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SOIL BIOLOGY & BIOCHEMISTRY

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

^a Washington State Univ., Irrigated Agricultural Research and Extension Center, 24106 N. Bunn Rd., Prosser, WA 99350
^b USDA-ARS, Vegetable and Forage Research Unit, 24106 North Bunn Rd., Prosser, WA 99350
^c USDA-ARS, 10300 Baltimore Ave., Bldg. 001, Room 140, BARC-West, Beltsville, MD 20705
^d Colorado State Univ., Natural Resource Ecology Lab., Fort Collins, CO

^* Corresponding author (hal.collins{at}ars.usda.gov).

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 (Zea mays 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 ¹³C 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 ¹³C 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 ¹³C 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.

Abbreviations: BRSP, Bradford reactive soil protein • CT, conventional tillage • KBS, Kellogg Biological Station • LF, light fraction • MRT, mean residence time • NT, no-till • OM, organic matter • POM, particulate organic matter • SOC, soil organic carbon • SOM, soil organic matter