Evaluating Soil Carbon Pools in Long-Term Organic and Conventional Farming Systems.

Organic farming practices are known to build soil organic matter and potentially reduce the risk of sediment and nutrient runoff losses into surrounding rivers and lakes. However, heavy reliance on tillage to control weeds in organic agriculture can promote soil degradation. Project objectives were to 1) determine the impact of long-term management practices on soil biological properties; 2) identify soil properties that correlate with runoff and nutrient losses under conventional and organic management; and 3) determine how management impacts total organic matter lost from runoff. The experiment was carried out on plots in Mills River, NC under continuous management for over 15 years. Treatments included 1) organic management + no tillage, 2) organic management  + conventional tillage 3) conventional management + no tillage 4) conventional management + conventional tillage, and 5) a control, replicated 4 times each. Soil samples from all plots were analyzed for particulate organic matter (POM) using density fractionation, and microbial biomass using chloroform fumigation extraction. Soil bulk density was assessed using an Uhland coring device. Plots were instrumented with automated electronic samplers and a runoff sample from each rain event collected, dried until water evaporated, and sediment analyzed for total carbon. POM in the organic no till plots was significantly higher than all other treatments. Organic no till plots had significantly more microbial biomass carbon when compared to a conventional system with tillage. Bulk density was highest in the conventionally-managed no till plots and lowest in the organic plots, which may be attributed to the high organic matter content in the organic plots. Results provide evidence that long-term organic management increases soil microbial activity and organic matter in agricultural soils, especially where tillage is absent, but that such soils on sloped land may also lose more of this carbon via surface runoff.