Effects Of Straw Returning And Water And Nitrogen Managements On Soil Carbon And Nitrogen Contents And Yield Formation In Paddy Field

To investigate the effects of different water and nitrogen management methods on soil carbon and nitrogen nutrition and the mechanism of improving yield under straw returning to the field.This study was conducted in the modern research base of Sichuan Agricultural University in Chongzhou City,Sichuan Province from 2021 to 2022.The former crop was wheat,and the indica hybrid Yixiangyou 2115 was used as the material.A two-factor spilt-plot trail was conducted.Two straw returning modes including non-returning to the field（S0）and straw overturning to the field（S1）were set as the main plot.No nitrogen（M0）,optimized water-nitrogen coupling mode（M1）,and water-nitrogen integration mode（M2）were used as subplot.The differences of water nitrogen yield,dry matter accumulation,soil organic carbon and nitrogen content,soil enzyme activity and the relative expression of ammoxide and bacteria in different returning modes and water-nitrogen management modes were studied.The relationship between soil organic carbon and nitrogen content and soil enzyme activity at heading stage was further discussed by redundancy analysis（RDA）,based on the correlation of soil organic carbon and nitrogen contents as dependent variables and soil enzyme activities as independent variables,the causes of soil organic carbon change were analyzed The main research results are as follows:1.Effects of straw returning and water-nitrogen managements coupling on soil carbon and nitrogen contentStraw returning increased soil total nitrogen and organic carbon content,and increased the contents of soil soluble organic carbon and readily oxidized organic carbon,and also improved the soil carbon pool management index.Soil organic carbon content in jointing stage was higher under water-nitrogen integration mode,while total organic carbon,soluble organic carbon and easily oxidized organic carbon were higher under optimized water-nitrogen coupling mode in each reproductive period(except for total organic carbon at heading stage in 2021 and maturity stage in 2022.The effects of optimized water-nitrogen coupling mode on soil nitrogen content were as follows:optimized water-nitrogen coupling mode>water-nitrogen integration mode>no nitrogen application.2.Effects of straw returning and water-nitrogen coupling on soil enzyme activityStraw returning increased the activities of invertase and urease（4.51%-29.25%and7.81%-25.09%）in soil at jointing and maturity,and increased the activities of nitrate reductase and nitrite reductase in soil at all stages.Among the water and nitrogen management modes,optimized water-nitrogen coupling mode of soil invertase with water and nitrogen was greater at heading stage,and the water-nitrogen integration mode was greater at maturity stage.The effects on soil urease,nitrate reductase and nitrite reductase were consistent,and the optimized water-nitrogen coupling mode was greater,followed by the water-nitrogen integration mode,and no nitrogen application was the least.3.Effects of straw returning to field and water-nitrogen managements coupling on ammonia oxidation archaea and bacteriaThe gene abundance of AOA and AOB in soil was increased（0.05～0.47 and0.10～0.12）,and the relative expression levels of AOA and AOB were significantly higher under optimized water-nitrogen coupling mode than under no nitrogen application and under water-nitrogen integration mode.4、Effects of straw returning and water-nitrogen coupling on rice yield and its component factorsCompared with not returning the straw to the field,the effective panicle number in2022 and the 1000-grain weight and seed setting rate in the two years were decreased,and the yield increased and decreased simultaneously（1.95%and-3.09%）.The effective panicle number,grain number per panicle and total spikelet number under optimized water-nitrogen coupling mode were larger among different water and nitrogen management modes（except the effective panicle number in 2022）.The 1000-grain weight and seed setting rate of water-nitrogen integration mode were greater,and the yield showed the optimized water-nitrogen coupling mode>water-nitrogen integration mode.Therefore,the optimized water-nitrogen coupling mode combined with straw overturning to the field was better in 2021(9378.06 kg hm^-2and 9526.96 kg hm^-2).5.Redundancy analysis of soil carbon and nitrogen content and soil enzyme activityRedundancy analysis showed that in 2021,S1M1 and S1M2 were significantly different from other treatments in the plot,indicating that the effects of water and nitrogen management on soil carbon and nitrogen content and soil enzyme activities were significantly different,TOC,TN,NH₄⁺-N and NO₃^--N were positively correlated with the activity of soil enzymes according to the ray length and angle between the content of soil carbon and nitrogen and the activity of soil enzymes.In 2022,S1M0,S1M1,S1M2 and S0M0,S0M1,S0M2 were separated from each other,which indicated that straw mulching and replanting had significant effects on soil carbon and nitrogen content and urease activity,TOC,TN,DOC,NO₃^--N,NH₄⁺-N and ROC were susceptible to INV,NIR,URE,NR and AOA.AOA was negatively correlated with NH₄⁺-N and ROC,soil enzyme activities were increased by increasing TOC,TN,DOC,NO₃^--N,NH₄⁺-N and ROC under straw returning and water and nitrogen management.In conclusion,soil organic carbon and total nitrogen were increased by straw returning and two water-nitrogen managements,then the soil C/N ratio changed,and the substrate of enzymatic reaction increased,soil urease,sucrase,nitrate reductase and nitrite reductase activity were promoted,and gene copy number and microbial activity of ammonia-oxidizing bacteria and archaea were increased,the enzymatic reaction increased the contents of soil active organic carbon and inorganic nitrogen,and the available carbon and nitrogen in soil,which were beneficial to crop absorption,crop dry matter accumulation,transportation and yield formation.The optimized water-nitrogen coupling mode is due to the high amount of nitrogen applied,soil carbon content,soil nitrogen content,soil enzyme activity,relative gene expression,dry matter accumulation and yield were better than water-nitrogen integration mode.Therefore,optimized water-nitrogen coupling mode under straw overturning to field is the best treatment in this study.