has changed, largely because of declines in both
biomass accumulation and stream N export, and more recent analyses have focused on the question of
unmeasured sinks rather than unmeasured sources. All of the trees in W6 at the HBEF have been
inventoried and biomass N accumulation calculated in 1965, 1977, and every five years since 1977.
These calculations indicate that biomass N accumulation slowed in the 1980s, and since 1992 the rate of
N accumulation has been near zero or negative (Fig. 2). During this same period since 1992, the stream
NO3- -N export has been at record low levels (Fig. 2), except for a brief spike in 1998 due to a damaging
ice storm on the watershed (Houlton et al. 2003). Dissolved organic N (DON) contributes up to 2.0 kg N
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ha-1 y-1 of N export (Campbell et al. 2000) but unlike nitrate, DON export does not vary with successional
status (Goodale et al. 2000). Stream export of ammonium is negligible.
Hubbard Brook is not alone in its recent low NO3- export. Goodale et al. (2003) remeasured the
NO3 , NH4+ and DON concentrations in 28 of the 57 streams in NH that were sampled by Vitousek
(1977), and reported that NO3- declined in all of the streams between the mid-1970s and mid-1990s. They
also found that the successional patterns from Vitousek and Reiners (1975) remained intact, with streams
draining old-growth forests having higher NO3- than streams draining younger forests. This regional
pattern of NO3- decline suggested that the causal factor or factors was widespread, and because N
deposition had changed little between the 1970s and 1990s, Goodale et al. (2003, 2005) suggested that
changes in climate or some other regional environmental factor affecting plant and soil processes may be
involved. The decline in NO3- at Hubbard Brook is consistent with the other streams in the region, but at
Hubbard Brook the decline is more complete, with the NO3- concentration frequently approaching the
limit of detection. This suggests that there could be both a regional signal and a more local, successional
signal in the Hubbard Brook NO3- data (Bernal et al. 2012).
Successional processes could be affecting NO3- export in several ways. Bernhardt et al. (2005)
hypothesized that maturation of the forest may provide more dead wood to the streams, and the resulting
debris dams could enhance stream denitrification losses. However, subsequent studies by Warren et al.
(2007) found no evidence for increased denitrification in streams draining older forests. Similarly, Bernal
et al (2012) found no evidence of high denitrification rates in this watershed, using evidence from 15N and
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O in NO3- from precipitation and streamwater.
If denitrification rates are currently low, then N must be accumulating in an internal sink within
the ecosystem. The three principal internal sinks for N in forests are live vegetation, detritus including
the forest floor (FF) and coarse woody debris (CWD), and soils. Based on tree censuses on W6, it is clear
the biomass (including trees, saplings, and standing dead) is no longer a sink for N and is in fact a net
source of N (Fig. 2). Two measurements of CWD mass, in 1978 and 1995, show no appreciable change
in this pool (Fahey et al. 2005), a finding that is consistent with expectations based on CWD turnover
rates (Arthur et al. 1993). Likewise, repeated measurements of FF mass since 1975 show no trend (Fahey
et al. 2005), and recent evaluation of the FF N pool shows a slope not significantly different from 0,
although because of difficulty in quantifying this pool, the 95% confidence limits on the slope are
sufficient to encompass the required N sink (Hamburg et al. in review). The N pool in the mineral soil
(~6 t N ha-1, Johnson 1995) is even larger and more difficult to measure precisely than that of the FF
(~1.3 t N ha-1, Johnson 1995). Because of the lack of sufficient measurements, evaluation of whether any
change is occurring in the mineral soil pool has not been attempted. In summary, an internal N sink in the
biomass is clearly not occurring, there does not appear to be a sink in the CWD, a sink in the FF is
possible but unlikely, and a sink in the mineral soil is clearly possible but unmeasured. Resolving the
dynamics of the mineral soil N pool during and after forest succession is critical, both for refining a longstanding and widely used conceptual model and for predicting the long-term effects of disturbance and
atmospheric deposition on forest growth and N losses to aquatic ecosystems.
Interchange Between Soil and Vegetation N Pools.
Figure 2 illustrates that, prior to about 1982, the forest at HBEF was accumulating N in
aboveground biomass faster than the rate at which N was made available through net N retention (wet
deposition + dry deposition – stream export). Considering the very low N fixation rates in this forest, this
budget suggests an internal source of the N accumulated in the vegetation prior to 1982. The most likely
source is extraction of N from the large N pools in SOM of the mineral horizons. This process is often
referred to as “mining” of soil N (Johnson 1992, Johnson 1996), and has been reported for young forests
growing on soils where considerable soil pools of N are present (Hamburg 1984, Gholz et al. 1985,
Richter et al. 2000, Hooker and Compton 2003). For instance, both Richter et al. (2000) and Hooker and
Compton (2003) reported reductions in mineral soil N and increases in vegetation and forest floor N over
the course of forest development in post-agricultural sites. While the occurrence of N mining can be
inferred from chronosequence studies or N budgets (e.g., Fig. 1), the mechanisms and the specific pool of
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organic matter accessed are not well understood. Plants may be able to access recalcitrant pools of N in
SOM either by directly releasing decomposition enzymes, by increasing C allocation to mycorrhizal
symbionts, or by exuding labile C compounds into the rhizosphere to “prime” N mineralization activity
by free-living microbes (e.g., Phillips et al. 2011).
Mining activity by a fast-growing forest would increase the decomposition of SOM and uptake of
the mineralized N, causing the soil N pool size to decline. If mining were then to cease as the forest
matures, there would be re-accumulation of the depleted N pool toward the pre-mining pool size, and this
re-accumulation could produce a large sink for N in the soil. This re-accumulation phase has not, to our
knowledge, been studied and, if present, represents an important omission from theories of nutrient
retention. The soil N sink would be most evident in recently mature forests, such as W6 at HBEF, in
which N no longer accumulates in the live biomass but continues to accumulate in the soils. Reaccumulation of N in depleted soil N pools could occur through several processes. First, partially
decomposed litter (foliage, roots and wood) can be buried in the soil as new SOM. Second, turnover of
microbial biomass can add the C and N from microbial structures and enzymes to SOM. Third, dissolved
organic C and N (DOC and DON) can adsorb to mineral soil surfaces as water percolates down through
the soil.
