Notes
-,
-,
waters, Pap. No. 5 1. In Gas-liquid chemistry of
natural waters. Proc. Conf. Brookhaven Natl. Lab.
NTIS Publ. BNL 51757.
J. W. MOFFETT, R. G. PETASNE, W. J. COOPER,
AND E. S. SALTZMAN. 1985a. Spatial and temporal variations of hydrogen peroxide in Gulf of
Mexico waters. Geochim. Cosmochim. Acta 49:
1173-l 184.
E. S. SALTZMAN, W. L. CHAMEIDES, AND D. 0.
DAVIS. 1982. H,O, levels in rainwater collected
Limnol. Oceanogr., 33(6, part 2), 1988, 1611-1617
and Oceanography,
0 1988, by the AmericanSociety of Limnology
--
1611
in south Florida and the Bahama Islands. J. Geophys. Res. 87: 5015-50 17.
AND W. J. COOPER. 19858. Hydrogen
peioxide c&centrations
in the Peru upwelling area.
Mar. Chem. 17: 265-275.
Submitted: 9 December 1987
Accepted: 27 July 1988
Revised: 29 August 1988
Inc.
The relative importance of denitrification and nitrate
assimilation in midcontinental bogs
Abstract-Denitrification
rates measured in a
Minnesota bog and a bog in western Ontario were
similarly low (~0.20-2.28 pg rn-* h-l as N). Nitrate addition stimulated denitrification
at the
Minnesota bog, but < 1% of added NO,--N (0.0 l0.1 g m-‘) was denitrified in 24 h. In the Ontario
bog, denitrification was not stimulated by NO,--N
application (0.08 g m-*). Rates of NO,--N
uptake by Sphagnum, measured by nitrate reductase activity assays and NO,- disappearance from
cultures, were much higher, lOO-24,000 pg rn-*
h-l (0.57-48.7 pg g-l h-l), than rates of denitrification at comparable NO,- loadings. Measurements of NO,- in pore water and moss throughfall confirm that N03- disappears in the top 510 cm of moss. Plant uptake appears to be the
dominant sink for NO,- in midcontinental
bogs.
The effects on North American peat bogs
of increasing atmospheric deposition of nitrate (Galloway and Likens 198 1) are not
known. Nitrate is efficiently retained within
peatlands as evidenced by the fact that outputs are generally < 5% of N03- inputs (Hemond 1983; Urban and Bayley 1986; Urban
and Eisenreich 1988), but the mechanism
Acknowledgments
This work was funded by NSF Grant DEB 79-22 142,
by the Canadian Department of Fisheries and Oceans,
and by a Doctoral Dissertation Fellowship from the
University of Minnesota to N.R.U. We acknowledge
the support and assistance of D. W. Schindler at the
ELA and E. S. Verry and A. Elling of the U.S. forest
Service.
of retention is unknown. Possibilities include denitrification,
assimilation by plants,
and dissimilatory reduction to ammonium.
Short-term rates of N03- assimilation
have not been measured in peatlands. Annual plant uptake of N ranges from 6 to 60
kg ha-l (Martin and Holding 1978; Rosswall and Granhalll980;
Hemond 1983; Urban and Eisenreich 1988). Vascular plants
in bogs have a low capacity to assimilate
N03- (Lee and Stewart 1978; Havill et al.
1974). Bryophytes have a greater capacity
to use N03- (Press and Lee 1982; Woodin
et al. 1985), but rates of uptake have not
been measured. Although dissimilatory
reduction to ammonium may be an important
process in a Massachusetts bog, consuming
25% of N03- inputs (Hemond 1983), this
pathway accounted for < 10% of N03transformations
in a freshwater
marsh
(Bowden 1986a).
Potential rates of denitrification
in peat
are high (Muller et al. 1980; Hemond 1983;
Urban 1983), but it is not known if nitrate
penetrates to anaerobic zones in the field.
