and soil carbon dynamics
C H A R L E S T . G A R T E N , J R , P A U L J . H A N S O N , D O N A L D E . T O D D , J R , B O N N I E B . L U , A N D D E A N N E J . B R I C E
Introduction
The purpose of this chapter is to examine how natural abundance measure- ments of stable N and C isotope ratios might be used as indicators of envi- ronmental processes that impact soil C storage in forest ecosystems. This is important because increasing atmospheric CO2concentrations have created substantial recent interest in science and technology needs for increasing global C sequestration (Lal 2004). Soil C balance, in particular, has been at the center of many science questions related to the exchange of CO2between the terrestrial biosphere and the atmosphere because most of the global C inventory resides in soils (Post et al. 1990; Schimel 1995). Environmental factors that produce even a small change in global soil C stocks (through changes in soil C inputs or outputs) have the potential to produce a dispro- portionately large change in levels of atmospheric CO2.
Enhanced uptake of atmospheric CO2 by terrestrial ecosystems through management of croplands, grasslands, and forests has been suggested as a low-cost and technologically achievable strategy to partially offset CO2emis- sions to the atmosphere from the continuing use of fossil fuels (Lal 2004;
Post et al. 2004). However, because below-ground processes are difficult to observe and measure, we have a relatively poor understanding of (i) soil C dynamics, (ii) the degree of certainty associated with estimates of below- ground processes, and (iii) the factors that regulate soil C sequestration potential at regional and global scales (Metting et al. 2001).
Forest soil C dynamics can be simply described as the difference between soil C inputs and outputs (Six & Jastrow 2002). Litterfall and rhizodeposition (i.e., root mortality and root exudation) are the primary contributors to soil C inputs. Decomposition of organic matter inputs by heterotrophic soil microorganisms is the primary contributor to soil C loss. Quantitative
*The submitted manuscript has been authored by a contractor of the U.S. Government under contract DE-AC05-00OR22725. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes.
62 C H A R L E S T . G A R T E N , J R , E T A L .
determinations of processes controlling forest soil C storage can be both costly and time-consuming. However, continuous-flow, light stable isotope ratio mass spectrometers (CF-IRMS) allow for rapid and precise measurements of C or N concentrations and isotope ratios (13C/12C, 15N/14N) at natural abun- dance levels in both soils and plants. Our hypothesis is that such measure- ments may be useful for preliminary assessments of environmental factors affecting forest soil C sequestration potential.
In practice, soil C sequestration potential has two broad aspects: (i) reten- tion and (ii) accumulation. Retention of existing soil C stocks means imple- mentation of land management practices that alter soil processes to minimize long-term reductions in soil organic matter (SOM). Accumulation involves application of management practices or technologies that alter soil processes to maximize the likelihood of increasing long-term soil C storage. Accumula- tion involves altering the soil C balance such that inputs exceed C losses, while the objective of retention is to ensure that soil C losses do not exceed inputs.
The focus of this chapter is forests because forests occupy ca. 25 percent of the total land area in the USA. Approximately 83 percent of forest re- sources in the southern USA are classified as “natural” forest while only 17 percent are classified as planted (i.e., tree plantations or “augmented forests”
with varying degrees of forest management) (Smith et al. 1997). The latter distribution mirrors a national trend and indicates that most forests in the USA are not intensively managed. Natural forest resources have an important role in strategies for soil C sequestration. Active management practices that promote soil C sequestration on a relatively small coverage of planted forests may be augmented or offset by minor changes in soil C sequestration under a large coverage of natural forests. One of the more important questions in evaluating the role of natural forest resources in C sequestration strategies is: What can natural abundance measurements of stable C and N isotope ratios tell us about forest soil C storage and dynamics?
Although there are multiple environmental factors that can impact forest soil C dynamics (such as temperature, moisture, and mineralogy), many studies indicate an important role of N availability and organic matter de- composition rates. The latter two factors are not necessarily independent;
nonetheless we first examine the potential importance of N availability on processes determining soil C dynamics and the use of 15N natural abundance measurements as an indicator of site N status. Second, we examine the use of 13C natural abundance measurements in soil profiles as an indicator of differences in SOM dynamics. Rapid assessments of forest soil C sequestra- tion potential require methodologies and tools that will provide easily meas- ured, reliable indicators of both soil C dynamics and site N status. Such information may permit relative comparisons of soil C sequestration poten- tial across different forest ecosystems, soil types, and climate regimes. Aside from issues surrounding C sequestration, measurements of stable N and C
isotopes may also provide a window into biogeochemical processes, such as N cycling and SOM dynamics, which are important to rapid assessments of forest health.
