Abstract
Agricultural soils are important sources of atmospheric N2O and CO2. However, in boreal agro-ecosystems the contribution of the winter season to annual emissions of these gases has rarely been determined. In this study, soil N2O and CO2 fluxes were measured for 6 years in a corn-soybean-wheat rotation in northeast China to quantify the contribution of wintertime N2O and CO2 fluxes to annual emissions. The treatments were chemical fertilizer (NPK), chemical fertilizer plus composted pig manure (NPKOM), and control (Cont.). Mean soil N2O fluxes among all three treatments in the winter (November–March), when soil temperatures are below −7°C for extended periods, were 0.89–3.01 µg N m−2 h−1, and in between the growing season and winter (October and April), when freeze-thaw events occur, 1.73–5.48 µg N m−2 h−1. The cumulative N2O emissions were on average 0.27–1.39, 0.03–0.08 and 0.03–0.11 kg N2O–N ha−1 during the growing season, October and April, and winter, respectively. The average contributions of winter N2O efflux to annual emissions were 6.3–12.1%. In all three seasons, the highest N2O emissions occurred in NPKOM, while NPK and Cont. emissions were similar. Cumulative CO2 emissions were 2.73–4.94, 0.13–0.20 and 0.07–0.11 Mg CO2-C ha−1 during growing season, October and April, and winter, respectively. The contribution of winter CO2 to total annual emissions was 2.0–2.4%. Our results indicate that in boreal agricultural systems in northeast China, CO2 and N2O emissions continue throughout the winter.
Highlights
Agricultural cropland is a significant source of the greenhouse gas nitrous oxide (N2O) accounting for about 60% of anthropogenic N2O [1]
Understanding the sources and temporal variations of N2O flux from cropland, as well as the underlying mechanisms for these emissions, is necessary in order to fully account for all the annual greenhouse gas emissions and devise mitigation strategies
Nitrous oxide emissions in agricultural soils result from nitrification and denitrification processes [2], which are regulated by microbial activity, soil moisture, temperature, mineralizable C and N content [3,4,5]
Summary
Agricultural cropland is a significant source of the greenhouse gas nitrous oxide (N2O) accounting for about 60% of anthropogenic N2O [1]. Understanding the sources and temporal variations of N2O flux from cropland, as well as the underlying mechanisms for these emissions, is necessary in order to fully account for all the annual greenhouse gas emissions and devise mitigation strategies. Nitrous oxide emissions in agricultural soils result from nitrification and denitrification processes [2], which are regulated by microbial activity, soil moisture, temperature, mineralizable C and N content [3,4,5]. The emissions of N2O and CO2 in high latitudes during winter have mostly been ignored as they are assumed to be small since soil microbial and root activity below 0 ̊C and in frozen soil conditions is low. While numerous intensive studies on terrestrial CO2 flux from frozen soil have been conducted (e.g. Shi et al [6]), much less work has been done to quantify N2O emissions in boreal cropland during winter [7, 8]
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