Compositional characterization of soil organic matter and hot-water-extractable organic matter in organic horizons using a molecular mixing model
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Microbial decomposition of soil organic matter (SOM) is generally believed to be heterogeneous, resulting in the preferential loss of labile compounds such as carbohydrates and proteins and the accumulation of recalcitrant compounds such as lipids and lignin. However, these fractions are difficult to measure directly in soils. We examined patterns in the biomolecular composition of SOM and hot-water-extractable organic matter (HWEOM) by using a molecular mixing model (MMM) to estimate the content of carbohydrates, protein, lipids, and lignin.
Materials and methods
Organic-horizon soils from Spodosols at the Hubbard Brook Experimental Forest in NH, USA were analyzed for this study. The MMM uses data from elemental analysis (C, H, and N) and 13C nuclear magnetic resonance spectroscopy with cross-polarization and magic-angle spinning to estimate the percentage of total C in the various classes of biomolecules.
Results and discussion
Carbohydrate content decreased from about 50 % of the C in recent litter to approximately 35 % in the bottom of the humus layer. Lipids accounted for about 18 % of C in recent litter and increased to 40 % in the lower humus layers. The HWEOM fraction of SOM was dominated by carbohydrates (40–70 % of C). Carbohydrates and lipids in HWEOM exhibited depth patterns that were the opposite of the SOM. The results from the MMM confirmed the selective decomposition of carbohydrates and the relative accumulation of lipids during humus formation. The depth patterns in HWEOM suggest that the solubility of carbohydrates increases during decomposition, while the solubility of the lipid fraction decreases. The MMM was able to reproduce the spectral properties of SOM and HWEOM very accurately, although there were some discrepancies between the predicted and measured H/C and O/C ratios.
The MMM approach is an accurate and cost-effective alternative to wet-chemical methods. Together, carbohydrates and proteins account for up to 85 % of the C in HWEOM, indicating that the HWEOM fraction represents a labile source of C for microbes. Humification resulted in a decrease in carbohydrate content and an increase in lipids in SOM, consistent with investigations carried out in diverse soil environments.
KeywordsCarbon Decomposition Forest soil Modeling Nuclear magnetic resonance spectroscopy Soil organic matter
This work was funded by the US Department of Agriculture National Research Initiative Competitive Grants program (award no. 2005-35107-16200). Support was also provided by the National Science Foundation, through the Long-Term Ecological Research program (grant no. 1114804). Jeff Baldock and Ron Smernik provided valuable advice on the application of the molecular mixing model and interpretation of the results. Ankit Balaria was supported by a W.L. Li Fellowship while preparing this manuscript. The comments of two anonymous reviewers greatly improved the paper. This is a contribution of the Hubbard Brook Ecosystem Study. The Hubbard Brook Experimental Forest is operated by the Northern Research Station of the USDA Forest Service, Newtown Square, PA.
- Balaria A (2011) Effects of calcium addition on structure and bioavailability of soil organic matter. Dissertation. Syracuse University, Syracuse, 228 ppGoogle Scholar
- Bohlen PJ, Groffman PG, Driscoll CT, Fahey TJ, Siccama TG (2001) Plant–soil–microbial interactions in a northern hardwood forest. Ecology 82:965–978Google Scholar
- Chen CR, Xu ZH, Mathers NJ (2004) Soil carbon pools in adjacent natural and plantation forests of subtropical Australia. Soil Sci Soc Am J 68:282–291Google Scholar
- Ekschmitt K, Kandeler E, Poll C, Brune A, Buscot F, Friedrich M, Gleixner G, Hartmann A, Kästner M, Marhan S, Miltner A, Scheu S, Wolters V (2008) Soil-carbon preservation through habitat constraints and biological limitations on decomposer activity. J Plant Nutr Soil Sci 171:27–35CrossRefGoogle Scholar
- Hatcher PG, Nanny MA, Minard RD, Dible SD, Carson DM (1995) Comparison of two thermochemolytic methods for the analysis of lignin in decomposing gymnosperm wood: the CuO oxidation method and the method of thermochemolysis with tetramethylammonium hydroxide (TMAH). Org Geochem 23:881–888CrossRefGoogle Scholar
- Kolattukudy PE (1980) Cutin, suberin, and waxes. Biochem Plants 4:571–645Google Scholar
- Schlesinger WH (1997) Biogeochemistry: an analysis of global change, 2nd edn. Academic, San DiegoGoogle Scholar
- Soil Survey Staff (2010) Keys to soil taxonomy, 11th edn. USDA-Natural Resources Conservation Service, Washington, 338 ppGoogle Scholar
- Stevenson FJ (1994) Humus chemistry: genesis, composition, reactions, 2nd edn. John Wiley & Sons, New YorkGoogle Scholar
- Wilson MA (1987) NMR techniques and applications in geochemistry and soil chemistry. Pergamon, OxfordGoogle Scholar