Influence Mechanism Of Freezing-thawing Action And Autumn-tillage Patterns On The Soil Quality Of Cotton Field With Long-term Drip Irrigation In North Xinjiang

Non-irrigation season is long in oasis agriculture area of Xinjiang,Northwest China.In this season,freezing-thawing action（FW）significantly changes soil water-salt transport,which greatly affects soil quality.We focused on the problems of soil salinization and soil quality decreasing in the oasis cotton region of Northern Xinjiang.In this study,we selected typical cotton fields varied from application time of mulched drip irrigation（7～22 a）and a nearby wasteland in Xiayedi irrigation area of Manas River Basin,to investigate the temporal and spatial distribution of soil physical structure,nutrient storage,salt distribution,and microbial characteristics under various scenarios（drip irrigation age,FW,and autumn-tillage pattern）.We aim to reveal the evolution of soil quality（physical,chemical,and biological quality）in cotton field under long-term drip irrigation;and clear the influence mechanism of FW and autumn-tillage patterns（no tillage and deep ploughing;cotton-straw total returning and without returning）on soil quality of cotton field under long-term drip irrigation.The results showed that:1.Clear the change characteristics of soil quality in cotton field under long-term application of mulched drip irrigationThe soil bulk density at 0～100 cm layer decreased year by year with the time extension of mulched drip irrigation（0～14 years）.However,the soil bulk density had an increasing tendency when the application time of mulched drip irrigation was greater than 16～22 years.After reclamation from wasteland into cotton field,the salt migrated to the deep soil gradually.And the characteristic of stratification was observed.The salt storage at 0～200 cm soil layer decreased year by year with the extension of drip irrigation years,and the desalination rate gradually decreased.Long-term mulched drip irrigation increased soil total carbon content,total nitrogen content,and available phosphorus content;and decreased soil salt content,promoting soil agglomeration.Reclamation affected soil microbial species composition,which significantly increased bacterial and fungal diversity.However,after 16～22 years of application of mulched drip irrigation,soil microbial abundance and diversity decreased.After the reclamation from wasteland into cotton field,the soil microbial biomass carbon and the high-quality sequence quantity of bacterial and fungi significantly decreased firstly（<7 years）,and then increased（7～15 years）to a relatively stable level（16～22 years）with the extension of drip irrigation years.On the basis of the redundant analysis,total nitrogen,total nitrogen,carbon-nitrogen ratio,salt content,p H and available phosphorus were important factors affecting the community structure of bacterial and fungi.The comprehensive soil quality index（SQI）was calculated using the methods of the total data set and the minimum data set（CO₂ cumulative emission,catalase activity,acid phosphatase activity,fungal Pielou-e index,bacterial high-quality sequence quantity and geometric mean diameter of water stable aggregates,according to correlation analysis and principal component analysis）.The two evaluation results both showed that the SQI increased year by year with the extension of drip-irrigation years（0～14 years）.However,soil quality decreased in the plots with 16～22 years of application of mulched drip irrigation.2.Clarify the influence mechanism of FW on soil quality of cotton field under long-term mulched drip irrigation during non-irrigation seasonsFT significantly reduced the soil bulk density and solid-phase proportion at 0～100 cm layer.On the contrary,FT significantly increased the total porosity,gas-phase and liquid-phase proportion.Meanwhile,the interaction among FT and reclamation years was significant.FT significantly reduced the mechanical aggregates content of>2 mm and 0.5～1 mm in cotton field under mulched drip irrigation but increased its content of 1～2 mm.FT significantly decreased the mechanical stability of soil aggregates,but the reductions decreased with the increase of soil depth.On the contrary,FT improved the proportion and stability of soil water-stable aggregates,the storage of available phosphorus,and total carbon in 0～40 cm soil layer.But the total nitrogen storage was decreased after thawing.During FT,the soil salt dynamics in 0～100 cm soil layer was divided into three stages,i.e.,salt accumulation in the freezing period,desalination in the early melting period,and salt accumulation in the late melting period.The application