B51E-01 INVITED 08:00h
Storage of Organic Matter as Related to Organic and Mineral Properties of Soils
This paper reviews the composition of organic matter stabilized in temperate soils as revealed from solid-state $^{13}$C NMR spectroscopy and chemolytic techniques of bulk soils and particle-size and/or density fractions. The stability of the organic matter in these fractions is assessed using stable isotope or radiocarbon analysis. An attempt is made to elucidate the relevance of the mechanisms that are responsible for the stabilization of organic matter in different soils. These include recalcitrance of organic matter, spatial inaccessibility of organic matter to decomposer organisms due to aggregation and vertical distribution within the soil profile, and stabilisation by interaction with mineral surfaces (oxide-characteristic reactive surfaces versus phyllosilicates).
B51E-02 INVITED 08:30h
Chemical Nature and Turnover of Carbon Associated with Diagnostic Aggregate Fractions
Recently, many studies have shown the importance of aggregation in controlling soil organic C dynamics and storage. Nevertheless, very few studies have characterized the chemical nature of aggregated associated C fractions to elucidate the origin and degree of microbial alteration of these C fractions. Here, I summarize several studies employing biomarker analyses for plant-derived lignin, bacterial-derived muramic acid, and fungal-derived glucosamine to aggregate associated C fractions. A comparison of different particulate organic matter (POM) fractions indicated that fine POM occluded within microaggregates-within-macroaggregates (mM) had the greatest amino sugar content, greatest ratio of glucosamine over muramic acid, and lowest phenolic CuO oxidation products. The latter result suggest that the fine POM is the most degraded POM fraction, which was confirmed by C isotope analyses. However, side chain oxidation of lignin compounds of fine POM was intermediate, suggesting an average microbial alteration of lignin. These results suggest a significant microbial contribution, especially fungal, to this relative older C fraction protected within the mM. Carbon and isotopic analyses of the mM confirmed that this structural unit within the soil protects C from fast decomposition and facilitates the long-term stabilization of C in undisturbed soil. Furthermore, amino sugar analyses indicated that microbial-derived C is stabilized in the mM, due primarily to a greater fungal-mediated improvement of soil structural stability and concurrent deposition of fungal-derived C. In conclusion, the characterizing the chemical nature and turnover of aggregate associated C fractions elucidated that the mM fraction plays an important role in the long term stabilization of C and seems to be an ideal indicator or diagnostic fraction for C sequestration potential in soils.
B51E-03 09:00h
Nature and Dynamics of Carbon Accrued in a Forest Soil During Five Years of Atmospheric CO$_{2}$ Enrichment
The potential for enhanced soil C storage to partially offset rising atmospheric CO$_{2}$ concentrations is being evaluated by long-term field CO$_{2}$ enrichment experiments. Although plant productivity is often stimulated in such experiments, the fate of increased detrital inputs to soil has yet to be definitively resolved, in part because detecting changes in soil C against the relatively large, spatially heterogeneous pool of existing soil organic matter has proven difficult. Even when significant changes in whole soil C are evident, predictions of the potential for long-term sequestration will require detailed studies of C dynamics and stability in functionally meaningful soil organic matter pools. In our studies at the free-air CO$_{2}$ enrichment (FACE) experiment on a sweetgum ({\it Liquidambar styraciflua} L.) forest plantation in Oak Ridge, Tennessee, we are using (1) repeated sampling over time, (2) the isotopic tracer provided by the highly depleted $^{13}$C signature of the CO$_{2}$ source used for fumigation, and (3) physical and chemical fractionation procedures to determine the fate and dynamics of FACE-derived C inputs to soil organic matter. After five years of CO$_{2}$ enrichment, soil C accumulated at a linear rate in both unprotected and aggregate-protected pools, suggesting that additional C inputs were being processed and cycled in much the same manner as under ambient conditions. However, selective analysis of the biopolymer composition (lignin, suberin, and cutin) and oxidation state of the organic matter in physically and chemically isolated soil fractions will be used to assess the source, nature and potential stability of the C accrued in protected and unprotected pools.
B51E-04 09:15h
Control of SOM Stabilization by Preferential Sorption of Nitrogenous Compounds
Sequential density fractionation separates organic, mineral, and organo-mineral particles mainly according to the ratio of organic to mineral material (organic loading). Previous studies all show a consistent increase in %N (decrease in C:N) with increasing particle density from about 1.6 to 2.5 g cm-3. Correlated with this increase is a decrease in polysaccharides and increase in alkyl and aromatic compounds. We interpret these trends as due to strong binding by amino-N and carboxyl-rich compounds to mineral surfaces creating a strongly sorbed N-rich inner layer onto which neutral polysaccharides and other less polar (and less strongly sorbed) compounds attach (via both electrostatic and covalent bonds) more readily than they would onto clean mineral surfaces. As a first test of this hypothesis, we fractionated a forest Andisol at 1.65, 1.8, 2.0, 2.25, and 2.6 g cm-3. Mass recovery was greatest in the 2.0-2.25 and 2.25-2.6 g cm-3 fractions. Samples were analyzed for total C and N, 13C, 15N, and 14C. Cu oxidation, TMAH, and other molecular, isotopic and spectroscopic techniques were used to determine the organic matter composition of the fractions.
B51E-05 09:30h
Chemical Stabilization of Soil Organic Nitrogen by Phenolic Lignin Compounds in Anaerobic Agrosystems
In tropical Asia, continuous cropping of paddy rice promotes the covalent binding of soil organic nitrogen (N) by phenolic lignin residues, which in turns appears to contribute to substantial long-term declines in availability of soil organic N and in grain yield. A newly developed technique of nuclear magnetic resonance spectroscopy that selects for carbon atoms bound to N was applied to a young humic fraction to directly observe an agronomically significant greater quantity of organic N (difference of 55 kg per hectare) that was bound by lignin residues in a triple-cropped rice soil compared to a nearby aerated soil. The resulting compound was an anilide. Crop residues are the parent material of soil organic matter in agricultural soils, and their anaerobic decomposition was found to slow microbially driven mineralization of both soil organic N and soil phenols during the rice season, compared to aerobic decomposition. Through use of 15N-labeled fertilizer, stabilization of soil organic N was shown to be more extensive than binding of inorganic fertilizer N. Similar results were gained in eastern Arkansas when comparing a more anaerobic continuous rice rotation to the conventional rice-soybean rotation. Future studies of covalently bound N will consider both its seasonal dynamics and its significance to long-term yield trends. Agronomic observations suggest that covalent binding of nutrients by lignin residues might also occur in other agrosystems with anaerobic decomposition of crop residues, including paddy taro in Hawaii and to a lesser extent no-tilled crops in regions with cool, wet springtime weather.
B51E-06 09:45h
Lessons from compound specific turnover of soil organic matter using 2H, 13C and 14C
Compound specific stable and radioactive isotopes at their natural abundance level are used to understand sources and processes involved in the formation of soil organic matter. Bulk 13C values of soil organic matter from time series and depth profile give first hints on the turnover of soil carbon. However, only isotope values of single compounds give the opportunity to understand the processes of soil carbon formation. Therefore, compound specific turnover times (13C and 14C) are calculated from the shift of carbon isotopes in labile and recalcitrant biomarkers extracted from soil after vegetation change. Additionally, for the first time we present compound specific turnover times of these biomarkers using Deuterium. Results from three different experimental sites offer a new insight of soil carbon formation and stabilization.
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