Long-Term Effects Of Different Straw Return Methods On Soil Carbon,Nitrogen And Phosphorus Turnover In Rice-Rice-Oilseed Rape Rotarion

Rice-rice-oilseed rape rotation（i.e.,early rice,late rice and oilseed rape）is a typical three cropping system in the middle reaches of the Yangtze River in China.Developing rice-rice-oilseed rape rotation is beneficial for fully utilizing winter fallow fields and ensuring national grain and oil security.The rice-rice-oilseed rape rotation system has a high degree of intensification and a large amount of straw production.The impact of long-term different straw returning methods on soil nutrient pool capacity and nutrient availability under frequent dry-wet alternation conditions is still unclear.This study chose four treatment from a 15 years’located experiment under rice-rice-oilseed rape rotation,i.e.,conventional tillage（CT）,conventional tillage with straw returning（CTS）,no tillage（NT）,and no tillage with straw returning（NTS）,aiming to explore（i）the annual temporal variation characteristics of dissolved organic carbon（DOC）,inorganic nitrogen,available phosphorus（AP）,microbial biomass carbon（MBC）and nitrogen（MBN）,and extracellular enzyme activity related to C,N and P in rhizosphere and non-rhizosphere soils;（ii）long-term effects of different straw returning methods on C,N and P pools in the 0-20 cm and 20-40 cm soil levels;and（iii）quantifying the relationship between soil C,N and P turnover and crop yield based on soil quality index（SQI）and ecosystem multifunctionality（EMF）,providing a theoretical basis for the long-term straw returning to enhance crop productivity under the rice-rice-oilseed rape rotation.The main research findings were as follows:（1）In rice-rice-oilseed rape rotation,soil DOC,ammonium nitrogen（NH₄⁺-N）,nitrate nitrogen（NO₃^--N）,AP,MBC and MBN contents and acquired enzyme activity were significantly affected by crop growth period and straw returning.Compared to the treatments without straw returning（CT and NT）,long-term straw returning（CTS and NTS）significantly increased DOC,NH₄⁺-N,NO₃^--N,MBC and MBN contents in rhizosphere and non-rhizosphere soils by 2.4%-50.4%,2.8%-44.5%,3.4%-76.0%,5.3%-61.2%and 2.1%-142.4%,respectively.The promotion of soil AP by long-term straw returning was mainly observed in the rice season.Soil DOC,NO₃^--N,MBC and MBN contents in rhizosphere soils of different treatments were significantly higher as compared with non-rhizosphere soils,while the NH₄⁺-N contents were significantly lower by 4.6%-58.6%in rhizosphere.In terms of annual chronological change characteristics,soil MBC and MBN contents showed an increasing trend and NH₄⁺-N showed a decreasing trend from dry season to water season.Soil N acquisition enzyme activity(N_-acq)showed an increasing trend from oilseed rape season to rice season,while P acquisition enzyme activity(P_-acq)showed a decreasing trend with the reproductive process in both oilseed rape and rice seasons,and the rate of decrease was faster in the oilseed rape season.The carbon acquisition enzyme activity(C_-acq)of rhizosphere soil was significantly higher by 36.2%-146.7%and the N_-acq was significantly lower by 3.5%-50.2%than that of non-rhizosphere soil.The C_-acq and N_-acq with long-term straw returning were significantly increased by 5.2%-42.7%and4.1%-27.9%,respectively.Correlation analysis showed that the C_-acq,N_-acq and P_-acq of the non-rhizosphere soil were mainly positively correlated with NH₄⁺-N and NO₃^--N in oilseed rape season,while the C_-acq,N_-acq and P_-acq the soil acquisition enzyme activities during rice season were influenced by many factors.（2）Long-term different straw returning methods significantly increased soil organic carbon（SOC）,total nitrogen（TN）,DOC,MBC and MBN contents by 14.1%-22.9%,8.4%-20.6%,1.2%-55.5%,12.6%-30.8%and 14.7%-21.7%in the 0-20 cm soil layer,respectively.There was no significant difference in soil AP content among the treatments.Long-term straw returning significantly increased soil C,N and P acquisition enzyme activities in 0-20 cm soil.The C_-acq,N_-acq and P_-acq were increased by 3.4-33.6%,22.0%-42.2%and 5.6%-14.6%under CTS and NTS treatments as compared with the CT and NT treatments,respectively.The effect of different tillage on SOC and TN were mainly in the 20-40 cm layer.Take the oilseed rape season for example,the SOC and TN contents in the 20-40 cm soil layer were increased by 13.4%-20.9%and 9.6%-25.2%in the conventional tillage treatments（CT and CTS）as compared with no-tillage treatments（NT and NTS）,respectively.（3）Crop yield of early rice,late rice and oilseed rape were all increased by long-term straw returning,especially for the late rice and oilseed rape.Compared with the period of 2007-2011,different straw returning methods（CTS and NTS）increased yield of late rice by 15.8%-16.7%in the 2012-2016 and 24.1%-25.6%in the 2017-2021,while oilseed rape increased yield by 3.2%-6.2%and 37.5%-40.1%.Long-term straw returning significantly the SQI by 22.2%-40.7%and 27.8%-35.3%in the oilseed rape season and the rice season,respectively.And the EMF was enhanced by 104.2%-141.1%during the oilseed rape season and 10.3%-84.1%during the rice season in the treatment with long-term straw returning.In the 20-40 cm soil layer,long-term tillage increased SQI by 24.8%-35.7%as compared with no-tillage conditions during the oilseed rape season.Based on structural equation modeling（SEM）analysis,we found that long-term straw returning had significant positive effects on SQI and EMF for both oilseed rape and rice seasons,while different tillage practices did not have significant effects on EMF during the rice season.Long-term straw returning methods promoted oilseed rape yield increment by enhancing the SQI value,while long-term straw returning in rice season had direct effects on both SQI and rice yield.In summary,the long-term different straw returning methods of the rice-rice-oilseed rape rotation system increased the supply of rhizosphere soil dissolved organic carbon and mineral nitrogen by stimulating soil carbon,nitrogen and phosphorus related extracellular enzyme activities during the crop reproductive period,promoting soil carbon,nitrogen and phosphorus turnover in topsoil and improving soil quality index and ecosystem multifunctionality,thus ensures the continuous increase in yield of early rice,late rice and oilseed rape.