Soil nitrous oxide and methane fluxes are low from a bioenergy crop (Canola) grown in a semi-arid climate

Abstract

Understanding nitrous oxide (N2O) and methane (CH4) fluxes from agricultural soils in semi-arid climates is necessary to fully assess greenhouse gas emissions from bioenergy cropping systems, and to improve our knowledge of global terrestrial gaseous exchange. Canola is grown globally as a feedstock for biodiesel production, however, resulting soil greenhouse gas fluxes are rarely reported for semi-arid climates. We measured soil N2O and CH4 fluxes from a rain-fed canola crop in a semi-arid region of south-western Australia for 1 year on a subdaily basis. The site included N fertilized (75 kg N ha1 yr1) and nonfertilized plots. Daily N2O fluxes were low ( 1.5 to 4.7 g N2O-N ha1 day1) and culminated in an annual loss of 128 g N2O-N ha1 (standard error, 12 g N2O-N ha1) from N fertilized soil and 80 g N2O-N ha1 (standard error, 11 g N2O-N ha1) from nonfertilized soil. Daily CH4 fluxes were also low ( 10.3 to 11.9 g CH4-C ha1 day1), and did not differ with treatments, with an average annual net emission of 6.7 g CH4–C ha1 (standard error, 20 g CH4–C ha1). Greatest daily N2O fluxes occurred when the soil was fallow, and following a series of summer rainfall events. Summer rainfall increased soil water contents and available N, and occurred when soil temperatures were 425 1C, and when there was no active plant growth to compete with soil microorganisms for mineralized N; conditions known to promote N2O production. The proportion of N fertilizer emitted as N2O, after correction for emissions from the no N fertilizer treatment, was 0.06%; 17 times lower than IPCC default value for the application of synthetic N fertilizers to land (1.0%). Soil greenhouse gas fluxes from bioenergy crop production in semi-arid regions are likely to have less influence on the net global warming potential of biofuel production than in temperate climates.