time of drip irrigation significantly affected the soil salt flux.After thawing,the 0～200 cm salt storage in the cotton field with 7～12 years of drip irrigation decreased by 7.56 Mg·hm^-2 on average.The average salt storage increased by 10.11 Mg·hm^-2in13～22 years.Temperature gradient and snow-water infiltration are the main driving factors of salt migration,and the salt increment was significantly correlated with the initial salt value,soil bulk density and porosity.FW significantly impacted microbial community structure.Compared with before freezing,soil microbial biomass carbon,high-quality sequences,OTUs,and diversity and richness of fungi decreased significantly after thawing.Total data set and minimum data set（bulk density,bacterial pielou-e index,and bacterial OTUs）were created.The results on the basis of 2 data sets both showed that the alternation of natural freezing and thawing in winter and spring promotes the degradation of soil quality.3.Reveal the regulated mechanism of autumn-tillage patterns on soil quality of cotton field under long-term drip irrigation during non-irrigation seasonsDeep plowing and cotton stalk returning significantly reduced soil bulk density and solid-phase proportion,and increased soil porosity,the proportion of liquid phase and gas phase.Cotton stalk returning and deep tillage significantly increased the proportion of soil>0.25 mm water-stable aggregates and decreased the proportion of 0.05～0.25 mm water-stable aggregates.Deep tillage and cotton stalk returning significantly reduced salt content in aggregates,increased bound organic carbon content in soil aggregates,resulting in the improvement of water stability of aggregates.Cotton stalk returning increased the storage of soil total carbon and available phosphorus in the plough layer.Deep tillage promoted the increase the storage of soil total carbon but reduced the storage of soil total nitrogen and available phosphorus.Cotton stalk returning reduced soil freezing depth and increased soil temperature by 1.00℃on average.On the other hand,deep tillage reduced the average soil temperature by 0.05℃during freezing-thawing process.The cotton stalk returning and deep plowing pattern promoted the infiltration of snow water in the melting period,which increased soil water storage,reduced the surface accumulation of soil salt in the non-irrigation season and salt storage in 0～80 cm soil layer.Cotton stalk returning and deep tillage significantly increased Chao1 index of soil bacteria and fungi,and increased soil microbial richness.Changes in soil quality of cotton field with long-term drip irrigation as affected by autumn-tillage patterns were quantitatively accessed,according to the total data set and minimum data set（soil respiration rate,urease activity,bacterial high-quality sequence quantity,fungal pielou-e index,fungal goods coverage index,and bacterial Chao1 index）.The results on the basis of 2 data sets both showed that deep tillage and cotton stalk returning improved soil quality,which mitigated the declined caused by seasonal freezing and thawing.This study analyzed the spatial and temporal distribution and evolution characteristics of soil-quality indicators in cotton field under long-term mulched drip irrigation in Xinjiang oasis.And soil physical quality,chemical quality,and biological quality of cotton field under long-term mulched drip irrigation as affected by FW and autumn tillage pattern were comprehensively revealed.The results have shown that after the reclamation from wasteland into drip irrigation cotton field（<14 years）,the soil physical structure and nutrient storage increased year by year under the influence of mulched drip irrigation,thereby increasing the abundance and diversity of soil microorganisms and improving the comprehensive quality of soil.However,with the increase of cotton continuous cropping years（16～22 years）,the soil quality showed a downward trend.The freezing and thawing process is beneficial to the improvement of some basic soil properties.But the soil quality after thawing reduced due to the salt accumulation at soil surface and the decline of soil microbial diversity.The autumn tillage pattern of cotton stalk returning and deep plough is conducive to reduce the decline of soil salt accumulation and microbial diversity during freezing and thawing,which improved soil quality.This study could provide important theoretical basis to the sustainable application of mulched drip irrigation in arid area,and it is of great significance to achieve the balance between stable and high yield of cotton and the ecological sustainable development of soil in Xinjiang.