Effects Of Straw Returning Methods On Soil Water And Heat Conditions And N₂O Emissions In Soybean Fields In Cold Region

Heilongjiang Province is a typical area in the cold region of Northeast Asia.It is the main producing area of maize and soybean in our country.Corn-soybean rotation is one of the main planting methods.In 2020,the yield of maize and soybean were 36.466 million tons and 9.320million tons,respectively.According to the seed-straw ratio is 1:1.2,the straw yield were 43.759million tons and 11.184 million tons,respectively.Due to the lagging development of straw returning technology,large-scale open burning of straw pollutes the environment.Straw returning to the field is an effective method to solve the surplus of straw resources at present.Straw returning to the field improves the topsoil structure and water and heat conditions,affects soil greenhouse gas emissions,and provides suitable conditions and nutrients for crop growth.Different tillage methods such as deep tillage,deep loosening,deep scarification and till with shallow depth,rotary tillage and no tillage have different effects on soil disturbance and soil structure.The comprehensive utilization and innovation technology of straw returning combined with tillage measures can not only promote crop yield,but also achieve the goal of improving soil quality of cultivated land with high efficiency.Farmland N₂O emissions have a great potential for global warming,and with the continuous increase of N₂O emissions,it poses a serious threat to human life and production in the future.Therefore,the effects of straw returning methods in cold regions on soil water,heat,hardness and N₂O emissions in soybean fields were studied to provide theoretical basis and technical reference for expanding the area of crop straw returning,promoting greenhouse gas emission reduction,fertilizing soil,promoting high crop yield,achieving sustainable and green development of agricultural.This experiment was carried out in Xiangyang Experimental Practice Base of Northeast Agricultural University in 2021.Based on the continuous positioning experiment started in 2017,the maize-soybean rotation mode was adopted for the study.With the traditional tillage（TT）as the control,four treatments were set under the condition of full straw returning,namely straw mulching and no-tillage（NT）,straw mulching and no-tillage sowing+seedling ridge furrow subsoiling（NTS）,in seedling stage,ridge deep loosening stubble machine+seedling ditch deep loosening（SCS）,digging in seedling stage+corrugation depth（DS）.The effects of different tillage methods on soil temperature,water content and hardness of soybean field under the condition of previous maize straw returning were studied.The effects of straw returning on soil N₂O emission,soybean growth and yield were analyzed.The experimental results show that:（1）Straw returning methods affects soil temperature.In May and June,the soil daily average temperature showed DS>TT>SCS>NTS>NT.The soil daily average temperature of NT treatment decreased significantly.In July,the soil daily average temperature of SCS treatment was significantly higher than that of other treatments.In September,NT treatment increased the soil daily average temperature.DS treatment increased soil accumulated temperature,NTS and NT treatment significantly reduced soil accumulated temperature,medium tillage and soil cultivation made SCS treatment accumulated temperature higher than TT treatment;from May to mid-July,NT treatment decreased the daily maximum soil temperature,DS treatment increased the daily maximum soil temperature,and SCS treatment increased the daily maximum soil temperature from mid-July to August.NT and NTS treatments reduced soil daily minimum temperature;the first occurrence periods of daily maximum and minimum temperatures of soil were concentrated in12:00 to 14:24 and 2:24 to 4:48,respectively.（2）Straw returning methods affects soil moisture.Soil moisture changed significantly with rainfall.NT treatment increased soil moisture before sowing and May,DS treatment increased soil moisture in June and July,NTS treatment was at a low level throughout the growth period,and SCS treatment was higher than DS treatment in August and September.NT treatment increased the daily maximum soil moisture before sowing and June;DS treatment significantly increased soil daily minimum moisture;the first occurrence period of the highest and lowest soil moisture was concentrated from 19:12 to 24:00 in each day.（3）Straw returning methods affects soil hardness.The soil hardness of each treatment varied greatly in 0～15 cm soil layer.The soil hardness of NT and NTS treatments was the highest in 0～15cm soil layer,and the lowest in 30～40 cm soil layer.The soil hardness of SCS treatment was the lowest in 0～30 cm soil layer.The ridge soil hardness of each treatment varied greatly in 0-20 cm soil layer.The ridge soil hardness of TT treatment was the lowest in 0～20 cm soil layer and the highest in 30～40 cm soil layer.The ridge-furrow soil hardness of each treatment varied greatly in0～20 cm soil layer,and the ridge-furrow soil hardness of NT treatment was the largest in 0～20 cm soil layer.（4）Straw returning affects methods the growth and yield of soybean.TT treatment promoted soybean emergence,NTS treatment delayed soybean emergence,NT treatment reduced plant height at flowering and podding stages,DS treatment increased soybean node number,pod number,grain number and grain weight,TT and NT treatment reduced soybean node number,pod number,grain number and grain weight,but the difference was not significant.There was no significant difference in soybean yield among treatments.（5）Straw returning methods affects soil N₂O emission and available nitrogen content.The soil N₂O emission of ridge and furrow in soybean field showed a single peak curve change,and reached the peak in mid-July.The N₂O emission of ridge and furrow was positively correlated with soil temperature and moisture,and negatively correlated with soil hardness.The total soil N₂O emission was DS>NT>NTS>TT>SCS.DS treatment increased the total N₂O emission during soybean growth period,reaching 1892.44 g·hm^-2.SCS treatment had the best effect on N₂O emission reduction.DS treatment increased the content of ammonium nitrogen and nitrate nitrogen in 0～15 cm soil layer,and other treatments had no consistent effect on the content of available nitrogen in 0～15 cm and 15～30 cm soil layer.