Denitrification
was not detected in a subarctic mire receiving low nitrate inputs
(Rosswall and Granhall 1980), and nitrate
generally is undetectable in bog waters
(Gorham et al. 1985) and peat (Waughman
1980). Denitrification
in peatlands could be
a significant source of N20 to the atmo-
1612
Notes
sphere (Bowden 1986b), although previous
measurements may be overestimates (Bowden 1987). Nitrous oxide, a major end product of denitrification
at the low pH characteristic of bogs (Delwiche
1979), may
contribute to global warming (Wang et al.
1976; Donner and Ramanathan 1980; Weiss
198 1) and stratospheric ozone depletion
(Crutzen and Howard 1978). Few measurements of N,O emissions from natural ecosystems have been made (Natl. Res. Count.
1981; Crutzen 1983; Bowden 1986b).
The objective of this study was to determine the depth at which nitrate is retained
in peat and to compare rates of denitrification in that depth of peak with rates of
nitrate uptake by Sphagnum. Our field measurements of denitrification
show the relative importance of N20 as an end product
and represent the first measurements of
summer emission rates of N20 from midcontinental bogs.
Denitrification
and plant uptake of N03were measured in two small peat bogs: the
Marcel1 S-2 bog in north-central Minnesota
(47”32’N, 93’28’W) and a bog in the Experimental Lakes Area (ELA) of western
Ontario (49”40’N, 93”43’W), 300 km north
of the Marcel1 bog. The peat deposits at the
Marcel1 and ELA sites are similar in size
(3.24 and 3.67 ha) and both receive runoff
from comparably sized terrestrial uplands
(6.24 and 8.76 ha, respectively). At both
sites, perched water tables exist that are isolated from the regional groundwater and that
show annual vertical fluctuations of about
30 cm (Verry 1984). With one exception at
the ELA site, pools of standing water are
absent. Vegetation is similar at both sites
and consists of a tree layer of black spruce
(Picea mariana), a shrub layer dominated
by Labrador tea (Ledum groenlandicum) and
leatherleaf (Chamaedaphne calyculata), a
herb layer of Carex trisperma, Carex oligosperma, and Smilacina trifolia, and a
ground layer of mosses (principally Sphagnum magellanicum on hummocks and
Sphagnum angustifolium in hollows) (Vitt
and Bayley 1984).
The ELA site has been experimentally
acidified since 1983. Once a month for 6
months of the year, 2.7 ha are sprayed with
a 1.74 : 1 (molar basis) mixture of nitric and
sulfuric acid at a pH of 3. The application
increases acid loading to the site lo-fold,
equivalent to reducing the mean pH of precipitation from 5.0 to about 4 (Bayley et al.
1987). The unacidified portion of the site
receives an equal volume of water without
acid.
Denitrification
in unsaturated peat was
measured with an acetylene block technique. Glass chambers (2 liters, 12.5-cm
diam) with a septum sampling port were
pushed into the moss until the bottom was
below the water table and injected with 100
ml of acetylene and 10 ml of Freon- 12 (10.7
ppmv) as an internal standard. Over 24 h,
the headspace was sampled periodically with
a 25-ml syringe. A volume of ambient air
equal to that withdrawn was injected into
the chambers after each sampling. Gas samples were injected into water-filled
15-ml
glass vials, displacing water through a second needle. Vials filled with standards were
used to check for gas leakage. Denitrification rates were calculated by regressing N20
concentrations vs. time.
Gases were analyzed with a Hewlett Packard 5 840A GC equipped with a 3-m column
(6.4-mm o-d.) with Porapak Q and a 63Ni
electron capture detector. Analysis was done
isothermally
at 55°C the detector was
maintained at 325°C and the Ar-CH, (95 : 5)
carrier gas flow rate was 35 ml min- l. Certified standards of Freon- 12 and N20 were
purchased from Matheson Gas Products.
Denitrification
was measured in August
and October at Marcell. From 7 to 16 chambers (two-thirds with acetylene, one-third
without) were used on each of 4 d. Background rates were measured on a transect
through the bog, and rates were measured
on plots that had just been sprayed with
varying amounts of N03--N (0.0 l-55 g m-2,
Table 1). The stability of N,O in the chambers was assessed by adding 100 ml of 1.04
ppmv N,O to eight acetylated chambers. At
the ELA site, denitrification
was measured
daily for 4 d in August. On the second day,
acid was sprayed on two-thirds of the bog
at a pH of 3 (0.080 g m-2 N03--N).
