The Exchanges Of Greenhouse Gases And Effect Factors In Winter Wheat/Green Onion Interplanting System

Agriculture as either sink or source of greenhouse gases nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) plays a crucial role in global warming. Therefore, quantifying greenhouse gas (GHG) emissions and searching for appropriate mitigation measures have become a relatively hot issue in international global climate change studies. Taking winter wheat/green onion interplanting field as an example, the fluxes of N2O,CH4and CO2from five treatments of control (CK), farmer's practice(FP), optimized fertilization (OPT), optimized fertilization+reducing fertilizer(OPT-N), and straw returned (CN) were measured year-round using static opaque chamber/gas chromatography (GC) method. The factors such as fertilization, soil moisture and temperature etc. were also discussed. At last, the net exchanges of greenhouse gases and the mitigation potential of different strategies were evaluated. The main results were summarized as follows.N2O emissions from winter wheat/green onion interplanting field changed seasonally with many peaks. In green onion growing season, N2O emission peaks followed each time of fertilizer application and irrigation. And the highest N2O emission flux was observed in FP treatment with507.11μg N m-2hh-1. There were two lower emission peaks in winter wheat growing season occurring after sowing in autumn and topdressing in spring, respectively. Annual cumulative N2O emissions in the five treatments ranged from1.71to4.6kg N ha-1. The OPT, OPT-N and CN treatments could significantly reduce N2O emissions, comparing with FP treatment. The CN treatment was regarded as the best management practice because it not only had a significantly effect on reducing N2O emissions, but also maintaining crop yield. The annual emission factor (EF) of N fertilizer ranged from0.31%to0.48%, with an average of0.43%. The N2O background emission was1.71kg N ha-1, with0.63kg N ha-1from winter wheat growing season and1.08kg N ha-1from green onion growing season, respectively. Multiple regression analysis revealed that soil temperature and soil moisture were also the factors affecting N2O emissions besides nitrogen fertilizer. When soil temperature changed in the range of10-20℃, N2O emissions increased greatly with temperature rise, but soil temperature changed in the range of20-30℃, soil moisture became a limiting factor and there was a significant positive correlation between N2O emissions and soil moisture.Winter wheat/green onion interplanting field was a net atmosphere CH4sink. The highest absorption rate mainly occurred in winter wheat growing season. The average CH4absorption rate was15.35±1.03μg C m-2h-1and cumulative absorption ranged from1.12to1.49kg C ha-1yr-1. Nitrogen fertilizer application could reduce the ability of the soil oxidation of CH4. But the differences among the treatments were not significant. The cumulative absorption rate of CH4in OPT-N and CN treatments reduced13%and22%, comparing with OPT treatment. The oxidation of CH4was mainly controlled by soil temperature annually.CO2emissions from winter wheat/green onion interplanting ecosystem showed significant seasonal variation in winter wheat growing season. The highest CO2emission flux occurred in mid-May with 406.84±10.42mg C m-2h-1。 CO2emissions from green onion growing season did not show significant regularity due to frequent farmland management practices. The annual CO2emissions ranged from11.84to13.46t C ha-1, but there were no differences among the treatments. The CO2emissions during non-growing season (i.e., net CO2exchange) accounted for5.3%-7.1%of annual CO2emissions. Soil temperature was the main factor affecting the ecosystem respiration while the soil moisture had no obvious effect. There was a very significant correlation between the CO2emission fluxes from winter wheat season and crop biomass. With the continuous increase in biomass of green onion, the flux of CO2emissions and biomass did not show significant correlation. According to DNDC simulated results, the contribution of wheat root respiration to soil respiration was97%, while the contribution of green onion root respiration was76%, respectively.Annual net exchanges of greenhouse gases depend on the net exchanges of N2O CH4and CO2between the ecosystem and atmosphere. In this study, we calculated the net ecosystem carbon balance (NECB) which can act as a carbon source or sink for a terrestrial ecosystem. The net exchanges of greenhouse gases in CK treatment was mainly determined by the NECB. CK treatment was a carbon dioxide source. The CN treatment could uptake1.9t CO2-eq ha-1annually, which was an appropriate mitigation measure among the treatments.