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- Supporting Information
Nitrogen (N) availability has strong impacts on community (Tilman, 1987; Zavaleta et al., 2003) and ecosystem (Vitousek & Howarth, 1991; Elser et al., 2007; LeBauer & Treseder, 2008) processes in many systems. Yet despite its importance, ecosystem-scale N cycling is difficult to study because of the number and complexity of ecosystem N pools and fluxes. Increasingly, natural abundance stable N isotope ratios have been used to study N dynamics (Houlton et al., 2006, 2007; Craine et al., 2009). Many ecosystem processes fractionate against heavier isotopes, resulting in 15N-enriched source pools and 15N-depleted sink pools. Therefore, natural abundance N isotopes provide a valuable integrative record of N dynamics (Högberg, 1997; Robinson, 2001).
The degree of fractionation is better known for some N transformations than for others (Dawson et al., 2002; Tcherkez & Hodges, 2008). For example, biological N fixation (Dawson et al., 2002) and nitrate leaching (Houlton et al., 2006) fractionate little, but ammonium volatilization fractionates strongly (Dawson et al., 2002). However, the degree of intrinsic fractionation during some processes is unclear because the observed fractionation depends on the degree of completion of the reaction as well as enzymatic discrimination. If a discriminating enzyme consumes all of its substrate, there is no net fractionation (Dawson et al., 2002). For example, plant uptake of both nitrate and ammonium fractionates when available N concentrations are high or N is not limiting, but not when available N concentrations are low or N is limiting (Högberg, 1997; McKee et al., 2002; Clarkson et al., 2005).
One use of natural abundance N isotopes is the study of N cycling along environmental gradients such as long-term soil chronosequences. On very young soils, soil N is scarce (Chapin et al., 2002), N fertilization frequently stimulates net primary production (NPP; Vitousek et al., 1993), and symbiotic N fixers are relatively common (e.g. Walker, 1993) and actively fixing dinitrogen gas (e.g. Vitousek & Walker, 1989). By contrast, very old, undisturbed soils are thought to be primarily phosphorus (P)-limited because the major P input – rock weathering – decreases over time, whereas N inputs from atmospheric deposition and some N fixers continue indefinitely (Beadle, 1966; Walker & Syers, 1976; Vitousek & Farrington, 1997; Vitousek, 2004; Wardle et al., 2004). Within the range of P-limited soils, N dynamics can vary. Many old tropical forests are N-replete (Hedin et al., 2009), whereas older temperate forests are often N-poor (Vitousek & Howarth, 1991; Hedin et al., 1995; Perakis & Hedin, 2002), which can have implications for N isotope patterns.
To our knowledge, the patterns of natural abundance N isotopes in soils and plants have been investigated in five chronosequences in three different biomes: boreal forests in Alaska aged 55–225 yr (Hobbie et al., 1999a), boreal forests in Sweden aged hundreds to thousands of years (Hyodo & Wardle, 2009), tropical rainforests in Hawaii aged 28–67 000 yr (Vitousek et al., 1989) and 300–4 100 000 yr (Martinelli et al., 1999), and temperate annual grasslands in California aged 3000–3 000 000 yr (Brenner et al., 2001). Along both Hawaiian chronosequences, the older of which is known to switch from N limitation to co-limitation to P limitation (Vitousek & Farrington, 1997), both foliar and soil δ15N (see the Materials and Methods section for δ15N definition) increased with soil age (Vitousek et al., 1989; Martinelli et al., 1999), likely as the result of an increase in losses that fractionate against 15N across the chronosequence (Hedin et al., 2003). Foliar and litter δ15N increased with forest age in Sweden, likely because of increased N fixation by free-living N-fixers, increased reliance on dissolved organic N, or both (Hyodo & Wardle, 2009). By contrast, foliar δ15N decreased through early succession in Alaska (Hobbie et al., 1999a), likely because of a shift in the importance of fractionating mycorrhizal N transfer to plants (Hobbie et al., 1999b, 2000), although soil δ15N showed no consistent pattern (Hobbie et al., 1999a). On the California grassland chronosequence, foliar δ15N showed no trend with soil age and soil δ15N increased slightly (Brenner et al., 2001). These studies demonstrate that long-term variation in natural isotope abundance differs among systems and is driven by a range of processes.