Significance of 15N-abundance to soil carbon sequestration
The role of nitrogen in forest soil carbon dynamics
The importance of 15N measurements in plants and soils as an ecosystem in- dicator emerges when considering the control that N availability has over soil C dynamics. Reviews of multiple studies demonstrate N fertilization generally increases forest soil C stocks (Johnson 1992; Johnson & Curtis 2001) through increased inputs and decreased losses of SOM. In addition to a widely re- ported increase in forest growth following N fertilization, studies across both hardwood and coniferous forest stands show that ca. 50 percent of the vari- ation in above-ground net primary production (ANPP) is explained by vari- ation in annual net soil N mineralization (Reich et al. 1997). Some studies along soil N availability gradients indicate annual leaf litter production in- creases with annual net soil N mineralization (e.g., Nadelhoffer et al. 1983).
Greater soil N availability also appears to increase forest fine root production and turnover (Aber et al. 1985; Nadelhoffer 2000). Both increased leaf litter production and greater fine root turnover can directly contribute to increased soil C inputs in N-rich forests.
There is also a growing body of evidence that N availability indirectly controls forest soil C dynamics through effects on organic matter decomposi- tion. Regional-scale studies in forest ecosystems indicate a significant effect of soil N availability on measures of litter quality, such as C-to-N ratios (Stump & Binkley 1993). Greater soil N availability potentially reduces the C-to-N ratio of above- and below-ground litter inputs by increasing plant tissue N concentrations. Various studies (Taylor et al. 1989; Janssen 1996;
Kuperman 1999; Silver & Miya 2001) indicate decomposition rates are in- versely related to litter C-to-N ratios. While low litter C-to-N ratios accelerate initial stages of litter decomposition, high litter N concentrations (i.e., low litter C-to-N ratios) appear to inhibit the latter stages, resulting in a greater amount of organic matter remaining near the terminus of decomposition (Berg & Matzner 1997; Berg 2000; Berg & Meentemeyer 2002). In some forests, N-rich leaf litter inputs appear to significantly increase humus accu- mulation (Berg et al. 2001).
Inhibition of phenol oxidase, a lignin-degrading soil enzyme, in high N environments is one mechanism that may promote more humus formation and possibly greater soil C storage when litter inputs have low C-to-N ratios (Berg et al. 2001; Berg & Meentemeyer 2002). Even though responses are sometimes ecosystem specific, elevated levels of inorganic soil N can alter soil
64 C H A R L E S T . G A R T E N , J R , E T A L .
microbial composition (Frey et al. 2004; Gallo et al. 2004), suppress phenol oxidase activity (Carreiro et al. 2000; Saiya-Cork et al. 2002; DeForest et al.
2004; Frey et al. 2004; Gallo et al. 2004; Matocha et al. 2004; Waldrop et al.
2004a) and inhibit soil respiration (Fisk & Fahey 2001; Franklin et al. 2003;
Bowden et al. 2004; Burton et al. 2004), indicating an overall lower rate of soil microbial activity and reduced organic matter decomposition in N-rich forests. Short-term soil C gains in some upland forest stands appear to be related to the suppression of phenol oxidase activity (Waldrop et al. 2004b).
In summary, greater soil C sequestration in N-rich forests results from in- creasing soil C inputs and/or decreasing rates of organic matter decomposi- tion. Field studies indicate both processes, altered inputs and decomposition rates, contribute to greater soil C sequestration under N2-fixing tree species (Resh et al. 2002). However, at least two studies (Neff et al. 2002; Swanston et al. 2004) indicate that enhanced soil N (through fertilization) promotes stabilization of organic matter in soil pools that have long turnover times.
Thus, greater N availability has the potential to shift soil C partitioning in favor of more refractory SOM, reduce the overall rate of total soil C turnover, and increase forest soil C sequestration potential. Site N status is a potentially important factor controlling forest soil C dynamics and high N availability can contribute to greater soil C storage.
Use of 15N-abundance for indicating site nitrogen status
Nitrogen-15 case study
Measurements of foliar 15N-abundance and soil-to-plant 15N enrichment fac- tors (EF) have been shown to be useful indicators of forest N status. The isotopic composition of soil N may vary from one site to another, thus the relationship between the N isotope composition of plants and soil is fre- quently presented in terms of an isotopic “enrichment factor” (e.g., Mariotti et al. 1981). Because soil N (the substrate) is a large reservoir relative to plant foliage (the product), the enrichment factor can be approximated as the difference between 15N-abundance in the substrate and the product, or EF= δ15Nleaf− δ15Nsoil. The use of enrichment factors helps to adjust potential differences in natural foliar 15N-abundance for existing differences in soil
15N-abundance.
