Soil nitrous oxide emissions from no-till canola production under variable rate nitrogen fertilizer management

Abstract

Precision farming methods such as variable rate or site-specific nitrogen (N) fertilizer application based on field management zones may represent a mechanism to mitigate soil nitrous oxide (N2O) emissions. To investigate the impact of site-specific N fertilization on soil N2O fluxes a study was conducted from spring 2013 through summer 2015 on no-till cropland under canola (Brassica napus) production near Brandon, Manitoba, Canada. The static chamber method was used to measure soil N2O fluxes across three management zones (low yield, average yield, and high yield) with three N application rates (50%, 100%, and 150% of target seed yields for each management zone) and unfertilized control treatments. A machine-learning approach (random forest analysis) for analyzing soil N2O fluxes confirmed conditions related to soil temperature, moisture, and nitrate availability were the most important predictors of individual flux events. Similarly, soil N availability and moisture content were found to be the most important variables in regression models of cumulative growing season emissions. Nitrogen fertilizer-induced N2O emission factors were lower for the high yield management zone (≈0.1% both growing seasons) compared to the low yield management zone (0.9% in 2013 and 0.2% in 2015) confirming that variable rate application can mitigate N2O fluxes from cropland soils. The high yield zones had the lowest emission factors despite receiving 50% more fertilizer than field average target yields suggesting more efficient crop N use in these areas with greater production potential. Conversely, the low yield zones had higher emission factors despite receiving 50% less fertilizer than field average target yields indicating a greater environmental burden associated with annual cropping these portions of the agricultural landscape. More studies are required to investigate the generality of these findings for different crops, soils and climatic conditions.