Conceptual models of N dynamics
This project bears on several aspects of the conceptual understanding of nutrient dynamics in
forested ecosystems. First, as discussed above, is the Vitousek-Reiners hypothesis about the pattern of N
accumulation and export during ecosystem succession and maturation. Vitousek and Reiners (1975)
proposed this hypothesis in response to an earlier hypothesis from Odum (1969) who believed that
ecosystem N budgets would become “tighter” (less loss of N) as the ecosystem matured. In contrast,
Vitousek and Reiners proposed that the lack of biomass N accumulation in the later stages of ecosystem
maturation would remove a critical N sink from the system, leading to greater N losses (Fig. 1). Vitousek
and Reiners included soil organic matter (SOM) within their definition of biomass, but their
interpretation of this model for secondary succession focused on the dynamics of live biomass, as have
most other interpretations since then, including textbooks (e.g., Chapin et al. 2002).
While the Vitousek-Reiners hypothesis has been generally acknowledged as valid, several caveats
have been noted over the years since it was proposed. For example, changes vegetation or soil C:N ratios
can decouple N accumulation from C accumulation in some cases (Rastetter et al. 1992). Research in
relatively unpolluted forests has shown that whether old-growth forests are considered “leaky” depends
on whether DON is considered, as DON may constitute the major fraction of N export from these
ecosystems (Sollins et al. 1980, Hedin et al. 1995). In some cases the linkage between forest age and N
leaching has been found to be weak (e.g., Fisk et al. 2002). Finally, and perhaps most importantly, the
NO3- export data from Hubbard Brook (Fig. 2) stand in direct contrast to the Vitousek-Reiners model.
A second conceptual model of nitrogen cycling that is called into question by the Hubbard Brook
data is the theory of nitrogen saturation. Aber et al. (1989, 1998) describe nitrogen saturation as a process
by which chronically elevated input of N to the ecosystem causes a predictable sequence of changes in
forest N cycling, including increasing foliar N concentration, followed by increasing N mineralization,
nitrification, and finally increasing NO3- leaching. According to this model, an ecosystem receiving
decades of N deposition should progress towards higher and higher levels of N leaching as the available
N begins to relieve N limitation in the system. Here, too, the data record at Hubbard Brook pose a direct
contrast with theory. Despite the fact that Hubbard Brook has received elevated levels of N deposition for
over half a century, high levels of N leaching are not occurring.
A New Conceptual Model
We hypothesize that the dynamics of the N pool in forest soils produce both a source of N for
rapidly growing vegetation in early and mid-succession (“soil mining phase”), and a sink for N in later
succession as N re-accumulates in the soil (“soil re-accumulation phase”) (Fig. 3b). Over the course of
succession, borrowing from and repaying this N “bank” in the soil allows the pattern of N output to be
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partially decoupled from the pattern of biomass accumulation. In contrast to the Vitousek-Reiners model,
inputs of N to the ecosystem would not equal outputs until soil N accumulation reaches zero, which could
lag decades or centuries behind the biomass N accumulation (Fig. 3 b and c).
We emphasize that these patterns would not be unique to Hubbard Brook, and should occur in
any forest growing in an area where N inputs are insufficient to meet biomass demands during some stage
of succession, and where a considerable pool of N is available in the soil. This encompasses most
secondary successional forests, except those in which N fixation or bedrock N weathering is a large input.
a.
Net Biomass
Accumulation
0
Plant N
Accumulation
b.
Nitrogen
accumulation
+
0
-
N Deposition
W101
Soil mining
phase
c.
Nitrogen
export
W6
Soil re-accumulation phase
Soil N mining or
accumulation
N Deposition
N Leaching
0
Time
MMOG
Figure 3 Revised
conceptual model of N
dynamics during
succession. (a) Biomass
accumulation. (b) N
accumulation in biomass
follows biomass
accumulation curve in
panel a. When biomass N
accumulation is greater
than N deposition, soil
mining occurs. When
biomass N accumulation
falls below N deposition,
re-accumulation of soil N
occurs. (c) N leaching
occurs only when combined
biomass + soil N sinks are
less than N deposition.
Estimated positions of
intensive study sites (W101,
W6 and MMOG) on time
axis are shown in panel b.
Hypotheses and Approach
Our new conceptual model leads to the following general hypotheses:
1) For young forests in which biomass N demand is greater than net N retention (atmospheric input –
streamwater output), subsoils will provide an N source for the growing biomass.
2) When biomass accumulation of N declines to levels lower than net N retention, soils will reaccumulate N, producing an N sink in the system.
We propose an integrated set of experimental and modeling studies to test these hypotheses and
evaluate the conceptual model presented in Figure 3b. Mineral soils are a challenging research subject
because of their heterogeneity, large pool sizes, and slow rates of change. Nonetheless, understanding the
dynamics of the N pool in mineral soils is critical to research in ecosystem nitrogen cycling. Our strategy
is to use a combination of related approaches, taking advantage of new soil coring methods, both tracer
and natural abundance isotopic analyses, and new ecosystem model. We will use a field experiment and
soil solution profiles at Hubbard Brook and a nearby site to determine if mineral soil N is being lost in
mid-successional forests (the soil mining phase of Figure 3b) and if (and how) it is regained in the mature
forests (soil re-accumulation phase). We will use a chronosequence study on two separate
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chronosequences to determine if mineral soil N pools follow the patterns suggested by the conceptual
model—depletion during the soil mining phase, and re-accumulation after soil mining ceases. We will
augment the chronosequence studies with natural abundance 15N isotope analysis to improve our ability
to interpret the pool changes in the chronosequences. Finally, we will use a modeling study to tie
together the mechanisms and budgets we measure from these experiments and project forward to estimate
total amounts of mined and re-accumulated N and the duration of the soil N accumulation period in the
ecosystem. Specific hypotheses relating to these studies are listed within the narrative for each study.
Question 1. Do rates of mineral soil N loss and accumulation differ among forests at different stages of
regrowth following disturbance?
