Microbial Biomass and Nutrient Dynamics in Transitional Systems in Minnesota.

A large-scale systems study comparing whole system organic or conventional management with nested tillage, rotation, and fertilizer treatments was established in 2002.  The treatments within each management system were: strip tillage vs. conventional tillage; two vs. four year crop rotations (corn-soybean, corn-soybean-wheat-alfalfa), and fertilized vs. not fertilized (inorganic under conventional management and manure under organic management). Soil samples to 15-cm depth were taken each spring and analyzed for microbial biomass C and N (MBC and MBN). In-situ N mineralization incubations were also conducted in a sub-set of plots from 2005-2007.  In the transition phase (2002-2004) there were no significant differences in microbial biomass C or N between conventional and organic management. By 2005, differences between management systems and treatments across management became more pronounced. By 2007, significant differences were observed with the trend of greater overall MBC and MBN in plots under organic management and for strip-tillage, four year rotations and fertilization treatments across management systems. In the subset of strip-tilled or conventionally-tilled and fertilized treatments net cumulative mineralization of N was typically higher under organic management; however it was very dependant on the cropping phase in the four year rotation.