Kong, Angela (Univ. of California, Department of Plant Sciences, PES Bldg., One Shields Ave., Davis, CA, 95616; Phone: 530-754-7537; Fax: 530-752-436; Email: 1aykong@ucdavis.edu)

Carbon Input, Aggregate Stability, and Carbon Stabilization in Mediterranean Cropping Systems

A. Y. Kong*, J. Six, R. F. Denison, C. Van Kessel

One of our current challenges is to provide credible evidence that agricultural practices can sequester significant amounts of C and to quantify the underlying mechanisms, capacity, and longevity of agricultural lands to serve as C sinks. The objectives of this study were to (1) quantify the relationship between C input and SOC sequestration in whole soil and SOM fractions and (2) evaluate mechanisms of long-term SOC stabilization across a gradient of C input levels. Soil samples from 10 cropping systems at the Long-Term Research on Agricultural Systems (Davis, CA) were separated into four aggregate size classes (large macroaggregates: greater than 2mm, small macroaggregates: 0.25-2mm, microaggregates: 0.053-0.25mm, and silt&clay: less than 0.053mm) and into three SOM fractions within large and small macroaggregates (cPOM: greater than 0.25mm, mM: 0.053-0.25mm, and silt&clayM: less than 0.053mm). Estimates of C input from corn, wheat, and tomato residues were empirically derived from relationships between yield and biomass-C (above- and belowground), while C input from legume cover crops and compost was estimated from annual application rates. We found a positive correlation between cumulative C input and SOC (r²=0.70, p less than 0.003), while aggregate stability increased linearly with greater C input (r²=0.75, p=0.001) and SOC (r²=0.63, p =0.006), respectively. We observed that aggregate-C shifted from the less than 0.053mm fraction in low C input systems to the large and small macroaggregate fractions in high C input systems, with a majority of the increase in C input-derived SOC preferentially stabilized in the mM fraction. Hence, the mM fraction an ideal indicator for C sequestration potential in sustainable agroecosystems.