1a) Field Experiment
We will use a small-plot field experiment with double-labeled litter (13C, 15N) to determine if
mineral soils from three sites at different positions long the mining/re-accumulation continuum (Fig. 3b)
are accumulating or losing C and N. The sites are (1) the area just west of W6, the reference watershed at
HB, which we expect to be in the re-accumulation phase, (2) Watershed 101 (W101), an area near W6
that was commercially clearcut in 1970, which we expect to be at the peak of the soil mining phase, and
(3) an old-growth hardwood stand at Mt. Moosilauke (MMOG), immediately west of the HBEF and with
similar bedrock and vegetation, which we expect to be near the end of the re-accumulation phase (Fig.
3b). These three intensive study sites are also used in the soil solution study (section1b) and are all
included in one of the chronosequences described in question 2 below. The sites fall outside the
monitored catchments at Hubbard Brook in which isotopic tracers are not allowed. We hypothesize that
the cumulative amount of soil organic N depletion, and thus the propensity of the soils to retain added N,
will vary in the order W6 > W101> MMOG. The soils in W6 have experienced ~70 y of mining and
perhaps 20 y of re-accumulation. The W101 soils have experienced ~30 y of mining, the approximate
length of time that deep rooted trees have dominated the site after the clearcut. The MMOG forests have
not been disturbed for hundreds of years, and the soils have presumably had time to re-accumulate
previously mined N. The estimated position of these sites on the soil mining/re-accumulation continuum
is shown in Fig. 3b. At each site, retention and loss of C and N will be measured both in the bulk soil and
in buried mineral soil bags, which contain homogenized and well-characterized mineral soils (see details
below), thus minimizing the spatial variation problems that often plague measurements of soil change.
We will establish 5 replicate 0.5 x 0.5 m plots at each of the three sites, for a total of 15 plots. On
the downhill side of each plot we will excavate a pit to allow us to sample soil and access the mineral soil
bags and lysimeters installed in the plots. Excavating the pit will also allow us to make measurements of
soil C and N pools and root mass by horizon. On these initial soil samples (by horizon) in each plot we
will perform the following suite of analyses: %C, %N, light and heavy fraction C and N, mineralassociated organic matter (MAOM) C and N (sensu Castellano et al. 2012), and microbial biomass C and
N, plus the isotopic composition (13C and  15N) of all of these fractions (see Chemical Analysis
Methods below). Roots will still be present in the plots, entering from the 3 undisturbed sides. In each
plot we will install mineral soil bags containing homogenized B-horizon soil from each of the three sites.
The homogenized soil from each site will be subjected to the suite of analyses listed above. The bags will
be constructed of 250 m mesh, which retains the soil but allows good water flow and allows access to
roots and mycorrhizae (Mitchell et al. 2003). Soils from all three sites will be installed in all the plots in
each site, in a 3-way reciprocal transplant design. Two mineral soil bags of each of the three soil sources
will be installed in the Bh horizon of each plot, for a total of 6 bags per plot. (The experimental unit in
this design is the plot, and the replicated bags within plots allow for possible damage to bags during the
experiment.) We will also install small zero-tension lysimeters below the forest floor to collect the
drainage water to estimate the flux and isotopic composition of dissolved C and N as they percolate down
from the labeled litter. The lysimeters will be made from 2.5 cm PVC pipe sections split longitudinally to
form a trough, inserted in the soil laterally, and draining to a bottle to the base of the pit (e.g. Fitzhugh et
al 2003). We will also establish 3 reference plots at each site, which will have lysimeters in three
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horizons to be used for assessing soil solution profiles (see section 1b below), but will not have the
labeled litter addition or the mineral soil bags. We will monitor dissolved N concentrations from the
lysimeters after installation and will not begin experiments until the concentrations have stabilized, which
often requires 4-6 months.
We will remove the Oi layer from the top of each of the 15 experimental plots and replace it with
double-labeled sugar maple litter created by labeling a small number of trees as described below. An
equal mass of litter will be applied to each plot, equivalent to the mean mass of the annual litterfall to a
0.5 x 0.5 m area. The plots will be covered with a coarse plastic mesh (2.5 cm mesh size) to keep the
labeled litter in place. The litter will leach DOC and DON compounds which will percolate downward
and be retained throughout the soil profile, including in the mineral soil bags. The bulk soil in different
horizons and the mineral soil bags will be sampled after one year, using the same suite of measurements
listed above for the initial sampling. We will also measure fine root biomass and the root C and N
concentration and isotopic composition in the bags. Soil solution from the lysimeters will be collected
monthly during the year and will be analyzed for concentrations of NH4+, NO3-, DON, 15N in organic and
inorganic N, DOC and 13C in DOC. With this approach, we expect to be able to observe an increase in
15
N and 13C in the bulk mineral soil in the plots, and we will be able to measure even more accurately the
increase in N in the mineral soil bags because of the homogeneity of the soil enclosed in the bags. We are
confident of this approach and our ability to carry it out. We have in the past labeled a tree with 15N by
stem-well infusion, applied the resulting labeled litter to small plots, and observed the enhanced 15N
signal in mineral soil (Christenson et al. 2002). Colleagues at Hubbard Brook recently double-labeled a
tree and observed both 15N and 13C accumulation in mineral soil (Fahey et al. 2011). The mineral soil bag
technique has been used successfully numerous times in the past (e.g., Lawrence et al 1999, Fernandez et
al. 1999, Mitchell et al. 2003), although to our knowledge it has never been used with stable isotope
additions or applied to this question.
Our specific hypotheses with regard to 13C and 15N in this experiment are as follows:
H1A: In the bulk mineral soil of the plots, the increase in 15N will be in the order W6>W101>MMOG
because of the root mining histories mentioned above. We expect that 13C and 15N will increase
in all plots because the DOC and DON draining through the plots will be more highly labeled
than the mineral soil, and some degree of C and N exchange will occur even in plots where there
is no net N retention, but the magnitude of increase will vary among sites in the order above.
H1B: In the mineral soil bags, we hypothesize the following responses:
i) Soils in bags incubated in the W101 site will have lower 15N accumulation than their
counterparts in the other two sites. We expect that 15N and 13C will accumulate in all
bags because of the highly labeled solution draining through the plots, but that intensive
root mining of N is occurring currently at the W101 site and the roots will remove some
of the 15N from the soils in the bags.
ii) In the soils incubated in the W6 and MMOG sites, where current root mining is expected to be
minimal, the accumulation of 15N in the bags will vary depending on the source of the
soil in the bag, following the order W6>W101>MMOG because of the root mining
histories of these soils as described above.