Each
day, 16 chambers were used: 5 in the control
portion of the bog, 11 in the acidified area.
As at Marcell, unacetylated chambers were
used to determine N20 emission rates, and
Notes
N,O standards were used to verify N20 stability in field chambers and during sample
storage. To determine if denitrification
occurred below the water table, we measured
nitrous oxide in 18 pore-water profiles at
the ELA bog and 2 at Marcell. Pore water
was sampled at the surface and at 5-cm intervals down to 35 cm with a hollow, stainless steel tube. Profiles were measured at
each site (four sites in 1984, two in 1985)
before, 1 h after, and 24 h after acid applications.
To determine the depth at which nitrate
is taken up in the Marcel1 bog, we cut Plexiglas tubes (4-cm diam) in half lengthwise
and inserted them horizontally into a hummock at various depths below its top.
“Rainfalls”
of various intensities were simulated by allowing rainwater collected at the
site to drip from a bucket with pinholes in
the bottom. Water samples obtained after a
simulated rain event were brought to the
lab, filtered immediately, and frozen until
analyzed by ion chromatography for N03-.
Six profiles were obtained from four field
trips (June, October).
Nitrate uptake by Sphagnum was examined in two ways: nitrate reductase activity
in Sphagnum was measured in the field, and
rates of disappearance of N03- from solutions with Sphagnum were measured in the
laboratory. Nitrate reductase activity (NRA)
was measured by a modification
of techniques used by workers in England (Lee and
Stewart 1978; Press and Lee 1982; Woodin
et al. 1985). About 12 stalks of green moss
were placed in each of six centrifuge tubes,
each with 10 ml of phosphate buffer (100
mM, pH 7.5). After a 1.5-h incubation in
the field in the dark, tubes were placed in
boiling water for 20 min, the liquid decanted for measurement of nitrite, and the
moss dried and weighed. Six tubes were
boiled immediately without incubation to
determine initial nitrite concentrations. Sets
of six tubes were taken from four plots that
had received nitrate applications of 0, 0.1,
0.05 and 0.10 g m-2 N03- -N (1 cm of simulated rainfalls of solutions of 0, 1, 5, and
10 mg liter-l N03- -N). Measurements of
NRA were made immediately following and
24 h after application of nitrate. Increases
in nitrite within moss during incubation are
1613
Table 1. Uptake of nitrate within
“Ram”
intensity
0.25*
Depth
(cm)
0
21
25
30
a hummock.
(cm h-l)
1.ot
2.0$
Depth
NXl§
100
<4
~4
<4
Depth
(cm)
[NW18
(cm)
WX18
0
5
10
12
15
17
100
64
6
38
4
17
0
5
10
100
58
70
15
86
25
4
25
85
35
7
* One replicate; 2 cm applied.
t Three replicates; 2-3 cm applied.
$ Two replicates; 2-5 cm applied.
§ NO,- concentrations
given as a percent of values at hummock
surface.
regarded as minimal estimates of rates of
N03- uptake.
Three laboratory experiments performed
at ELA examined nitrate uptake by measuring N03- disappearance from water containing live Sphagnum (without peat). In
the first experiment, live, green S. magellanicum (~0.36 g dry wt) was placed into
1OO-ml jars containing 50 ml of dilute nitric
acid. Concentrations
of N03- were measured before addition of Sphagnum and 14
h after addition. Three replicates were measured at each of five initial nitrate concentrations (0.2, 2.1, 4.8, 9.6, and 19.2 mg liter-l N03- -N). Initial pH values ranged
from 2.86 to 4.84. Jars were kept in the light
at room temperature.
The second experiment was to determine
if high uptake rates were maintained over
extended periods. A mixture (pH = 3.4) of
nitric acid (760 pg liter-l N03- -N) and sulfuric acid (6.0 mg liter-’ SO4 2- -S) was
added to three replicate jars with S. magellanicum and three with S. angustifolium.