Here we present a site-specific example for the use of natural 15N- abundance as an indicator of site N status based on ongoing studies at three forest stands on the U.S. Department of Energy’s Oak Ridge Reservation (36°58’N; 84°16’W) near Knoxville, Tennessee. Overstory trees at the upland sites (one ridge and one slope) are predominantly oak (Quercus spp.) with scattered pine (Pinus echinataandP. virginiana) and mesophytic hardwoods (Liriodendron tulipifera, Fagus grandifolia, Acer rubrum). One site is located in a
valley and dominated by mesophytic hardwoods (primarily L. tulipifera). The difference in elevation between ridge and valley at this location is ca.
100 m.
We made conventional measurements (Hart et al. 1994) of potential net N mineralization in surface (0–10 cm) mineral soil samples collected during the spring of 2002 and 2003 from the three forest sites. Composite soil sam- ples from nine replicate plots at each site were used for the analysis. Results were expressed as µg N produced g−1dry soil per week and were normalized for bulk soil N concentrations (µg N g−1dry soil) to estimate a weekly rate of potential net soil N mineralization.
Potential net soil N mineralization in 12-week aerobic laboratory incuba- tions, expressed as either N production (µg N g−1) or production normalized for soil N concentration (i.e., percent soil N mineralized), was significantly different between study sites and years (Table 3.1). Site disparities in produc- tion of inorganic N indicated greater N availability in soils from the valley (F2,48=78,P≤0.001). Significant site differences in N production normalized for soil N concentrations followed a similar pattern (F2,48=37, P≤ 0.001), and indicated N availability across the three sites increased in the following order: ridge <slope<valley.
Site-to-site measurements of soil N availability were consistent in 2002 and 2003 (Table 3.1). Nitrate accounted for all of the inorganic N produced during aerobic incubations of valley soils and net soil nitrification was sig- nificantly greater in the valley soils than at the ridge or slope sites (F2,48= 224,P≤0.001). The observed topographic patterns in soil N availability were in agreement with prior studies that indicate riparian forests on the Oak Ridge Reservation are more N-rich than upland forests (Garten 1993; Garten et al. 1994).
Table 3.1 Mean (±SE) potential net N mineralization and potential net nitrification in aerobic laboratory incubations (12 weeks) of surface (0–10 cm) mineral soil samples collected from ridge, slope, and valley forests in 2002 and 2003. Means in the same row with different alphabetic superscripts are significantly different (P≤0.05).
Variable Units Year n Ridge Slope Valley
Potential net soil N µg N g−1 2002 9 1.23a±2.06 9.32a±4.22 51.0b±2.46 mineralization 2003 9 4.42a±2.83 5.21a±1.00 22.7b±3.90 N production % 2002 9 0.13a±0.18 1.34b±0.55 4.13c±0.26 normalized for 2003 9 0.43a±0.29 0.95b±0.14 1.86c±0.35 soil N
concentration
Potential net soil µg N g−1 2002 9 0.43a±0.30 2.12a±1.04 53.8b±2.85 nitrification 2003 9 0.59a±0.25 0.71a±0.27 24.7b±4.10
66 C H A R L E S T . G A R T E N , J R , E T A L .
Leaf litterfall was also collected at the ridge, slope, and valley site in the autumn of 2002 and 2003. Litterfall samples were combined by plot (three sampling plots at each site), oven-dried (70°C), and subsampled. Soils (passed through a 2-mm sieve to remove gravel, live roots, and coarse debris) were ball milled and litterfall samples were ground to a powder in a sample mill prior to analysis by combustion methods for total C and N using a LECO CN-2000 (LECO Corporation, St Joseph, MI). Stable isotope ratios for C and N were measured by continuous-flow, isotope ratio mass spectrometry (Integra-CN, SerCon Ltd, Cheshire, UK).
Forest N status was evaluated using 15N enrichment factors (EF) in leaf litterfall:EF= δ15Nlitterfall− δ15Nsoil. The 15N-abundance in the surface mineral soil was used to calculate the enrichment factor. Prior studies (Garten 1993;
Kolb & Evans 2002) have shown there is little isotopic discrimination against
15N associated with foliar N reabsorption prior to leaf senescence. Conse- quently, analysis of leaf litterfall yields an accurate estimate of foliar 15N- abundance in the forest canopy for a particular growing season.
The three forest sites were significantly different in leaf litterfall N con- centrations, C-to-N ratios, 15N-abundance, and enrichment factors (Table 3.2). Leaf litterfall C-to-N ratios were inversely related to soil N availability.