In addition to observing the relative magnitude of change and evaluating support for these
hypotheses, our measurements will allow us to infer the mechanisms of soil C and N accumulation. The
proportion of the 13C and 15N recovered in the bags in the mineral-associated fraction will indicate the
extent of C and N accumulation via adsorption of DOC and DON percolating through the soil horizons,
or by adsorption of dissolved microbial products such as extracellular enzymes. (We will measure the
mineral-associated fraction both by density fractionation (“heavy fraction”) and by particle size
fractionation (MAOM); see Chemical Analysis Methods below.) Further, by comparing the 13C:15N ratios
in the mineral associated fraction with the ratios in dissolved organic matter (soil solution) and microbial
biomass, we will be able to infer which of the two processes dominates the adsorption. The proportion of
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the isotopic recovery in the root biomass and the soil’s light (non-mineral-associated) fraction will
indicate the C and N accumulation by biomass turnover (roots and microbes).
Producing double-labeled litter. We will create double labeled (13C, 15N) sugar maple litter to
apply to the field plots. To label the litter, we will choose several small (~ 5 cm dbh) sugar maples
growing in the open near Hubbard Brook but outside of the Experimental Forest. We will determine the
number of trees to be labeled based on their estimated foliage mass and the amount of litter we need for
the experiment (allowing for a roughly 10% mass loss prior to abscission). Following the methods of
Christenson et al. (2002), at the beginning of budburst, we will apply 99% enriched (NH4)2SO4 solution
by stem-well infusion, allowing the plant to imbibe the label solution, which then spreads the label
throughout the canopy. We will label the foliage with 13C by enclosing the crown of each tree in a
chamber made from clear polyethylene sheeting for short periods on sunny days early in the growing
season, and adding 13C labeled CO2 to the air in the chamber. Following the methods of Horowitz et al.
(2010) and Fahey et al (2011), we will scrub the air in the chamber of CO2 to about 50-60 ppm, then add
labeled 13CO2 (40 atom % enriched) to bring the CO2 in the chamber up to about 500 ppm. We will then
keep the chamber on the tree as the CO2 concentration is drawn down by photosynthesis and then
stabilizes, typically 0.5- 1.0 h. We will then remove the bag from the tree, and repeat the procedure on
subsequent sunny days for a total of about 10 days. At the time of leaf fall in the autumn, we will enclose
the canopy in a large net to collect all the shed litter. This litter will be well mixed, samples taken to
measure the 13C and 15N, and the labeled litter will be added to the experimental plots that same autumn.
These techniques can enrich the litter to ~300 per mil 13C and ~1500 per mil 15N (Christenson et al. 2002,
Fahey et al. 2011), providing sufficient label to track the isotope movement into the mineral soil.
We considered adding an isotopic label to the plots by transplanting labeled forest floor material
from other research sites or by creating a “tea” of isotopically enriched, dissolved organic matter and
percolating it through the soil. We rejected both approaches because they would either have produced too
little 15N enrichment or would have introduced chemical and hydrological artifacts that could have
compromised the results.
Mineral soil bags. We will collect Bh horizon mineral soil from the pits we dig adjacent to each
plot. We will composite the soils by site, pass them through a 2 mm sieve, and thoroughly homogenize
the samples. Five initial samples of each of the three mineral soil composites (W6, W101 and MMOG)
will be taken for initial characterization of texture, %C, %N, light and heavy fraction C and N, mineralassociated organic matter C and N, and microbial biomass C and N, plus the isotopic composition (13C
and  15N) of all of these fractions (see Chemical Analysis Methods below). Approximately 500g of the
composited B horizon soil will be added to 5 x 5 cm mineral soil bags made of 250 m nylon mesh to
make 90 bags: 3 soil sources x 3 incubation sites x 5 plots/site x 2 reps/plot = 90. The bags will be
incubated in the field plots as discussed above.
1b) Soil Solution Profiles
At the same sites used in the forest floor experiment (section 1a: W6, W101 and MMOG), we
will also use a complementary approach to measure differences in the rates of mineral soil N
accumulation or loss between mature and successional forests—we will quantify the net retention of N in
each soil horizon by combining soil solution chemistry with a simple water balance model and other N
fluxes needed to construct N budgets for each horizon. Soil solution chemistry has been used to quantify
N fluxes between soil horizons at a variety of forested sites (e.g. Dittman et al. 2007, Sleutel et al. 2009).
For example, a 12-year record of soil solution chemical fluxes at Hubbard watershed 6 (W6) indicated
that dissolved N fluxes decreased by 70-80% between the bottom of the forest floor and the bottom of the
B horizon, with losses likely due to some combination of root uptake, denitrification, and adsorption to
the mineral soil (Dittman et al. 2007). In contrast, we hypothesize that:
H1C: the vertical profiles of N flux in the early-successional site (W101) will show equal or greater
reduction in dissolved N flux in the B horizon compared to W6 because of the higher plant uptake
10
in W101, while the old growth site (MMOG) will show less of a reduction in dissolved N flux
than either W6 or W101 because these soils have re-accumulated their previously mined N.
Our approach will be as follows. Samples from a total of 27 zero-tension lysimeters (3 sites x 3
plots per site x 3 horizons per plot) will be collected at 2-3 week intervals for 8 months a year to measure
soil solution concentrations of NO3-, NH4+, DON, and DOC as well as δ15N and δ18O of NO3, draining
from the 3 soil horizons (Oa, Bh, Bs). These small plots are the reference plots mentioned in section 1a,
and will be installed uphill from the experimental plots at enough distance to prevent introduction of 15N.
Water fluxes through the forest floor and mineral soil horizons will be calculated monthly using the
PnET-SOM model (see question 3 below), which estimates water dynamics for each of the horizons to be
examined in our study as part of its water balance calculations.
Mechanisms of N retention in mineral soils differ for DON and inorganic N. DON is primarily
retained by adsorption onto mineral surfaces while NH4+ and NO3- are primarily retained through
microbial immobilization (Qualls 2000). Losses of N from soil solution can also occur via plant uptake
and denitrification. Measurements designed to assess these losses will allow us to determine the degree to
which measured N retention is driven by N accumulation in mineral soils. First, enrichment of both δ15N
and δ18O in NO3 and can be used to indicate the amount of denitrification in soil water (e.g. Fukada et al.
