1
Appendix S1 Description of experiments and predictor characterization
2
3
Experimental methodology: Differences in experimental protocols are summarized in
4
Table S1. Further details on experiments and sites can be found in the literature for ARC
5
(Shaver et al. 1996; Gough et al. 2002a; Gough & Hobbie 2003), CDR (Tilman 1987;
6
Wedin & Tilman 1996), JRG (Shaw et al. 2002; Zavaleta et al. 2003), KBS (Huberty et
7
al. 1998), KNZ (Seastedt et al. 1991; Blair 1997; Turner et al. 1997; Collins et al.
8
1998b), NWT (Bowman et al. 1995; Theodose & Bowman 1997; Seastedt & Vaccaro
9
2001), SEV (Noy-Meir 1973; Kieft et al. 1998), and SGS (Lauenroth et al. 1978;
10
Milchunas & Lauenroth 1995).
11
12
Climatic, soil, and biogeochemical properties: We used data from either online sources
13
or the published literature to characterize climatic, soil, and biogeochemical properties.
14
Where possible, we used data that utilized similar methodologies at multiple sites over
15
the same time period. Site temperature was represented by the USDA Hardiness factor
16
(http://www.usna.usda.gov/Hardzone/ushzmap.html), which are categories (1-11) based
17
on the average annual minimum temperature from 1974-1986. Precipitation for all sites
18
was estimated by the annual rainfall averaged over a decade or more from individual
19
monitoring programs. Precipitation values were extracted from the site LTER websites
20
for the seven LTER sites (http://www.lternet.edu/), and from the literature for Jasper
21
Ridge (Dukes et al. 2005). Soil pH was available from the literature and unpublished
22
data sources for ARC (Giblin et al. 1991; Gough et al. 2002b; Hobbie et al. 2002), CDR
23
(Pastor et al. 1987), JRG (Bohannan, unpublished data), KBS (Johnson et al. 2003), KNZ
1
1
(Blair, unpublished data), NWT (Bowman 2001), SGS (Johnson et al. 2003), and SEV
2
(Johnson et al. 2003). All measurements were taken from surface soil layers relevant to
3
the local site (which varied in depth by site) using predominantly a 1:1 ratio of soil:water
4
to measure pH (18 of 23 communities). The remaining 5 sites (all ARC) used either a
5
1:10 ratio of soil:water (2 communities) or a 1:10 ratio of soil:0.01MCaCl2 (3
6
communities). Increases in soil:water ratios do not largely affect pH readings due to
7
internal buffering in the soil suspension in either acidic (from hydrolysis of Al or H) or
8
calcareous (from hydrolysis of basic cations) soils (Sparks 1996). Measurements of pH
9
using water versus 0.01M CaCl2 have been shown to be highly correlated (r > 0.9),
10
though these studies were with large sample sizes (Gascho et al. 1996). Data for net N
11
mineralization used more heterogeneous methods, and were assembled using unpublished
12
data and the literature for ARC (Giblin et al. 1991; Hobbie & Gough 2002), CDR (Wedin
13
& Tilman 1996), KBS (Robertson et al. 1997), KNZ (Blair, unpublished data), NWT
14
(Bowman 2001), SGS (Dodd et al. 2000), and SEV (Kieft et al. 1998). Mean values
15
were inputted for the two communities for which data was unavailable (JRG and one
16
community at ARC, dry heath tundra). Most studies used intact soil samples separated
17
from surrounding soil matrix (either with polyethylene bags or PVC sleeves), incubated
18
monthly or longer, and then extracted with KCl (17 of 23 communities). The remaining
19
4 sites used resin bags sampled monthly (2 communities) or laboratory incubations over
20
similar periods (2 communities). Studies have shown that N mineralization estimated
21
from intact samples may differ from estimates using other methodologies (Raison et al.