Leaf litterfall N concentrations were significantly greater (F2,77 = 156, P ≤ 0.001) and litterfall C-to-N ratios were significantly less (F2,77 = 156, P ≤ 0.001) in the valley (Table 3.2). Differences between years in leaf litterfall N concentrations were also statistically significant (F1,77= 7.1, P ≤ 0.01), but they were consistent in both years (thus data from both 2002 and 2003 were combined for the analysis). Greater soil N availability in the valley produced higher leaf litterfall N concentrations and lower litterfall C-to-N ratios.
Table 3.2 Mean (±SE) surface soil 15N abundance, leaf litterfall enrichment factors (EF),15N abundance, N concentrations, and C-to-N ratios at three forest sites on the Oak Ridge Reservation. Means in the same column with different alphabetic superscripts are significantly different.
Site Surface soil Leaf litterfall
n δ15N (‰) n EF δ15N (‰) N (%) C : N
Ridge 3 3.42 ±0.63 15 −7.21a±0.12 −3.79a±0.12 0.75a±0.01 65.5a±1.1 Slope 3 4.11 ±0.50 13 −8.25b±0.10 −4.14a±0.10 0.81a±0.04 60.8a±2.4 Valley 3 3.82 ±0.42 14 −4.99c±0.20 −1.17b±0.20 1.27b±0.05 37.8b±1.6 Statistics
F-value 124.2 119.7 64.3 79.3
Probability 0.001 0.001 0.001 0.001
There were also statistically significant differences in leaf litterfall 15N- abundance and enrichment factors among the three forest sites (Table 3.2).
Even though surface soil 15N-abundance was similar among the three loca- tions, site-specific surface soil δ15N values were used to calculate leaf litterfall
15N enrichment factors. Both the litterfall δ15N value and the 15N enrichment factor were less negative at the valley site (Table 3.2). More positive (or less negative) foliar δ15N values and 15N enrichment factors closer to zero are in- dicative of more N-rich environments (Garten & Van Miegroet 1994), but how do such spatial patterns relate to soil C storage?
There was a tendency toward greater mineral soil C stocks (0–30 cm) at the valley site, relative to the upland forests, but the site differences are not statistically significant (Table 3.3). However, the C stock associated with soil silt and clay (i.e., mineral-associated organic matter) was significantly greater in the valley, as was the total mineral soil N stock (Table 3.3). As discussed in the following section, greater soil N availability (in addition to other en- vironmental factors) may contribute to the partitioning of soil C to mineral- associated organic matter and greater stabilization of mineral soil C stocks in the N-rich valley. Carbon associated with silt and/or clay has been shown to have a longer turnover time than C in more labile fractions of SOM (Balesdent 1996).
In summary, the N-rich valley had higher potential rates of net soil N mineralization, lower leaf litterfall C-to-N ratios, more positive 15N enrich- ment factors (i.e., EF approached zero), and a greater partitioning of C to the soil silt and clay fraction than N-poor forests located on ridges or slopes. An observed relationship between soil N availability and foliar 15N-abundance or soil-to-plant15N enrichment factors is not new (see e.g., Garten 1993; Garten
& Van Miegroet 1994). Garten & Van Miegroet (1994) proposed several pos- sible mechanisms that might underlie changes in the isotopic composition of
Table 3.3 Mean (±SE) N stocks and C stocks in whole mineral soil or the silt and clay fraction at three forest sites on the Oak Ridge Reservation. Means in the same column with different alphabetic superscripts are significantly different (NS =not statistically significant).
Site n g N m−2 g C m−2
Mineral soil Mineral soil Silt and clay
Ridge 12 133a±7 2941a±127 2045a±90
Slope 12 114a±5 2637a±150 1839a±88
Valley 12 257b±17 3123a±224 2568b±198
Statistics
F-value 50.2 2.0 7.7
Probability 0.001 NS 0.01
68 C H A R L E S T . G A R T E N , J R , E T A L .
foliar N along gradients of N availability in addition to a model that predicts how foliar δ15N values are affected by (i) varying uptake of soil ammonium-N and nitrate-N, (ii) the isotopic composition of different soil N pools, and (iii) relative rates of soil N transformations. In particular, foliar δ15N values appear to increase in direct association with the relative importance of net soil ni- trification. In this case study, soils from the valley had the highest rates of net soil nitrification (Table 3.1) and the least negative foliar δ15N values (Table 3.2). Results from three forest sites on the Oak Ridge Reservation exemplify the utility of natural 15N-abundance for distinguishing N-rich and N-poor forests that share similar climates.
