Increased N2O emissions during soil drying after waterlogging and spring thaw in a record wet year

Global climate change is expected to increase the frequency and intensity of extreme precipitation events, which can dramatically alter soil nitrous oxide (N2O) emissions. However, our ability to predict this effect is limited due to the lack of studies under real-world conditions. We conducted a field experiment in a maize-cultivated black soil in northeast China with six treatments: control without nitrogen (N) application (CK) and N-fertilized treatments with the ratio of urea N to manure N at 100:0 (NPK), 75:25 (OM1), 50:50 (OM2), 25:75 (OM3) and 0:100 (OM4). The experimental year was the wettest on record with an extreme rainfall event of 178 mm occurring in summer 2013. Annual N2O emissions from CK and NPK were increased by 168% and 171%, respectively, relative to normal wet years. Extreme rainfall saturated soils, resulted in low N2O fluxes (<20 μg N m−2 h−1) lasting for 25 d. However, N2O flux peaked (169-264 μg N m−2 h−1) in all treatments as the soil dried. Total N2O emissions were 0.43-0.74 kg N ha−1 over the drying period, accounting for 47.5-51.2% of the annual budget. High N2O fluxes occurred when the ratio of soil nitrate (NO3−) to dissolved organic carbon was 0.07-0.10 mg N mg−1 C, NO3− concentration was >3 mg N kg−1 and water-filled pore space was 67-76%. Distinctly higher N2O fluxes were also identified during the spring thaw period, accumulating to 20.1-49.4% of the non-growing season emissions. Emissions upon thawing were likely related to denitrification induced by high moisture conditions as a result of lag effect of the extreme rainfall. Annual N2O emissions progressively reduced as the ratio of urea N:manure N shifted towards manure, which was also the case during soil drying after waterlogging. Total N2O emissions were reduced by 25.6% for OM4 than NPK. Overall, our results suggest that soil N2O emissions were increased in the record wet year but a shift from urea towards manure with more N applied as starter N can minimize the N2O losses.