Nitrous oxide emissions following seasonal freeze-thaw events from arable soils in Northeast China

Abstract Seasonal soil freeze-thaw events may enhance soil nitrogen transformation and thus stimulate nitrous oxide (N 2 O) emissions in cold regions. However, the mechanisms of soil N 2 O emission during the freeze-thaw cycling in the field remain unclear. We evaluated N 2 O emissions and soil biotic and abiotic factors in maize and paddy fields over 20 months in Northeast China, and the structural equation model (SEM) was used to determine which factors affected N 2 O production during non-growing season. Our results verified that the seasonal freeze-thaw cycles mitigated the available soil nitrogen and carbon limitation during spring thawing period, but simultaneously increased the gaseous N 2 O-N losses at the annual time scale under field condition. The N 2 O-N cumulative losses during the non-growing season amounted to 0.71 and 0.55 kg N ha −1 for the paddy and maize fields, respectively, and contributed to 66 and 18% of the annual total. The highest emission rates (199.2–257.4 µg m −2 h −1 ) were observed during soil thawing for both fields, but we did not observe an emission peak during soil freezing in early winter. Although the pulses of N 2 O emission in spring were short-lived (18 d), it resulted in approximately 80% of the non-growing season N 2 O-N loss. The N 2 O burst during the spring thawing was triggered by the combined impact of high soil moisture, flush available nitrogen and carbon, and rapid recovery of microbial biomass. SEM analysis indicated that the soil moisture, available substrates including NH 4 + and dissolved organic carbon (DOC), and microbial biomass nitrogen (MBN) explained 32, 36, 16 and 51% of the N 2 O flux variation, respectively, during the non-growing season. Our results suggested that N 2 O emission during the spring thawing make a vital contribution of the annual nitrogen budget, and the vast seasonally frozen and snow-covered croplands will have high potential to exert a positive feedback on climate change considering the sensitive response of nitrogen biogeochemical cycling to the freeze-thaw disturbance.