2003). We will measure soil solution δ15N and δ18O of NO3- and take advantage of intensive work
currently underway at Hubbard Brook using these isotopes to assess the importance of denitrification
(NSF DEB 0919047; Lead PI PM Groffman, Cary Institute; Co-PIs include Goodale and Ollinger). To
quantify plant N uptake, we will estimate total plant N uptake and partition by horizon using the
distribution of fine roots in each horizon (McMurtrie et al. 2012). Total plant N uptake will be calculated
using measured N fluxes to foliage, wood and roots for 10m radius plots encompassing the soil pits used
for measuring soil solution profiles. At each plot, we will collect leaf litterfall in 8 litter baskets and
measure litter N concentrations to calculate annual litterfall N as an indicator of annual N uptake to
foliage. N uptake to wood will be used by combining estimates of annual wood growth with wood N
concentrations (to be measured as part of question 2, described below). Wood growth increment will be
estimated using standard methods involving allometric equations applied to repeat tree diameter
measurements (using all trees > 5 cm DBH). Estimates of N uptake to roots will be derived by combining
measured root biomass and root N concentrations with root turnover rates measured at Hubbard Brook by
Tierney and Fahey (2005).
With the above information on soil solution N fluxes and N losses, we will construct an N budget
for each soil horizon at each of the three sites. The net N accumulation or loss from each horizon will be
compared with N accumulation or loss rates measured from the co-located chronosequence plots (see
question 2 below) for each soil horizon, and with the measurements of tracer 15N incorporation into
mineral soils calculated from the co-located labeled-litter plots (section 1a above). The use of all three
approaches to quantify rates of N accumulation in mineral soils-- soil solution chemistry, successional
patterns of soil N pools, and tracer 15N incorporation-- will allow us to directly compare approaches and
evaluate the consistency of the measured responses. For example, rates of N retention in mineral soils of
W6 at Hubbard Brook were estimated at ~10 kg N/ha/yr (Dittman et al, 2007). This rate of N retention is
similar in magnitude to N accumulation rates (13 kg N/ha/yr) measured in soils from a chronosequence
study in New England (Clark and Johnson 2011). Rates of N accumulation can be difficult to detect
against the large size of soil N pools, but by using multiple independent techniques, we expect to see
significant measurable differences in mineral soil N accumulation rates across the successional gradient.
Question 2: Are the successional patterns of plant and soil N and 15N consistent with our main
hypotheses, that young forests extract N from soil as they aggrade, and that soil N stocks are replenished
after stand maturation?
Specifically, we hypothesize that over the course of forest succession:
11
H2A: Soil N pools should be largest in old-growth stands, and will be smallest near the end of the plant
biomass accumulation phase of succession due to plant-induced “mining” of soil N, after which
soil N should begin to re-accumulate.
H2B: Soil 15N should become more enriched over the course of plant biomass accumulation, as plants
accumulate N relatively depleted in 15N through processes of fractionation during N uptake.
Following stand maturation, soil 15N should become less enriched in 15N as the N pool reaccumulates because of addition of N from 15N-depleted plant litter and atmospheric deposition.
We will test these hypotheses through measurements of plant and soil N across two
chronosequences of sites in the White Mountains: one chronosequence at Hubbard Brook and adjacent
Mt. Moosilauke, and one at the Bartlett Experimental Forest, about 30 km from Hubbard Brook. Both
chronosequences have their strengths and weaknesses, and we think using both provides the strongest test
of the hypotheses. The Hubbard Brook-Moosilauke sites have the benefit of many past and ongoing
measurements (see below) and they encompass the Hubbard Brook watersheds that provide the long-term
data that frame the questions we are asking. However, these sites span a distance of ~11 km. By contrast,
the Bartlett Experimental Forest, which is physically and floristically similar to Hubbard Brook, allows
location of multiple stands from 10 to > 200 years old within close proximity to one another and on a
common soil type relatively poor in N (Ollinger et al. 2002). Moreover, the Bartlett stands all have
repeated biomass inventories dating to 1932, allowing complete reconstruction of biomass accumulation
on all of these sites.
The Hubbard Brook-Moosilauke chronosequence (Table 1) includes several of the sites sampled
by Vitousek and Reiners (1975) and re-sampled by Goodale et al. (2003) to determine the role of
ecosystem development in N export. In addition, watersheds 2, 5, and 6 (W2, W5, W6) at Hubbard
Brook have unparalleled records of information on precipitation N input and stream N export that date to
the early 1960s (Likens & Bormann 1995). Quantitative sampling of soil N stocks has been conducted at
many of the sites (Table 1), allowing us to leverage many existing quantitative measurements of soil N
stocks. Moreover, the repeated plant and soil sampling at W5 since its harvest in 1983-84 provides a
temporal sequence that will allow us to evaluate part of the space-for-time substitution of the
chronosequence approach (e.g., Pardo et al. 2002). All of these sites have mixed northern hardwood
vegetation and occur on well-drained podzolic soils.
Table 1: Proposed Hubbard Brook-Moosilauke chronosequence, all mixed northern hardwood sites.
Intensive
Age
Cut
Soil N Stock
Stream N Export
Site Name
study plot
(2012) Date
Measurements
Measurements
for Q1?
