B12A-01 INVITED
Biochemically-resistant carbon compounds in physically- and chemically-protected soil C pools
Soil organic matter can be protected from decomposition through physical protection within soil aggregates, chemical protection through association with mineral particles, and biochemical stabilization through biotic or abiotic creation of decay-resistant compounds. However, the methods most often used to differentiate soil organic matter usually lead to composite fractions in which one or more of the mechanisms of protection contribute to stabilization. We developed a new approach to isolate and quantify biochemically-protected carbon within physically- and chemically-protected soil organic matter fractions. Blending physiochemical isolation of soil carbon fractions with subsequent biochemical characterization enabled us to investigate the ways that biochemical stabilization complements physical and chemical stabilization. We applied this approach for soil samples from three cultivated fields and used plant-derived shifts in 13C signatures to understand how biochemically-protected carbon contributes to overall resistance of that fraction. Results show that a variety of biochemically resistant compounds are found in all of the physical fractions isolated. Isotopic analyses indicate that these compounds are significantly older than average soil C in these fractions and that they contribute a great deal to the 14C age of these fractions.
B12A-02
Redistribution and Destabilization of Forest Soil Carbon by Earthworm Invasion
Soils of temperate forests in the northern Great Lakes region have developed in the absence of earthworms, which were largely eradicated during the last ice age. European earthworms, such as Lumbricus terrestris, are spreading and their effect on forest soil C has not been widely studied. We examined soils along a chronosequence of worm activity (wormosequence) ranging from no apparent activity (Low) to high activity over several decades (High). In the soil profile, including organic horizons, there was only a small loss (4%) of C with high worm activity. There was a 71% decrease in the Oe and no remaining Oa in the High site. Conversely, the mineral soil contained 25% more C in the High site, with most of that in the top 20 cm. We densimetrically separated the mineral soil into primarily organic free light fraction (FLF) and occluded light fraction (OLF), a mineral-associated intermediate fraction (IF), and a mineral-associated dense fraction (DF). We measured fraction mass, C, nitrogen, and 14C content. In comparison to Low sites, each depth of High worm soil had less FLF C and more IF C. With worms, more C was stored in aggregates in the upper soil but less in the lower soil. The presence of worms had a very large effect on the 14C content of soil organic matter, with shifts of > 60 per mil in several fractions presumably due to a combination of vertical redistribution and more rapid transfer into and out of some fractions. For example, from the Low to High sites OLF 14C increased 100 per mil at 0-10 cm depth, while DF 14C decreased 80 per mil at 20-50 cm. The effect of worms on C cycling can be explored using a multi-pool, multi-depth model that treats 14C as a conservative (i.e., stable) tracer. Our results suggest that worms play a major role in redistributing soil C in these forest soils by pathways that differ with depth: the upper soil loses FLF C and incorporates organic horizon C into OLF and IF, whereas in the lower depths FLF is lost and the previously stable OLF C pool is redistributed into IF and DF. We conclude that the introduction of worms to these forest soils has fundamentally changed the storage, distribution, and turnover of soil organic C.
B12A-03
Nitrogen Effects on Organic Dynamics and Soil Communities in Forest and Agricultural Systems
Human activities have doubled the global flux of biologically available N to terrestrial ecosystems but the effects of N on soil organic matter dynamics and soil communities remain difficult to predict. We examined soil organic matter chemistry and enzyme kinetics in three soil fractions (>250, 63-250, and <63 μm) following six years of simulated atmospheric N deposition in two forest ecosystems with contrasting litter biochemistry (sugar maple/basswood and black oak/white oak). Ambient and simulated atmospheric N deposition (80 kg nitrate-N/ha/y) were studied in three replicate stands in each ecosystem type. Using pyrolysis-gas chromatography/mass spectroscopy, we found striking, ecosystem-specific effects of N deposition on carbohydrate abundance. Furfural, the dominant pyrolysis product of polysaccharides, was significantly decreased by simulated N deposition in the sugar maple/basswood system (15.87 versus 4.99%) but increased by N in the black oak/white oak system (8.83 versus 24.01%). There were ca. 3-fold increases in the ratio of total lignin derivatives to total polysaccharides in the >250 μm fraction of the sugar maple/basswood system but there were no changes in other size classes or in the black oak/white oak system. We also measured significant increases in the ratio of lignin derivatives to N-bearing compounds in the 63-250 and >250 μm fractions in both ecosystems but not in the <63 μm fraction. We compare these results to a study looking at changes in enzyme activities and soil communities along a N fertilizer gradient in a corn-based cropping system. Our results demonstrate that changes in soil organic matter chemistry resulting from atmospheric N deposition or fertilization are directly linked to variation in enzyme responses to increased N availability across ecosystems and soil size fractions.