22
1987); unfortunately, this heterogeneity was out of our control. Soil bulk density was
23
measured similarly for all sites, with a large diameter core taken to minimize compaction
2
1
which was then dried to constant weight. These data were available from the literature
2
and unpublished data for ARC (Giblin et al. 1991; Hobbie & Gough 2002), CDR (Grigal
3
et al. 1974), JRG (Cleland, unpublished data), KBS (Johnson et al. 2003), KNZ (online
4
data, http://www.konza.ksu.edu/), NWT (Sherrod & Seastedt 2001), SGS (Dodd et al.
5
2000), and SEV (Kieft et al. 1998). We were unable to obtain information on the
6
methodology employed for CEC at every site and thus do not report these methods here.
7
8
Plant community properties: Community properties represent conditions in non N-
9
treated plots. Most community properties were calculated using the PDT-Net dataset,
10
including productivity, the proportional number of species that were rare, and the relative
11
abundance of different functional groups. Additional details of this dataset are available
12
in prior publications (Gough et al. 2000; Suding et al. 2005). Our estimate of species
13
richness (number per square meter) required adjustment for different methodologies
14
employed. When measurements were made on a 1 m2 basis or larger, actual field data
15
were used, corrected to 1 m2. When smaller measurements were made, we used nested
16
species area accumulation curves collected in the summer of 2003 to estimate the number
17
of species per square meter.
18
19
3
1
Table S1 Descriptions of experimental protocols. Experiments differed in the amount of
2
N annually added, form of N, time of year N was added, number of replicates, the
3
measure on which relative abundance was based (Response measure), sample area, and
4
the specific year(s) used in this analysis (from experiment initiation). Multiple values
5
refer respectively to the different experiments within a site (ordered by Experiments
6
column).
7
8
4
1
System and
location
Arctic tundra,
Toolik Lake,
AK (ARC)
Sand prairie
or old field,
Cedar Creek,
MN (CDR)
Annual
grassland,
Jasper Ridge,
CA (JRG)
Old field,
Kellogg
Biological
Station, MI
(KBS)
2
Tallgrass
prairie,
Konza
Prairie, KS
(KNZ)
Alpine dry
meadow
tundra, Niwot
Ridge, CO
(NWT)
Desert
grassland,
Sevilleta, NM
(SEV)
Upland
shortgrass
steppe, CO
(SGS)
Number of
Form of N Time added replicates
Response
measure
Sample
area (m2)
Yrs used
100
NH4-NO3
pellet
June
16, 16, 24,
32, 24
Cover (%)
1
14, 14, 5,
10, 10
95
NH4-NO3
pellet
May, June
6
Biomass
0.3
Ave 17-19*
70
Ca-NO3
pellet
Nov., Dec.
8
Cover
(# hits)
0.5
5
120
NH4-NO3
pellet
June
6
Biomass
1
Ave 10-12*
100
NH4-NO3
pellet
May-June
4-8
Cover (%)
1-10
5-13
100
Urea, NH4NO3 pellet
July
5, 4
Cover (hits)
1
10, 4
Desert grassland (1)
100
NH4-NO3
pellet
May, July
10
Biomass
(allometric
estimates)
4
10
Ungrazed, with and
without increased
water (2)
60
NH4-NO3
prilled
Early April 100 (2 true)
Cover (%)
0.1
4
Experiments
(# per site)
N added
(kg ha-1yr-1)
dry heath sag, moist
sag, moist nonacidic
and acidic tussock,
dry heath tundra (5)
native savanna, old
fields abandoned
from agriculture in
1957 or 1934 (3)
Coastal range
grassland, ungrazed
(1)
Old field abandoned
from agriculture
1989, untilled since
(1)
Upland: burned,
burned and watered.
Lowland: six
combinations of
burning, irrigation
and mowing (8) †
Dry and moist
meadow tundra (2)
3
* Species abundances were determined from three year averages in these studies because
4
the small area annually sampled produced high variability between years (the same area
5
was never sampled twice).
6
† For more detail on the eight experiments from Konza see prior publications (Seastedt et
7
al. 1991; Blair 1997; Turner et al. 1997; Collins et al. 1998a).
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25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
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