1. Moosilauke Old Growth
Yes (MMOG)
>200
(none)
X
1974, 1997 (Goodale)
Yes (W6)
95
1917
2012 (Goodale)
1964-present (Likens)
3. HB W101 Mature
4. Moosilauke 1938
Hurricane
5. HB W2
no
95
1917
2010 (Goodale)
X
no
69
1943-7
X
1974, 1997 (Goodale)
no
46
1965-6
X
1964-present (Likens)
6. HB W101 Mid-age
Yes (W101)
42
1970
X
7. HB W5
no
28
1983-4
8. Moosilauke recent harvest
no
15
1997
2010 (Goodale)
repeated
(Johnson)
X
2. HB West of W6
1964-present (Likens)
None
To test the hypotheses above, we will measure the soil and vegetation N stocks and 15N values
on both chronosequences. For those sites on the Moosilauke-Hubbard Brook chronosequence that lack
existing sampling (Table 1), we will sample soils and vegetation at 4 plots per site stratified to match
12
known gradients in soil and vegetation between 525 and 775 m elevation (Huntington et al. 1988, Bohlen
et al. 2001). At Bartlett, we will sample 20 plots ranging from 10 to >200 y old. At each plot, we will
record the dbh and species of all trees > 5 cm in diameter. Wood and foliage will be collected from at
least two individuals of each of at least three dominant tree species, including at least one tree species
with arbuscular (red or sugar maple) and one with ectomycorrhizal (American beech, yellow birch)
association. These two types of mycorrhizal associations yield contrasting patterns of plant and soil
accumulation of N and 15N (Hobbie and Ouimette 2009). Ectomycorrhizae have the capacity to acquire N
from SOM through production of enzymes that facilitate the breakdown of recalcitrant organic matter,
whereas arbuscular mycorrhizae lack this capacity (Olsson et al. 2002). In addition, ectomycorrhizae
discriminate against 15N during transfer of N to plants more than do arbuscular mycorrhizae.
Wood and bark will be collected by 5 cm increment borer (e.g., McLauchlan et al. 2007) and
foliage will be collected from multiple positions in the canopy by shotgun (e.g., Ollinger et al. 2002).
Quantitative samples of soils and roots will be collected with a gas-powered, diamond-bit soil auger that
drills through roots and rocks. This recent improvement in soil sampling technology enables more
extensive spatial replication than is typically possible with the traditional quantitative pit sampling (Rau
et al. 2011). In measuring changes in mineral soil N stocks, the challenge has long been in how to detect
relatively small changes in a large and spatially heterogeneous N pool (at Hubbard Brook, ~6 + 0.3 t N
ha-1; Huntington et al. 1988, Johnson 1995). We will collect seven 9.5-cm diameter soil cores per plot,
with soil horizons separated into organic (forest floor) and mineral soil (0-10, 10-20, 20-30, 30-50 cm)
depths layers. Recent sampling with this corer at Hubbard Brook and Bartlett Forest demonstrates that
these soil horizons each contain 1-2 t N ha-1, and that this sampling design should allow a sampling
precision (standard error) of approximately 200 kg N ha-1 (C. Goodale, unpublished data). This precision
will not allow detection of expected changes in soil N over short time periods, but should suffice to detect
the ~500 kg N ha-1 expected be transferred to aboveground vegetation over the course of stand
development. Using a similar approach with 3 quantitative soil pits per site for post-agricultural forests in
Rhode Island and near Hubbard Brook, respectively, both Hooker and Compton (2003) and Hamburg
(1984) detected N losses from surface mineral soils (0-20 cm) averaging 14-15 kg N ha-1 yr-1 over 70-115
year chronosequences.
Concurrent with our measurements of soil N across the two chronosequences, we propose to
compare the distribution plant and soil 5N, and of soil N into light and heavy fractions, as well as the
activity of microbially produced carbon-degrading enzymes, especially phenol oxidase and peroxidase,
the enzymes most associated with the breakdown of recalcitrant soil organic matter (Sinsabaugh 2010). If
plants accelerate N mineralization by priming of decomposition by saprotrophic or ectomycorrhizal fungi,
we would expect the activity of phenol oxidase and peroxidase in the mineral soil to be greatest during the
phase of rapid plant N accumulation, and lowest in late succession, when plant N demand is the lowest.
Our analysis of plant, soil and whole-ecosystem 15N over succession will follow Compton et al.
(2007), who found that over the course of a century of old-field succession, in which N was mined from
the mineral soil and transferred to vegetation and surface litter, white pine foliage became less enriched in
15N, while both organic and mineral soil horizons became more enriched. We will examine whether the
same patterns occur in post-harvest successional forests, and whether these patterns reverse as expected
during the second century of stand development, after plant biomass accumulation ceases. These 15N
measurements will enable more detailed examination of the internal redistribution of N than afforded by
N pool measurement alone, through both examinations of 15N mass balance (e.g., Compton et al. 2007)
and the more sophisticated modeling proposed below.
Question 3. How do these processes alter N dynamics in an ecosystem model, and how does their
inclusion affect predictions of forest C and N cycling under scenarios of future climate, N deposition and
timber harvesting?
To examine the longer-term implications of mineral soil N source/sink dynamics, we will add
algorithms corresponding to mechanisms of soil mining and re-accumulation, parameterized by the data
13
collected in this project, to a newly revised version of the widely used PnET-CN model (Aber and
Driscoll 1997) called PnET-SOM (Figure 4; Tonitto et al. in prep.). The original PnET-CN is a forest
ecosystem model designed to simulate the effects of climate change, rising CO2, disturbance and
biogeochemical perturbations on forest coupled C, and N and H2O cycling (Aber et al. 1997; Aber &
Driscoll 1997; Aber et al. 2002; Ollinger et al. 2002). The model runs at daily to monthly time steps and
combines generalized relationships for processes such as photosynthesis, respiration, transpiration, litter
production, decomposition and N mineralization with climate, site and vegetation inputs to estimate C, N
and water fluxes. The model uses a multi-layered canopy sub-model of photosynthesis and phenology
initially developed for the PnET-Day and PnET-II models by Aber et al. (1995, 1996). Added to this in
PnET-CN are allocation and accumulation of C and N in live biomass, litter compartments and soil
organic matter, as well as algorithms for N mineralization and nitrification, plant N uptake and leaching
losses to produce complete cycles for C, water and N.
PnET-SOM replaces the single SOM pool of the PnET-CN model with representation of organic
matter dynamics by soil horizon (O, A, B) and organic matter form. Plant litter is divided into three
fractions of varying quality and N dynamics during decomposition were derived using data from the
LIDET experiments (Parton et al. 2007). SOM is divided into measurable humus (light fraction) and
mineral-associated (heavy fraction)
pools in the forest floor (Oa) and two
mineral horizons (A and B) . Rather
than relying on calibration, turnover
rates for northeastern U.S. SOM pools
are based on the 14C-based estimates of
Gaudinski et al. (2000).