B12A-04
Stabilization and Destabilization of Soil Carbon with Nitrogen Additions in Two Tropical Forests
Nitrogen (N) deposition is known to effect carbon (C) cycling in temperate ecosystems, but less is known about the effects of added N in tropical forests, where N is not generally limiting to plant growth. We examined changes in soil C dynamics with N fertilization in two tropical forest types (lower elevation and montane) in the Luquillo Mountains, Puerto Rico. We hypothesized that increased N would accelerate the decomposition of labile C pools, while decreasing losses of more recalcitrant C compounds. We measured C and C:N in bulk soil C and C fractions (free light, occluded light, and heavy fractions) as measures of C content and chemical properties in fertilized and control plots. To address our hypotheses, we conducted several measures of microbial activity, including extracellular enzyme activities and respiration during a long-term soil incubation. We included measurements of 14C of CO2 respired during the soil incubation to determine whether added N changed the age of respired C. After 3.5 years of N fertilization, plots with added N had higher C content (42.3 ± 6.8 and 40.7 ± 4.7 g/cm2, lower elevation and montane respectively) than control plots (34.2 ± 5.9 and 34.3 ± 1.3 g/cm2) at 0 – 10 cm depth. While the labile fraction of C declined with added N as a proportion of total soil weight, the C concentration of the heavy fraction increased in fertilized plots (2.9 ± 0.3 and 4.0 ± 0.7%) relative to control plots (2.6 ± 0.4 and 2.8 ± 0.5 %), helping explain the increase in bulk soil C content. The soil incubation revealed changes in microbial respiration with added N, and a trend toward higher 14C of CO2 in fertilized plots for the lower elevation forest. Together, these results indicate that rates of C stabilization in the heavy fraction exceeded the increase in respiration of older C with N additions.
B12A-05
How can we measure the soil carbon stabilization capacity?
Soils represent an important potential sink for atmospheric CO2 through the actions of various stabilization mechanisms. However, previously published evidence indicates that soils may have a limited capacity to stabilize organic matter. However, determining what the stabilization capacity (and conversely, the saturation deficit) is for a given soil remains a significant challenge. We propose determining the specific surface area (SSA, m2 g-1) of whole soils and physically isolated soil fractions, and the determination of organic C loadings (g C m-2) as a promising mechanistic approach to estimating soil carbon stabilization capacities.
B12A-06
Tracking the incorporation of 15N from labeled beech litter into mineral-organic associations
Nitrogen containing organic compounds are thought to have a role in the complex web of processes that control the turnover time of soil organic matter. The sequential density fractionation technique is increasingly used for the purpose of investigating the association of organic materials with the mineral matrix. Organic materials in the denser fractions (>2.0 kg L-1) typically show 13C NMR signals indicative of carbohydrate and aliphatic structures, an absence of lignin and tannin structures and a narrow C:N ratio, suggesting a microbial origin of organic matter in these fractions. Here we take advantage of a labeling experiment conducted at two different sites in Germany and in France to investigate the incorporation of organic nitrogen into physical fractions of increasing density, representing a proximity gradient to mineral surfaces. 15N labeled beech litter was applied to two acidic forest topsoils 8 and 12 years ago. Although there are differences in the distribution patterns between the two soils, and the majority of the organic nitrogen was recovered in fractions representing organic matter of plant origin and not bound to the mineral matrix, our data clearly show that after a decade, significant amounts of the nitrogen had been incorporated in mineral-organic fractions of supposedly slow turnover. It remains to be shown to which extent the N in the densest fractions was incorporated by soil microbiota and associated with mineral surfaces in organic form or adsorbed to mineral surfaces in inorganic form (NH4+).
B12A-07
Differentiation of organic C and N forms between density fractions: does the presence of Fe matter?
15N labeled beech litter was applied to a forest topsoil 12 years ago, and a density fractionation procedure carried out to separate the soil into density fractions representing a proximity gradient to mineral surfaces. It was observed that although the majority of the 15N label was recovered in unaltered plant materials, significant amounts of the nitrogen had been incorporated in mineral-organic fractions of supposedly slow turnover. Scanning Transmission X-ray Microscopy (STXM) at the C, N, and Fe edges was used to determine whether qualitative changes observed in the chemistry of particle associated organic matter were related to the presence of Fe. Here we show how the speciation of mineral-associated nitrogen differs between iron free and iron rich particles. We further demonstrate the variation of carbon chemistry with the presence of Fe within individual microaggregates.
B12A-08
First order decay models fail to represent soil organic carbon dynamics in both single-pool and in multiple-pool models based on size fractionation
Soil organic carbon (SOC) models often rely on analytical solutions derived from litter bag experiments in which C losses follow first–order kinetics of the type (Ct= Co . e-kt). In this model, k is the decomposition or decay factor and is considered constant over time. Several authors pointed out that one-pool exponential SOC models fail to reproduce long-term SOC dynamics, and that multiple pools or time-dependent k values are needed to capture SOC behavior given its heterogeneous composition and turnover. As a consequence, models that separate organic matter in multiple pools with their own specific k values, expected to remain constant through time, have been proposed. Whether these multiple pool models with constant k values through time are able to represent SOC trajectories is unknown. We explored the stability of k values through time for total SOC and its more labile fraction known as particulate organic carbon (POC) across 15 grasslands sites in which shifts in the stable isotopic composition of C inputs after grazing exclosure (change from C4 to C3 dominated vegetation) allowed us to trace SOC dynamics. Time elapsed after grazing exclusion ranged from 4 to 30 years. We sampled soils and roots in grazed-ungrazed paired plots and measured SOC, POC and 13C composition. Using 13C and SOC changes we estimated k and carbon mean residence times (MRT) after grazing exclusion. As expected k (mass of annual C losses per unit of mass of C storage) values for total SOC were variable between sites but always lower (0.03 to 0.11) than k values of the POC fraction (0.09 to 0.19), and the opposite was true for MRT. For the total SOC, k values varied with time elapsed since grazing removal (r2= 0.71), suggesting that a variable k through time is needed to reproduce SOC dynamics with a one-pool model. Surprisingly, k values for the POC fraction were also significantly correlated with time (r2= 0.75), suggesting that even the POC fraction can not be accurately modeled with a first order kinetic model with a constant k that is successfully applied to litter bag decomposition experiments. Our results suggest that the widely used POC fraction does not separate a sufficiently homogenous pool with constant k values, or either that the exponential decay model does not apply for SOC dynamics.