Each day for 22 d, the solution was decanted
and replaced with 50 ml of fresh solution
at the initial concentration. Nitrate concentrations were measured in the decanted solutions. Jars were kept under ambient conditions of light and temperature.
The third experiment was designed to saturate the moss with nitrate. The procedure
was identical to experiment 2, but the concentration was increased to 10.6 mg liter-’
N03- -N (pH = 2.95). Only S. magellani-
Notes
1614
Table 2.
Site
Summary of denitrification
Standard
recovered
from vials
(%I
measurements.
Freon recovery in
field chambers
Mean
SE(n)
Nitrate-N
apphed
k m-7
Mean$
0
55
0
0.01
0.05
0.10
2.28
52.2
0.3
0.25
0.51
1.09
1.39(6)
16.3 (4)
0.17(7)
0.06(3)
0.09(3)
0.12(3)
Control
Exptl§
Control
Exptlg
Control
Exptl§
Control
Exptl§
1.6
0.5
co.20
0.36
co.20
co.20
1.19
2.39
1.06(4)
0.35(5)
Denitrificatron
Nitrous oxide
emission rate+
rate*
SE(n)
Mean+
SE(n)
0.61
0.28(5)
Marcel1
Aw
100
0.79
0.06( 14)
Ott
89
0.84
0.03(24)
60
0.84
0.02(64)
ELA
Before
acidification
Day of
acidification
1 d after
acidification
2 d after
acidification
(4)
0.24(6)
co.20
(3)
(4)
(10)
0.80(4)
1.85(11)
co.20
(3)
* Only rates baesd on four or more samples from a given chamber were included in the mean. Units are pg mm2 h-l as N.
t Nitrous oxide emission rate determined
from chambers without acetylene. Units are pg mm2 h-r as N.
f Values of half the detection limit were used for all measurements
not significantly
different from zero.
4 Experimental
acid application
at ELA supplied 0.08 g mm* NO,--N.
cum was used (in triplicate) and the experiment was continued for 13 d.
Our results suggest that nitrate inputs in
rain do not reach waterlogged, anaerobic
peat. Nitrate reaching the surface of S. magellanicum-covered hummocks in simulated rain was efficiently removed in the top
10 cm (Table 1) at Marcel1 bog. Only at
unrealistically
high rainfall intensities did
nitrate penetrate to greater depth. Insertion
of the Plexiglas collection troughs may have
separated the moss and enhanced channeling of rainfall. These experiments thus may
have overestimated the depth to which nitrate normally penetrates. Similarly, at the
ELA bog nitrate concentrations
in pore
waters increased only after two of six applications of nitric acid (Bayley et al. 1987).
Thus, although potential rates of denitrification in water-saturated
peat are high
(Muller et al. 1980; Hemond 1983; Urban
1983), for denitrification
to be a major loss
mechanism for nitrate it must occur in anaerobic microsites
in unsaturated
peat
(Bowden 1986b; Parkin 1987).
Direct field measurements of denitrification in peat above the water table revealed
low background rates at both the Marcel1
(0.3-2.3 pg me2 h-l) and ELA (<0.2-l .6
pg me2 h-l) bogs (Table 2). Leakage from
incubation chambers was minor (< 20%),
and N,O concentrations were adjusted according to the mass of Freon-12 present.
Low rates were not due to nitrous oxide
consumption within acetylated field chambers; recovery of N20 spikes was essentially
100% throughout the incubation. At Marcell, rates of denitrification in chambers with
N,O spikes (1.53kO.59 pg m-2 h-l) were
greater than in chambers without spikes
(0.16kO.43 pg m-2 h- ‘). Were N20 being
consumed, the opposite would be predicted.
Thus, low background rates and lack of response to HN03 addition at ELA are not
artifacts of the technique, but may result
from low availability of nitrate. Extractable
nitrate was not measured at these sites but
is typically undetectable (~2 pg ggl dry wt)
in bog peat (Waughman 1980) and surface
bog waters (Gorham et al. 1985).
Concentrations of nitrous oxide in pore
water were similar at each site and usually
showed little variation with depth (range,
7-27 nmol liter-‘). There was no consistent
change in concentration following acid applications. The largest concentration
increase observed was ~0.5 pg liter-l as N.
