Growing and non-growing season nitrous oxide emissions from a manured semiarid cropland soil under irrigation

Dairy manure is used in semiarid southern Idaho to improve soil fertility, but campaigns to measure resulting nitrous oxide (N2O) emissions over the complete year have not been conducted to date. The objective of this study was to measure N2O fluxes throughout the growing (April to Sept) and non-growing (Oct to Mar) seasons in 2020 (sugarbeet) and 2021 (silage corn and triticale) in a field that received inorganic N fertilizer or was previously treated with dairy manure solids on an annual and biennial basis for 8 years. Gas fluxes were measured daily using automated chambers that were connected to a gas chromatograph for in situ analysis of N2O. The N2O emissions were found to be highly episodic and major pulses were associated with irrigation during the growing season, warming events in the winter, and soil disturbance at harvest. Emissions were greatest from soil that had received manure at the highest annual application rate of 52 Mg ha−1 (dry wt.), with cumulative totals of 3.6 and 3.0 kg N2O-N ha−1 in 2020 and 2021, respectively. These cumulative totals were about 3-fold greater than emissions from plots treated with inorganic fertilizer or manure at 17 Mg ha−1 annually or 35 Mg ha−1 biennially. This outcome can be attributed to high concentrations of nitrate produced through mineralization of organic N in manure. Emission factors indicated that up to 1.2% of the total N applied was lost as N2O-N, with the greatest loss from inorganic fertilizer treated soil. When breaking down the emissions by season, anywhere from 49%–63% (2020) and 37–58% (2021) of the N2O-N emissions occurred during the non-growing season. Growing and non-growing season N2O emissions were found to be statistically equivalent for each of the respective fertilizer or manure treatments. This finding stresses the need to also measure N2O emissions during the non-growing season as a way to improve the accuracy of annual emission estimates.