Biological and Physico-chemical Processes of Soil Organic Matter Cycling in Diverse Soils

Soils comprise the largest biologically active terrestrial pool of organic carbon (OC). The top meter of soil contains 1500 Pg of OC which is 3 times that present in vegetation and two times the CO2-C present in atmosphere. Current soil C models simulate soil C pool sizes and turnover rates on post-hoc basis and the mechanisms governing soil OC cycling have not been integrated in such models. Therefore the scale of applicability and accuracy of predictions of current C models are questionable. Our current efforts are focused on developing a mechanistic framework of soil C cycling processes and its linkage to global C model. As part of this effort, we seek to understand the important cycling and interactive processes of OC compounds with the soil minerals and microbial community on a global suite of soils from temperate, tropical and arctic ecosystems. The selected OC compounds are glucose, cellulose, stearic acid and vanillic acid which are representative of SOM composition that contains 5-15% sugars, 20-50% starch, 10% proteins, 20-30% lignin and 2-5% lipids. We hypothesize that physico-chemical interactions between OC compounds and soil minerals determines the biological stability and distribution of such compounds in soils. Cycling of the selected 14C-labeled OC compounds were investigated as a function of soil type, soil depth and functional components of SOM (dissolved organic carbon, DOC; particulate organic matter, POM; and mineral associated organic matter, MAOM). This presentation will consist of the results from sorption and long-term incubation experiments conducted on diverse soils by the addition of 14C-glucose. Sorption of 14C-glucose on soil minerals was determined by batch equilibration experiments of MAOM fraction at a solid-to-solution ratio of 1:60 for 8 hours. A series of initial glucose solutions containing 0-100 mg C/L unlabeled C and 4000 dpm/ml labeled C were used. Maximum sorption capacity (Qmax) and affinity coefficient (K) were determined by fitting the experimental data to the Langmuir model. Results indicated that C sorption potential varies across different climates, soil types and soil horizons. Tropical Oxisol from Costa Rica exhibited the lowest Qmax (12 mgC kg-1) and temperate Alfisols from United States exhibited the highest Qmax (4893 mgC kg-1) for the added glucose. Another interesting finding is that the MAOM derived from the surface soil likely possess higher sorption capacity than that of subsoil. The biological cycling of C through microbes via microbial uptake and mineralization processes are currently being undertaken by monitoring the 14CO2 evolution from the long-term incubation experiments. Additionally, the evidence of priming as a result of glucose addition will also be tested and presented at the meeting. The ultimate outcome of this study is the development of a mechanistically-based and globally-relevant soils C model that is linkable into widely-used global circulation models.