In all but one of the profiles from the ELA
and Marcel1 bogs, concentrations of N,O in
pore water were close to saturation (Weiss
Notes
and Price 1980) with ambient air concentrations at the temperature (8”-12°C) of pore
water in the Marcel1 site. At the low pH (z 4)
of the pore water, N20 is thermodynamically a favored end product of denitrification (Delwiche 1979). Low concentrations
of N,O in pore water therefore suggest that
background rates of denitrification
beneath
the water table are low also.
Enhanced denitrification
in unsaturated
peat occurred in the Marcel1 bog following
application of NO,- -N (Table 2), but maximum rates were only three times background rates. Denitrification
rate was
strongly correlated with nitrate concentration (P < 0.01) and constant over 24 h. In
no case was > 1% of applied nitrate recovered as N20 within 24 h in acetylated
chambers. The small denitrification
response to nitrate addition suggests that denitrification
in unsaturated peat is a minor
sink for nitrate inputs. Even if the response
were 10 times greater and denitrification
took place 365 d yr-‘, only 5% of annual
nitrate inputs would be denitrified. Denitrification rates may be elevated in spring
when nitrate in snowmelt reaches saturated,
anaerobic peat. Because only 1O-20% of annual nitrate inputs occur in snow (Urban
1983), 20% (0.8 kg ha-l yr- ‘) of total nitrate
input is probably an upper limit for denitrification losses of N at the Marcel1 bog.
At the ELA site neither rates of denitrification in unsaturated peat nor concentrations of N20 in pore waters increased in
response to acid application. Given the high
N03- -N loading in the acid application
(0.08 g m-2), the lack of denitrification
response is at variance with results observed
at Marcell. It is not known whether denitrifiers are less abundant at the ELA bog or
whether the plants take up nitrate more rapidly at this site. Acid application has stimulated moss growth (Bayley et al. 1987).
At both sites background rates of N,O
emission were low, and N20 accounted for
~25% of total denitrification
end products
(Table 2). In contrast, Hemond (1983) reported that N20 constituted 100% of end
products from laboratory incubations
of
peat, and Urban (1983), in similar laboratory studies, found N20 to represent 80%
of end products. Both studies used water-
1615
Table 3. Uptake of nitrate by Sphagnum angustifilium at Marcel1 bog as measured by nitrate reductase
activity.
Nitrate-N
loading
(g m-T
0
0.01
0.05
0.10
Uptake is expressed per gram dry weight.
Time after
NO,- addition
(h)
1
20
1
20
1
20
1
20
Uptake rate
(pg g-’ h-7
0.005
0.005
0.567
0.002
0.621
0.038
0.884
0.198
$6)
0.010
0.010
0.238
0.013
0.181
0.022
0.083
0.088
saturated peat from below the water table.
The low fraction released as N,O in our field
chambers may reflect the presence of anaerobic microsites with higher pH above the
water table. The low rates of denitrification
measured at both bogs (~0.2-2.4 pg m-2
h-l as N), even after nitrate applications,
suggest that summer emission rates of N20
from midcontinental
bogs are low relative
to emission rates from other ecosystems
(0.4-142 PLgm-2 h-l; Crutzen 1983).
Experiments at Marcel1 and ELA suggest
that plant uptake is more rapid than denitrification. At Marcell, denitrification
rates
ranged from 0.2 to 1.3 pg m-2 h-l in plots
receiving nitrate amendments (Table 2).
Plant uptake (NRA) rates in the same plots
were 800-2,200 times greater (567-884 PLg
m-2 h-l) 1 h after nitrate addition and were
still 8-l 80 times greater than denitrification
rates 20 h after nitrate additions (Table 3).
The NRA measurements are minimal estimates of nitrate uptake because nitrite was
not shown to be conservative during the
incubations. If nitrite reductase activity increased substantially over the 24-h period
between measurements of NRA, total plant
uptake may have been much greater than
estimated above. Future work with 15N is
needed to more accurately measure the relative rates of these two processes, but the
preliminary field measurements at Marcel1
indicate that plant uptake rates are much
higher than rates of denitrification.
