Effects Of Straw Returning Methods On Greenhouse Gas Emissionand Related Enzyme Activities In Soybean Soil Under Climate Change

The world is experiencing climate warming characterized by rising air temperature and elevatedatmospheric carbon dioxide(CO2)concentration.CO2,CH4 and N2 O are the most important greenhouse gases in the atmosphere,and the contribution rate to greenhouse effect is nearly 80%,and they are still rising.Farmland soil is an important emission source of greenhouse gases.Different tillage methods,including different straw returning methods,can affect greenhouse gas emissions from dry farming fields.In this study,we simulated the different environment condition in which the atmospheric CO2 concentration increased by200μmol·mol-1and the temperature increased by 2℃ in the controlled air chamber,and the dynamic changes of soil greenhouse gas emissions,enzyme activities and mineral nitrogen atdifferent growth stage of soybean were studied by setting up straw ploughing and returning(F)and straw mulching and returning(M).This willprovide theoretical basis for making reasonable straw returning methods to reduce greenhouse gas emissions under the environment of future climate change.The main results are as follows:1.Except the environment of elevated atmospheric CO2 concentration,the total amount of seasonal total CO2 emissions from straw mulching and returning to the field was less than that from straw ploughing and returning to the field.Under the environmental conditions of increased temperature and the combination of elevated CO2 concentration and temperature,the seasonal total N2 O emission from strawploughing returning was significantly increased,while the total N2 O emission was decreased under straw mulching and returing.CH4 emission in soybean soil was mostly negative,indicating that the soil absorbed CH4.In the future climate change environment,compared with straw ploughing returning,the absorption of CH4 under straw mulching was greater.2.Under the background of climate change in the future(elevated CO2 concentration,elevated temperature and combined conditions),straw ploughing and returning can increase soybean yield.Under different climatic environment conditions,the soybean yield under straw mulching and reaturing was higher than that under ploughing and straw returning.Compared with straw ploughing and returning,straw mulching and returing can reduce greenhouse gas emission intensity of soybean soil system under different environment conditions.3.The increase of atmospheric CO2 concentration hadlittle effect on the nitrate nitrogen content of soybean in each fertilityperiod,but itincreased the ammonium nitrogen content of soil in each fertility period.Increased temperaturesignificantly improvednitrate nitrogen content,but the effect on ammonium nitrogen contentwas inconsistent.The ammonium nitrogen content was increased at mature stage in 2018.while was decreased atfiling stage in 2018.Under most environmental conditions,the contents of nitrate nitrogen and ammonium nitrogen in soil treated by straw mulching and returing were higher than those treated by ploughing and straw mulching.The change of soil nitrate nitrogen in 2018 wasbasically consistent with N2 O emission.4.The increase of atmospheric CO2 concentration increased the content of urease,protease,sucrase and phosphatase in soybean soil.Increased temperature improved soil urease and phosphatase content,and decreased protease content,but had no significant effect on sucrase content.The content of urease,protease and sucrase in soil was increased with the increase of CO2 and temperature.In most cases,the contents of urease,protease,sucrase and phosphatase in the soil after straw mulching and returingwere lower than those of straw ploughing and reuring.The relationship between these enzyme activities and greenhouse gas emissions needs further verification.5.In 2018,with the increase of atmospheric CO2 concentration,Nitratereductase(NR)under ploughing and returing Nitrite reductase(NIR)at filing stage was significantlyincreased,while NIRwas significantlydecreased at the mature stage.Increased temperaturesignificantly increased NR under ploughing and returing,decreased NIR at mature stage under ploughing and returing,decreased Ammonia monooxygenase(AMO)during the filing stage under ploughing and straw returing,and increasednitrous oxide reductase(NOS)duringfilling stage.The results showed that NR affected N2 O emission by influencingthe process of NO3-reduction to NO2-,while soil NIR,AMOand NOS played little role in N2 O production.To sum up,straw mulching and returing can not only increase the yield,but also reduce the emission of CO2 and N2 O,and increase the absorption of CH4,thus reducing the intensity of greenhouse gas carbon emission.This returning straw mulching allows forthe unity of crop yield and environmental effect,which is a better way of strawreturnedin the future climate change background.