Long-Term Effects of Organic Agriculture on Soil Organic Carbon Stocks up to One Meter Depth.

Organic agriculture (OA) is practiced on about 1 percent of the global agricultural land area and its importance continues to grow. It is perceived by many as having less negative effects on the environment than conventional agriculture because applications of soluble mineral fertilizers, and synthetic herbicides and pesticides are prohibited. However, yields under OA are about 19 percent lower and the attendant lower soil carbon (C) inputs together with tillage for weed control may contribute to lower profile soil organic carbon (SOC) stocks under OA. We studied SOC stocks up to one meter depth at long-term field experiments comparing OA and conventional agriculture at sites in Canada (Glenlea, Manitoba), USA (Lamberton, Minnesota; Rodale, Kutztown, Pennsylvania) and the U.K. (Nafferton, Stocksfield). Preliminary data analysis indicated that SOC stocks to 0.6 meter depth at Morris, Minnesota, were similar among the most widely used conventional system (147.7 Mg C ha-1) in comparison to those in the OA system (150.5 Mg C ha-1). However, the conventional system had higher SOC stocks than those in the OA system (167. 1 vs. 157.8 Mg C ha-1, respectively) for systems designed to maximize the agronomic production. There is an urgent need to strengthen the database on impacts of OA on profile SOC stocks by establishing and studying long-term field experiments in all major biomes and principal soils. The scientific interest in OA is less than a century old, and there is significant potential to lessen its environmental impacts while methods derived from OA can contribute to sustainable intensification of conventional agricultural systems.