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Fractionation and Long-Term Laboratory Incubation to Measure Soil Organic Matter Dynamics

Haile-Mariam, S., Collins, H. P., Wright, S., Paul, E. A.
Soil Science Society of America journal 2008 v.72 no.2 pp. 370
soil organic matter, agricultural soils, fractionation, soil separates, carbon, nitrogen, particles, particle size, glomalin, conventional tillage, no-tillage, stable isotopes, clay fraction, silt fraction, sand fraction, Zea mays, crop residues, Corn Belt region
Soil organic matter (SOM) in agricultural soils comprises a significant part of the global terrestrial C pool. It has often been characterized by utilizing a combination of chemical dispersion of the soil followed by physical separation. We fractionated soil samples under continuous corn (L.) rotations at four long-term sites in the Corn Belt to determine the concentration of C and N associated with soil fractions (light fraction [LF], particulate organic matter [POM], silt size, clay size, and Bradford reactive soil protein [BRSP]) and to identify the change in C concentration and δC signal of each fraction using laboratory incubations. Light fractions comprised 3 to 5% of the soil organic carbon (SOC), with no significant difference between conventional tillage (CT) and no-till (NT) treatments. The POM fraction accounted for 5 to 11% of the SOC in the soils with >30% clay and 17 to 23% for the soils with <20% clay. The clay-size fraction contained the highest proportion of SOC. Measurement of C during long-term incubation showed that the average mean residence time (MRT) of corn-derived C in the LF was 3.5 yr, whereas the POM fractions ranged from 6 to 12 yr. The C changes during incubation show that both fractions consist of a mixture of active and resistant materials, with movement between fractions. The BRSP has long MRTs except in the NT Hoytville soil. Measurement of the dyna mics of these fractions provides a basis for C models to test the impacts of land use and management on C sequestration.