The effect of nitrogen fertilization and no-till duration on soil nitrogen availability and greenhouse gas emissions
With a world population now greater than seven billion, it is imperative to conserve the arable land base, which is increasingly being leveraged by global demands for producing food, feed, fibre, and fuel. A key component of sustainable agriculture involves the restoration of unproductive lands that have been rendered unsuitable for agricultural production through anthropogenic soil degradation. The objective of this study was to determine the effect of varying fertilizer N rates on soil N availability and N2O and CO2 emissions of three soils collected at adjacent locations with contrasting management histories: native prairie or short-term (10 years) and long-term (32 years) no-till continuous multi-crop (wheat-pea-canola) cropping systems receiving five fertilizer N rates (0, 30, 60, 90, and 120 kg N/ha) for the previous nine years. Intact soil cores were collected from each site, maintained at field capacity, and incubated (22 oC) for six weeks. Weekly assessments of soil nutrient availability and N2O and CO2 emissions were completed to assess the impact of prolonged variable rates of fertilizer N and duration of no-till management of degraded agricultural soil relative to an adjacent native prairie soil. At the end of the six-week incubation, there was no significant difference (P > 0.15) in cumulative soil N supply rate between the unfertilized long-term no-till soil and native soil. Annual fertilizer N additions of 120 kg N/ha for the previous nine years were required to restore the N supplying power of the short-term no-till soil to that of the native soil, through the build-up of mineralizable N levels. As expected, repeated applications of fertilizer N increased the residual soil N levels in the cultivated soils compared to the native soil. The estimated cumulative CO2-C and N2O-N emissions at the end of the six-week incubation ranged from 231.8-474.7 g/m2 to 183.9-862.5 mg/m2, respectively. Repeated applications of ≥ 60 kg N/ha supported larger N2O-N fluxes in the long-term no-till soil compared to the unfertilized control. Highest CO2-C fluxes from the native prairie soil are consistent with its high organic matter content and contributions from root respiration. Surprisingly, the native prairie soil N2O-N emissions were equal to those from LTNT and STNT soils receiving repeated fertilizer N applications at typical agronomic rates and is probably characteristic of rapid denitrification rates during the dormant vegetative period after snow melt prior to the growing season within temperate native grassland environments. The use of modern no-till continuous multi-crop 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 associated with repeated fertilizer N applications is imperative for properly assessing the net global warming potential of any land management system.
CO2 and N2O fluxes
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