Effects of Alternative Farming Systems on Soil Organic Matter Pools and Nitrous Oxide Emissions.

A majority of the world’s anthropogenic nitrous oxide emissions come from agricultural soils. Because emissions levels depend on a combination of factors including soil organic matter (SOM) levels and mineral N availability, it is not clear how farm management strategies may affect net production of this potent greenhouse gas. Furthermore, it is vital to understand which of several diverse SOM pools is driving N₂O emissions. The objectives of this study are to examine different rotations for their impact on soil N₂O emissions, and moreover, to elucidate the soil C and N dynamics driving these differences. Both organic and conventional annual cropping systems have been managed since 1999 in the Coastal Plain of North Carolina using reduced-till (RT), clean till (CT), or a 3 year rotation with pasture (LR). Data collected from field soil (0-15cm) include microbial biomass carbon and nitrogen (MBC/MBN), and dissolved organic carbon (DOC). Laboratory incubations were also conducted to quantify N₂O emissions, soil respiration, and N mineralization from the same samples. In the soybean year of rotation, farming systems differ in the amount of N₂O that is emitted following rewetting soils (70% WFPS) collected at harvest when soy nodules are likely senescing, but not during the growing season. In fertilized soils from a corn year, N2O emissions differed following the first fertilization and also at corn harvest.  MBN of organic LR also exceeded that of conventional CT in early spring, while organic RT MBN was higher than that of conventional NT at harvest.  No differences were found in MBN or DOC.