The effect of nitrogen fertilization and no-till duration on soil nitrogen supply power and postspring thaw greenhouse-gas emissions

With a world population now > 7 billion, it is imperative to conserve the arable land base, which is increasingly being leveraged by global demands for producing food, feed, fiber, fuel, and facilities (i.e., infra-structure needs). The objective of this study was to determine the effect of varying fertilizer-N rates on soil N availability, mineralization, and CO2 and N2O emissions of soils collected at adjacent locations with contrasting management histories: native prairie, short-term (10 y), and long-term (32 y) no-till continuous-cropping systems receiving five fertilizer-N rates (0, 30, 60, 90, and 120kg N ha1) for the previous 9 y on the same plots. Intact soil cores were collected from each site after snowmelt, maintained at field capacity, and incubated at 20 degrees C for 6 weeks. Weekly assessments of soil nutrient availability along with CO2 and N2O emissions were completed. There was no difference in cumulative soil N supply between the unfertilized long-term no-till and native prairie soils, while annual fertilizer-N additions of 120kg N ha1 were required to restore the N-supplying power of the short-term no-till soil to that of the undisturbed native prairie soil. The estimated cumulative CO2-C and N2O-N emissions among soils ranged from 231.8474.7 g m2 to 183.9862.5 mg m2, respectively. Highest CO2 fluxes from the native prairie soil are consistent with its high organic matter content, elevated microbial activity, and contributions from root respiration. Repeated applications of 60kg N ha1 resulted in greater residual inorganic-N levels in the long-term no-till soil, which supported larger N2O fluxes compared to the unfertilized control. The native prairie soil N2O emissions were equal to those from both short- and long-term no-till soils receiving repeated fertilizer-N applications at typical agronomic rates (e.g., 90kg N ha1). Eighty-eight percent of the native soil N2O flux was emitted during the first 2 weeks and is probably characteristic of rapid denitrification rates during the dormant vegetative period after snowmelt within temperate native grasslands. There was a strong correlation (R-2 0.64; p < 0.03) between measured soil Fe-supply rate and N2O flux, presumably due to anoxic microsites within soil aggregates resulting from increased microbial activity. The use of modern no-till continuous diversified cropping systems, along with application of fertilizer N, enhances the soil N-supplying power over the long-term through the build-up of mineralizable N and appears to be an effective management strategy for improving degraded soils, thus enhancing the productive capacity of agricultural ecosystems. However, accounting for N2O emissions concomitant with repeated fertilizer-N applications is imperative for properly assessing the net global warming potential of any land-management system.