Frozen Cropland Soil in Northeast China as Source of N(2)O and CO(2) Emissions

Agricultural soils are important sources of atmospheric N(2)O and CO(2). 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 N(2)O and CO(2) fluxes were measured for 6 years in a corn-soybean-wheat rotation in northeast China to quantify the contribution of wintertime N(2)O and CO(2) fluxes to annual emissions. The treatments were chemical fertilizer (NPK), chemical fertilizer plus composted pig manure (NPKOM), and control (Cont.). Mean soil N(2)O 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 N(2)O emissions were on average 0.27–1.39, 0.03–0.08 and 0.03–0.11 kg N(2)O(–)N ha(−1) during the growing season, October and April, and winter, respectively. The average contributions of winter N(2)O efflux to annual emissions were 6.3–12.1%. In all three seasons, the highest N(2)O emissions occurred in NPKOM, while NPK and Cont. emissions were similar. Cumulative CO(2) emissions were 2.73–4.94, 0.13–0.20 and 0.07–0.11 Mg CO(2)-C ha(−1) during growing season, October and April, and winter, respectively. The contribution of winter CO(2) to total annual emissions was 2.0–2.4%. Our results indicate that in boreal agricultural systems in northeast China, CO(2) and N(2)O emissions continue throughout the winter.