Evidence from other studies
Numerous studies lend support to the use of natural abundance measure- ments of stable N isotopes in plants or calculated 15N enrichment factors as indicators of ecosystem N cycling and/or site N status (Högberg 1990; Garten 1993; Högberg & Johannisson 1993; Garten & Van Miegroet 1994; Nasholm et al. 1997; Emmett et al. 1998; Martinelli et al. 1999; Vervaet et al. 2002;
Koba et al. 2003). Several possible mechanisms are potentially responsible for such an association. First, the product of nitrification is 15N-depleted ni- trate-N (Mariotti et al. 1981). High rates of net soil nitrification and elevated levels of nitrate-N leaching can contribute to a gradual enrichment in 15N- abundance in forests with open and leaky N cycles because there is a chronic loss of 15N-depleted nitrate (Högberg 1997). In an experimental test of the foregoing hypothesis, nitrate leaching from soils following forest clear-cutting was associated with more positive foliar δ15N values and changes in foliar
15N-abundance through time coincided with temporal changes in stream- water nitrate concentrations (Pardo et al. 2002). Second, in soils prone to high nitrification rates, denitrification is a natural process of nitrate loss char- acterized by a relatively large isotopic fractionation (Mariotti et al. 1981) that leaves remaining soil nitrate isotopically enriched in 15N. The utilization of that nitrate by plants can also contribute to more positive foliar δ15N values in N-rich settings. Last, in N-poor environments, greater reliance on N de- rived from mycorrhizal fungi contributes to lower foliar δ15N values while in N-rich environments, less reliance on N derived from mycorrhizal fungi re- sults in higher foliar δ15N values (Hobbie et al. 2000; Hobbie & Colpaert 2003). In field studies, the relative importance of these various mechanisms (nitrate leaching, denitrification, and mycorrhizal N fractionation) can be difficult to ascertain.
The foregoing mechanisms may be site-specific and they may work inde- pendently or in combination, but the overall relationship of foliar δ15N to site N status is unchanged. Despite the complexity of the N cycle and potential isotopic fractionations associated with various soil N transformations (Högberg 1997; Bedard-Haughn et al. 2003), N-rich forests (particularly sites
with elevated net soil nitrification) have been repeatedly differentiated from N-poor forests by more positive foliar δ15N values and more positive 15N-en- richment factors (Garten 1993; Garten & Van Miegroet 1994; Pardo et al.
2002; Koba et al. 2003). For example, δ15N values are generally more nega- tive in N-poor temperate forests with closed N cycles than in relatively N-rich tropical forests with more open N cycles (Martinelli et al. 1999). The utility of natural abundance 15N measurements in distinguishing site N status has also been demonstrated experimentally using gradients of both N fertilization (Högberg 1991; Johannisson & Högberg 1994) and atmospheric N deposition (Emmett et al. 1998).
Vertical changes in soil 13C-abundance and soil carbon dynamics
Decomposition and vertical soil profiles of 13C-abundance
Organic matter decomposition causes a decline in soil C concentration leaving progressively older C and eventually terminating at a steady-state pool of refractory soil C (Wang et al. 1996). This is one reason why rates of organic matter decomposition decline with increasing soil depth (Paul et al. 1997;
Van Dam et al. 1997). Carbon-14 measurements demonstrate that both the age and turnover time of forest SOM change over the soil profile such that the oldest and most refractory soil C is generally found in the deepest soils (Wang et al. 1996; Paul et al. 1997; Van Dam et al. 1997; Gaudinski et al.
2000; Torn et al. 2002; Gaudinski & Trumbore 2003). Carbon in mineral- associated organic matter (i.e., soil silt and clay) is also usually older than bulk soil C (Gaudinski & Trumbore 2003).
There is a widely reported occurrence of increasing 13C-abundance with greater soil depth or a similar change along a continuum of progressively more decomposed fractions of SOM (e.g., see Balesdent et al. 1993; Ehler- inger et al. 2000; Garten et al. 2000; Powers & Schlesinger 2002; Schweizer et al. 1999). Possible causal mechanisms for greater 13C-abundance with in- creasing soil depth have been discussed in detail by other authors (Nadelhof- fer & Fry 1988; Ehleringer et al. 2000). Briefly, these mechanisms include:
1 changing isotopic ratios in atmospheric CO2over the past 200+years;
2 the enrichment of soil C in 13C as a result of fractionation during organic matter decomposition;
3 the mixing of new C inputs with older SOM that has a different δ13C value;
4 preferential decomposition of organic matter by soil microorganisms.
Opinions vary, but there are a large number of recent studies indicating discrimination against 13C, relative to 12C, during decomposition of organic substrates by heterotrophic soil microorganisms (Mary et al. 1992; Schweizer