The Franz Josef chronosequence on the west coast of New Zealand’s South Island is a well-studied chronosequence in temperate forest, a biome for which no chronosequence N isotope data have been reported to our knowledge. Another novel aspect of the Franz Josef chronosequence is that it spans the full range of ecosystem development stages (Peltzer et al., 2010), including young progressive sites (< 10 yr), maximal biomass sites, and relatively old retrogressive sites (120 000 yr) (Richardson et al., 2004). Total soil N : P and nonfixer foliar N : P generally increase with soil age (Stevens, 1968; Richardson et al., 2004), consistent with a switch from N to P limitation. However, soil available N and foliar N decline in the older sites (Richardson et al., 2004, 2005), suggesting that the older sites are N-poor even if they are not N-limited (see also Table 1).
Table 1. Site characteristics and δ15N measurement species list
| ||Age (yr)|
| ||5 (site 1)||60 (site 2)||130 (site 3)||500 (site 5)||12 000 (site 7)||120 000 (site 9)|
|Annual precipitation1 (mm)||6520||6576||6188||6278||3706||3652|
|Total soil N : P1 (g g−1)||0.07||2.8||6.8||17.5||17.5||33.0|
|Foliar N : P1 (g g−1)||16.8||15.3||8.2||9.8||13.9||16.1|
|Nonfixer foliar N : P1 (g g−1)||8.4||14.7||8.2||9.8||13.9||16.1|
|Maximum tree height1 (m)||0.9||6.8||7.6||23.8||31.8||11.2|
|Bulk N deposition2 (kg ha−1 yr−1)||1.5||1.5||1.5||1.5||0.9||0.9|
|N fixation2 (kg ha−1 yr−1)||22.1||15.3||5.5||4.7||9.7||2.5|
|Amax3 (μmol m−2 s−1)||19.6||16.3||12.5||8.0||5.3||3.9|
|Dominance weighting1 (%)|
| Coriaria arboreaNf,e,a||89*||25*|| || || || |
| Olearia avicenniifoliae,a||9*|| || || || || |
| Aristotelia serratad,a||2*||21*|| || || || |
| Schefflera digitatae,a|| ||46|| || || || |
| Melicytus ramifloruse,a|| ||8*||1*|| || || |
| Griselinia littoralise,a|| ||< 1||47*||2*|| || |
| Coprosma lucidae,a|| || ||28|| || || |
| Pseudopanax colensoie,a|| || ||11|| || || |
| Coprosma ciliatae,a|| || ||2|| || || |
| Carpodetus serratuse,a|| || ||1|| || || |
| Weinmannia racemosae,a|| || ||10*||20*||30*||13*|
| Prumnopitys ferrugineae,c|| || || ||12|| || |
| Metrosideros umbellatae,a|| || || ||66*||< 1*||3*|
| Dacrydium cupressinume,c|| || || || ||44*||14*|
| Quintinia acutifoliae,a|| || || || ||26||14|
| Manoao colensoie,c|| || || || || ||15|
| Phyllocladus alpinuse,c|| || || || || ||40|
| Podocarpus halliie,c|| || || || || ||1|
|% community occupied by sampled species||94||93||81||68||69||80|
In this study, we examined natural abundance N isotopes across the Franz Josef chronosequence. Specifically, we report δ15N in foliage and abscised leaves; mineral, organic, and bulk litter soil horizons; lichens and bryophytes; and bulk deposition and actinorhizal root nodules, the last of which is an indication of symbiotic N fixation N inputs. We used these data to resolve two interlinked questions. First, how do each of these pools vary with soil age? And second, how do these pools compare within each site? These data will fill a gap in our knowledge of N isotope patterns along temperate forest chronosequences. Additionally, they will help us to determine the generality of N isotope patterns observed along chronosequences in other biomes.