Rates of disappearance of N03- measured in the ELA laboratory (1,000-48,000
pugrnp2 h-l; Table 4) were even higher than
rates of NRA measured at Marcell. Rates
Notes
1616
Table 4. Uptake of nitrate by Sphagnum in laboratory experiments at ELA. Uptake is expressed per
gram dry weight.
Nitrate-N
Concn
(j4g liter-l)
Loading
(g m-7
189
2,145
4,825
9,575
19,200
715
759
10,950
10,600
* Average
hourly
Time
after NO,addition
(‘0
Total
uptake
(/.cz g-‘1
0.005
0.053
0.120
0.239
0.480
Experiment
14
14
14
14
14
0.03
0.60
Experiment 2
51
24
22-d
960
0.53
5.03
Experiment 3
24
425
13-d
1,359
uptake
1
28
249
486
552
682
Uptake rate
bs g-’ h-9
SE
(n = 3)
1.99
17.8
34.7
39.4
48.7
0.26
3.02
3.72
0.28
4.56
2.14
1.82*
17.7
4.4*
n=2
n=2
1.02
0.95
over 22- or 13-d period.
of disappearance measured in these experiments were comparable to potential rates
of NRA (56-70 pugg-l h-l) measured by
Woodin et al. (1985) in response to application of 14 mg liter- 1of N03--N to Sphagnum &scum hummocks, but without the
growth inhibition and toxicity noted by Press
and Lee (1982) and Woodin et al. Rates
were concentration-dependent
and maximum potential rates were not achieved in
the 14-h experiment. High rates of disappearance were sustained over a 22-d period,
and considerable stimulation of growth was
noted in the 22-d experiment with 0.76 mg
liter- l of N03- -N. This finding suggests that
active uptake rather than ion exchange was
responsible. An anion exchange capacity
> 10 meq (100 g)-1 would be necessary to
account for the nitrate that disappeared. Although denitrification
may have occurred
in these experiments, it is unlikely that it
accounted for much of the NO,- loss. To
minimize microbial effects and anaerobic
microsites, we used only green moss without peat. Measured rates of N03- disappearance (l-4-49 pg g-l h-l) greatly exceeded the maximum
potential rate of
denitrification
in water-saturated peat from
Marcel1 (0.28 pg g- ’ h- ‘; Urban 1983), Thoreau’s Bog (1 pg g-l h-l; Hemond 1983), or
southern Finland (1.4 pg g-l h-l; Muller et
al. 1980). Enrichment of microbial populations seems to have been a minor effect
since rates of N03- disappearance did not
increase with increasing duration of experiments (Table 4). Although these experiments do not yield accurate measurements
of plant uptake rates, they do support the
claim that rates of N03- uptake by Sphagnum are greater than rates of denitrification.
Although experiments at Marcel1 and ELA
show that nitrate is taken up by Sphagnum,
British and German studies (Lee et al. 1987;
Rudolph and Voigt 1986; Press and Lee
1982) show that high concentrations of both
nitrate and ammonium inhibit Sphagnum
growth. Sphagnum grown in “unpolluted”
areas of Britain and Germany experiences
higher deposition rates of N than does moss
in midcontinental
North America. The
N-deficient condition of North American
peatlands may account for the discrepancies
between our results and those observed in
Europe.
These results suggest that more of the nitrate reaching the bog surface in throughfall
is taken up by bryophytes than is lost to
denitrification.
The importance of dissimilatory reduction of nitrate to ammonium
is unknown. Both denitrification
and dissimilatory reduction to ammonium may be
more important during snowmelt when the
water table is near the moss surface and
more nitrate may reach anoxic sites. Our
study suggests that increased deposition of
anthropogenic nitrate will fertilize North
American peat bogs.
N. R. Urban
S. J. Eisenreich
Environmental
Engineering Program
Department of Civil and Mineral
Engineering
University of Minnesota
Minneapolis 5 545 5
S. E. Bayley
Department of Botany
University of Manitoba
Winnipeg R3T 2N2
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Submitted: 19 March 1987
Accepted: 19 May 1988
Revised: 5 August 1988