For the proposed work, the
PnET-SOM model will be used both to
integrate results from the forest floor
15
N addition experiment and the soil
chronosequence studies, and to
quantify the long-term implications of
our new conceptual model (Fig. 3)
under future scenarios of climate and
forest management. First, we will use
results from the soil solution chemistry
work to refine parameters for N
transfers between SOM pools and rates
of N uptake. This will require addition
of 15N fractionation values and
Figure 4. Structure of the PnET-SOM model, showing
algorithms to account for N losses to
vegetation (green), soil (brown) and water (blue) pools and
denitrification. Although both of these
associated fluxes for each element. Arrows shown in red
modifications can be challenging, we
represent processes that will be modified as a result of our field
will draw from a related effort
measurements. 15N isotopic fractionation rates will be added to
involving the PnET- DNDC model (Li
all N transfers and transformations.
et al. 2004), which is being used as
part of ongoing denitrification research
at Hubbard Brook (on which both Goodale and Ollinger are co-investigators).
After improvements to PnET-SOM are in place, we will apply the model to the sites examined in
the soil chronosequence studies. Given the relatively short-term nature of the soil solution chemistry
measurements, we anticipate that refinement of N transfer parameters may be required. If so, we plan to
make use of the 15N and soil N pool data from the HB-Moosilauke chronosequence only, leaving the
Bartlett chronosequence data as independent validation.
14
With improvements to the model in place, we will estimate the magnitude and temporal dynamics
of the soil N source/sink relationships over the course of multiple disturbance/recovery sequences and
explore the impact of future scenarios of climate, rising CO2 and climate change. Future scenarios of
forest harvesting will include return intervals of 80-100 years (reflecting current best management
practices for the region) as well as the shorter rotations that may occur if pressures for forest biomass
energy production continue. Future climate scenarios will come from a set of statistically downscaled
GCM simulations that are already available for HBEF (Hayhoe et al. 2008) and have been used in
previous analyses with PnET (Ollinger et al. 2008). Projections of future N deposition will be based on
“business-as-usual” scenarios as well as estimated reductions in NOx emissions due to the EPA’s new
Cross-State Air Pollution Rule.
Chemical analysis
Soil solution chemistry will be determined using colorimetric auto-analysis for NO3- (azo dye),
+
NH4 (indophenol blue), and DON (persulfate oxidation followed by azo dye) on an Astoria II
Autoanalyzer at the University of New Hampshire. Total dissolved nitrogen (TDN) and DOC will be
quantified via thermal combustion using a Shimadzu TOC/TN analyzer at Cornell University. δ15N and
δ18O of NO3- will be determined following chemical denitrification to gaseous N2O following methods
outlined in McIlvin and Altabet (2005). Briefly, NO3- will be quantitatively reduced to nitrite using
spongy cadmium and converted to N2O using acetic azide prior to cryogenic pre-concentration on a
Thermo Finnigan Precon trace gas analyzer and analysis on a DeltaPlus XP isotope ratio mass
spectrometer at UNH.
Soil and plant13C and 15N will be measured using on a DeltaPlux isotope ratio mass
spectrometer coupled to a Costech 4010 elemental analyzer at the UNH Isotope Laboratory. Density
fractionation of soil into light and heavy fractions of will be determined using the methods of Crow et al.
(2007). Briefly, the fractions are separated by suspending in liquid sodium polytungstate (SPT) with a
density of 1.65 g/cm3. The fractions are physically separated and analyzed for %C, %N, 13C and 15N as
above. Mineral associated organic matter (MAOM) will be determined by the methods of Castellano et al
(2012), which involves separating the mineral-associated fraction by passing a soil suspension through a
53 m sieve and analyzing the retained fraction for %C, %N, 13C and 15N as above. The “heavy fraction”
of soil and MAOM are essentially two techniques for isolating the same fraction of SOM, and we will
compare the techniques and determine which seems most appropriate for our study. Microbial biomass C
and N will be determined by the chloroform fumigation-extraction technique (e.g., Brookes et al. 1985)
and 13C and 15N will be measured on the extraction solutions using the soil solution methods above.
The oxidative enzyme assays will follow Allison and Jastrow (2006).
Broader Impacts.
This research presents opportunities for a broad spectrum of impacts within our discipline and in
the wider society. Within the discipline, this work is likely to have a broad impact on ecosystem science
because it addresses a fundamental conceptual model of how ecosystems work and offers a potential
revision of that model. If our hypotheses are supported, the result will change how we perceive linkages
between terrestrial and aquatic ecosystems and how those linkages are managed, modeled, and discussed
in classrooms. Our revision of the PnET model also provides an opportunity to generalize this work and
make it available to more students and scientists.
This project also presents educational opportunities at several levels. We propose a teacher
training program associated with the Hubbard Brook Research Foundation (HBRF) and its
Environmental Literacy Program (http://hubbardbrookfoundation.org/environmental-literacy-program/
), which uses the research activities and findings of the Hubbard Brook Ecosystem Study to support the
teaching of science-inquiry skills to middle and high school students. HBRF education staff have
developed extensive contacts with high schools and teachers in NH (an EPSCOR state) and have
developed teaching modules and brought teachers to Hubbard Brook for hands-on research training. We
propose to include two teachers in this research by funding their participation in the summer field
15
research, to which they would contribute by doing their own independent project under the supervision of
HBRF staff and a project PI.
In a small teacher “focus group” we held during the development of this proposal, we discovered
considerable enthusiasm for development of a simple forest growth model, with which students and
teachers could adjust perhaps six or seven different parameters (such as CO2, temperature, rainfall,
nitrogen deposition, harvesting, etc. ) and simulate the effects on forest growth. We will develop such a
model, with the intent of giving the students perspective on both the power and the pitfalls of computer
simulation (to which the public is increasingly exposed in news stories about global change), while also
teaching about the multiple interacting factors affecting forests. The output of the model will be graphical,
and will allow students to ask questions, test hypotheses, and interpret and evaluate output in accordance
with the cross-cutting themes between math and science expected in the national Next Generation Science
Standards. These national standards are due to be piloted next school year and may be adopted widely in
following years. The project PIs will lead the development of this model, in collaboration with HBRF
education staff and targeted teachers from NH high schools. The teachers will “beta test” the model in
their classrooms and recommend revisions to make it most effective as a teaching tool. The final model
will be made available on the HBRF web site. HBRF education staff have experience developing
simulation models as teaching tools, and are using one in a teaching module about migratory birds
(http://hubbardbrookfoundation.org/migratory_birds/index.html ). Co-PI Ollinger also has experience
developing student-friendly models, through his efforts as lead of the NSF-GLOBE program’s K-12
Carbon Cycle Science project (globecarboncycle.unh.edu). We have budgeted for a subcontract with
HBRF in years 2 and 3 for this work and summer stipends for the teachers discussed above.
