Nitrous oxide fluxes determined by continuous eddy covariance measurements from intensively grazed pastures: Temporal patterns and environmental controls

Abstract

Nitrous oxide (N2O) is a prominent greenhouse gas. Our understanding of environmental controls on N2O fluxes has mainly come from small-scale experiments, for example, static chamber measurements on plots or lab incubations. However, studies of the environmental controls for N2O fluxes at ecosystem scales have been limited. Using eddy covariance (EC) measurements, this study evaluated the environmental drivers of N2O fluxes for a one-year period at a farm grazed year-round by dairy cows in the Waikato region, New Zealand. We identified an optimum soil moisture/temperature zone that favours maximal N2O emissions, demonstrating maximum N2O fluxes at ∼70% water-filled pore space (WFPS) and moderate soil temperatures. Our measurements consistently identified significant N2O flux pulses associated with rainfall following grazing events in warm-dry months. In contrast, during cold-wet months when WFPS was consistently high, pulses after rainfall did not occur. A clear positive temperature response for N2O fluxes was observed above 70% WFPS while a negative relationship was detected when WFPS was less than 70%. Distinctive diurnal flux patterns emerged in both pulses and background fluxes, implying that soil temperature regulates N2O fluxes at sub-daily timescales. Over the annual period, N2O emissions were 6.5 kg N2O-N ha−1. We found the highest cumulative rates (maximum 35.7 g N2O-N ha−1 day−1) in autumn but the rates were low during both summer and winter. Our results highlighted the combined effects of environmental factors on N2O fluxes, and quantified N2O flux variations at seasonal and daily scales, suggesting that continuous measurement techniques, such as EC, could serve as an alternative in national N2O inventories.