Nitrous oxide emissions from black soils under a continuous soybean cropping system in northeast China

A large number of natural wetlands in northeast China have been reclaimed as farmland in the last few decades, and soybean is the main rain-fed crop here. For the depth understanding of nitrous oxide (N2O) emission from reclaimed soybean fields, using static opaque chamber method, we conducted a four-year N2O flux measurement at two adjacent soybean fields cultivated after wetland drainage in 1987 and 1993, respectively, in the Sanjiang Plain of northeast China Using static opaque chamber method,. Both sites had two treatments including soybean cropped and bare soils (i.e., SF87, BS87, SF93 and BS93). The results showed that soil N2O emission from all of the plots was severely inhibited by the low temperature in winter (November to March), while a N2O emission pulse occurred during the spring thaw (April and May). Temporal variation of the N2O fluxes during the growing season varied over all the four years but was mainly affected by soil water-filled pore space (WFPS). Intense rainfall events increased the intensity and duration of N2O pulses during the growing season, and most high fluxes were occurred at WFPS > 45%. The mean annual N2O emission from all treatments over four years was 4.8 +/- 1.2 kg N ha(-1) (ranges: 1.9-19.8), and one third of the emission originated from the spring-thaw. In addition, soybean growth did not increase N2O emissions during the growing season, which support the cancellation of N2O emission calculations from nitrogen fixed by legumes in the 2006 IPCC Guidelines for National Greenhouse Gas Inventories.