At the post-secondary level, this project will also result in the training of teams of undergraduate
students assisting with field and laboratory work, two graduate students (one each at UNH and Cornell)
and a postdoctoral associate at the Cary Institute. The PIs expect to mentor 1-2 undergraduate students
per year through the Hubbard Brook REU program.
Finally, the proposed research is highly relevant to forest management and public policy, because
it deals with the linkages between forest growth, water quality and the sustainability of forest harvest
practices with respect to soil N pools. We have an exceptional opportunity to strengthen the scientific
foundation of the management of National Forest lands because the revision of the management plans for
both the White Mountain and Green Mountain National Forests, which happens only every 10-15 years, is
due to begin in the period of this grant. We propose to host a workshop for forest scientists and land
managers, targeted to meet the information needs of USFS forest managers and natural resource planners
with the White Mountain and Green Mountain National Forests. Forest managers in the region are under
pressure to increase whole tree harvesting for biomass energy production while protecting soil and water
quality. While they strive to use the best available science, they often face the challenge of fragmented
findings scattered across the scientific literature. The workshop, which will be held in VT or NH in 2015,
will facilitate a two-way exchange that will allow us to learn more about their specific management
issues. Scientists at the workshop, including the project PIs as well as other forest scientists in the region,
will share the current state of the science related to forest management and water quality. We
will prepare a scientific summary relevant to the management concerns of the forest managers at the
workshop. To host this workshop, we will collaborate with Northeast Science and Policy Consortium, a
new regional initiative for science-policy linkage in the Northeast
(http://harvardforest.fas.harvard.edu/policy-regional ). A letter of collaboration from Kathy Fallon
Lambert of the Northeast Science and Policy Consortium is included. We have budgeted for the
workshop and report preparation in year 3 of the project.
Project management and PI responsibilities
All PIs will share in the management, decision making, analysis and publication of the research in
this project. Lovett will be the overall project director and, with the assistance of the Cary postdoc, will
take primary responsibility for the labeled-litter field experiment. Goodale will have primary
responsibility for the chronosequence studies, including the interpretation of the isotope measurements,
16
assisted by Ollinger. Ollinger will have primary responsibility for the soil solution measurements and the
modification and testing of the of the PnET-SOM model, assisted by Goodale. Goodale and Ollinger will
oversee the training of graduate students at Cornell and UNH, respectively. Lovett will supervise the
HBRF subcontract for teacher training and development of the forest model teaching tool, and the
Northeast Science and Policy Consortium for organizing the workshop with National Forest Managers.
Goodale and Ollinger will participate in both activities. Project meetings will be quarterly, associated
with quarterly meetings of the Hubbard Brook LTER project, and will be supplemented by frequent
phone and email contact. Field personnel will be shared among institutions to expedite periods of intense
field work. Data sharing is described in the attached data management plan.
Timeline
Year 1: Site selection, chronosequence sampling, installation of small plots field experiment and
lysimeters, double-labeling of trees, application of labeled litter, processing of soil from intensive sites.
Year 2: Mineral soil bag installation, soil solution monitoring, coring of soils and measurement of trees
in chronosequences, initial model development. RET and REU training. Begin chemical analysis.
Year 3. Complete chemical analysis and data analysis. RET and REU training. Workshop for forest
managers. Development of forest model teaching tool, and testing of revised PnET-SOM model.
Manuscript preparation.
Results from Prior NSF support
DEB 9981503 Effects of an Introduced Pest on the C And N Dynamics of a Northern Hardwood Forest.
$974,748, 3/00–2/05. Major findings: tree species of the Catskill mountains differ markedly in foliar N,
litterfall N, N mineralization, soil C:N ratio, and nitrification (Templer et al. 2003, Lovett et al. 2004,
Templer et al. 2005), resulting in different NO3- concentrations in headwater streams (Lovett et al. 2002);
patterns are modified by beech bark disease severity (Griffin et al. 2003, Hancock et al. 2008 , Lovett et
al. 2010). This work has resulted in 19 published papers and trained 1 PhD student, 3 MS students, 1
Post-doc, and 7 undergraduates.
NSF-GEO-0627916; Collaborative Research: Exploring Ecosystems and the Atmosphere in the
Classroom: A Plan to Integrate NASA Carbon Cycle Science with GLOBE. $968,166, 8/06-7/12. The
NSF GLOBE award involves building educational activities that connect students in middle to high
school levels with current research on the carbon cycle. We have developed over 30 student activities
involving field measurements of carbon stocks and fluxes, classroom experiments and computer
simulation models designed specifically for students. We have trained over 100 teachers in North
America, Europe and Africa. The full set of activities are being prepared for formal release to the GLOBE
network (www.globe.gov), which includes over 24,000 schools in 112 countries. This work has been
presented at over a dozen national and international meetings and is scheduled to be released in full in
2012. The GLOBE Carbon project is described in Sallade et al. (2012) and at
http://www.globe.gov/explore-science/field-campaigns/essp/carbon-cycle
DEB 845451 NSF CAREER Mechanisms of Forest Nitrogen Retention Over Seasons, Sites, and
Succession $536,700, 08/09 – 07/14. This ongoing project has supported studies of seasonal 15N
retention at Arnot Forest, NY; studies of DOC and nitrate interactions in the Adirondack and Catskill
Mountains, NY; and lab and field studies of the effects of nitrogen and acidification on forest C and N
balance. To date, it has supported 12 undergraduate researchers and resulted in 12 national or
international presentations and three publications (Curtis et al. 2011, Lovett & Goodale 2011, Stone et al.
2012), including one led by an undergraduate student researcher, Madeleine Stone, as well as Goodale’s
contribution to editing a special issue on N effects on ecosystems (Goodale et al. 2011). A postdoctoral
associate will be developing science communication exercises for teachers over the coming year, and 4
manuscripts are in progress for anticipated submission within the next six months.