Wayne R Roper III, Deanna L Osmond, Joshua L Heitman, Michael G Wagger and S Chris Reberg-Horton, Department of Crop and Soil Sciences, North Carolina State University, Raleigh, NC
Accumulating soil organic matter (SOM) is a major goal of soil health management because of the various beneficial soil ecosystem services that involve SOM. Due to its complex composition, SOM is difficult to directly measure, and soil organic carbon (SOC) is instead used as a surrogate. It is assumed that SOC accumulates from certain agronomic practices, but that assumption has not been widely tested for long-term management in Southeastern U.S. soils. To compare the presence of SOC in different agronomic systems, we used soils from long-term (16-31 yr.) agronomic trials in the mountain, piedmont, and coastal plain physiographic regions of North Carolina. Agronomic trials consisted of management practices ranging from no-till with cover cropping to deep tillage with moldboard plowing. Soil was collected from the top 15 cm and homogenized before analysis. Methods used to quantify SOC were automated gas chromatography (AGC), mass loss on ignition (LOI), and Walkley-Black (WB). All soil samples were collected after crops were harvested in the fall. Average results of SOC analyses using AGC, LOI, and WB ranged from 0.53-1.23%, 0.99-2.01%, and 1.07-1.64%, respectively. The LOI and WB procedures typically measured more SOC than AGC. There was weak correlation between AGC and either LOI (r2 = 0.29) or WB (r2 = 0.30). Conversions between measured SOC for different procedures could be off by as much as 0.28 %SOC, and may not be reliable predictors of SOC results from different procedures. Among the agronomic systems, there was not more SOC from no-till soils compared to tilled soils in any trial, which means that conservation management did not accumulate more SOC than tillage. If SOC will be a significant determinant of soil health, interpretations of SOC content should be relative to a specific procedure and should not be compared to different procedures.