N2O Emissions From Two Agroecosystems: High Spatial Variability and Long Pulses Observed Using Static Chambers and the Flux‐Gradient Technique.

With the addition of nitrogen (N), agricultural soils are the main anthropogenic source of N2O, but high spatial and temporal variabilities make N2O emissions difficult to characterize at the field scale. This study used flux‐gradient measurements to continuously monitor N2O emissions at two agricultural fields under different management regimes in the inland Pacific Northwest of Washington State, USA. Automated 16‐chamber arrays were also deployed at each site; chamber monitoring results aided the interpretation of the flux gradient results. The cumulative emissions over the six‐month (1 April–30 September) monitoring period were 2.4 ± 0.7 and 2.1 ± 2 kg N2O‐N/ha at the no‐till and conventional till sites, respectively. At both sites, maximum N2O emissions occurred following the first rainfall event after N fertilization, and both sites had monthlong emission pulses. The no‐till site had a larger N2O emission factor than the Intergovernmental Panel on Climate Change Tier 1 emission factor of 1% of the N input, while the conventional‐till site's emission factor was close to 1% of the N input. However, these emission factors are likely conservative. We estimate that the global warming potential of the N2O emissions at these sites is larger than that of the no‐till conversion carbon uptake. We recommend the use of chambers to investigate spatiotemporal controls as a complementary method to micrometeorological monitoring, especially in systems with high variability. Continued monitoring coupled with the use of models is necessary to investigate how changing management and environmental conditions will affect N2O emissions. Plain Language Summary: Nitrous oxide (N2O) is a greenhouse gas and stratospheric ozone depleting substance that is emitted by soils. Agricultural soils tend to emit more N2O than natural soils due to the addition of nitrogen fertilizers. N2O emissions are not well understood on the scale of individual farms, as emissions are difficult to measure at this resolution because they are irregular over time and space. This variability is due to the dependence of N2O production and emission on soil properties, that is, moisture, nitrogen, and the microbiome. In this study we monitored N2O emissions from two agricultural fields under different tillage regimes using two complementary methods: the flux‐gradient technique and automated chambers. The flux‐gradient technique measures N2O emissions at the field scale, which is relevant to agronomic management. Using both techniques together improves confidence in our results, which give us information on total N2O emissions from these fields, as well as the relationships between N2O emissions and rainfall, temperature, and carbon dioxide respiration. Key Points: Total emissions from 1 April to 30 September 2015 were 2.4 ± 0.7 and 2.1 ± 0.2 kg N2O‐N/ha at the no‐ and conventional‐till sites, respectivelyN2O fluxes had similar responses to temperature and precipitation at both sitesHigh spatial variability at both sites (CVs > 140%) limited the ability of the chamber systems to characterize field‐scale N2O emissions [ABSTRACT FROM AUTHOR]