Soil organic carbon fractions and aggregation in the Southern Piedmont and Coastal Plain

Quantification of the impact of long-term agricultural land use on soil organic C (SOC) is important to farmers and policymakers, but few studies have characterized land use and management effects on SOC across physiographic regions. We measured the distribution and total stock of SOC to a depth of 20 cm under conventional tillage (CvT), conservation tillage (CsT), and pasture in 87 production fields from the Southern Piedmont and Coastal Plain Major Land Resource Areas. Across locations, SOC at a depth of 0 to 20 cm was: pasture (38.9 Mg [ha.sup.-1]) > CsT (27.9 Mg [ha.sup.-1]) > CvT (22.2 Mg [ha.sup.-1]) (P [less than or equal to] 0.02). Variation in SOC was explained by management (41.6%), surface horizon clay content (5.2%), and mean annual temperature (1.0%). Higher day content and cooler temperature contributed to higher SOC. Management affected SOC primarily at the soil surface (0-5 cm). All SOC fractions (i.e., total SOC, particulate organic C, soil microbial biomass C, and potential C mineralization) were strongly correlated across a diversity of soils and management systems (r = 0.85-0.96). The stratification ratio (concentration at the soil surface/concentration at a lower depth) of SOC fractions differed among management systems (P < 0.0001), and was 4.2 to 6.1 under pastures, 2.6 to 4.7 under CsT, and 1.4 to 2.4 under CvT; these results agree with a threshold value of 2 to distinguish historically degraded soils with improved soil conditions from degraded soils. This on-farm survey of SOC complements experimental data and shows that pastures and conservation tillage will lead to significant SOC sequestration throughout the region, resulting in improved soil quality and potential to mitigate C[O.sub.2] emissions. Abbreviations: CMIN, potential C mineralization; CsT, conservation tillage row cropping; CvT, conventional tillage row cropping; MLRA, Major Land Resource Area; MWD, mean-weight diameter; POC, particulate organic carbon; SMBC, soil microbial biomass carbon; SOC, soil organic C; SOM, soil organic matter.