Accumulation Characteristics And Optimal Regulation Of Water,Salt And Nutrient In Film-Mulched Cotton (Gossypium Hirsutum L.) Field Under Drip Irrigation In Arid Area

The interaction between farmland water and salt and nutrients in arid areas and the environmental problems such as soil salinization and chemical fertilizer residue are the current international research hotspots.The technology of drip irrigation under mulch solves the problem of transforming desert into oasis in arid area of Xinjiang,and realizes the effect of water saving,fertilizer saving,yield increasing and land saving.However,long-term drip irrigation under mulch changed the water cycle process of farmland ecosystem,resulting in the particularity of salt and nutrient distribution in time and space,and environmental problems such as soil salt stress and fertilizer accumulation.In this thesis,the farmland ecosystem under mulched drip irrigation in Manas River Basin of Xinjiang was taken as the research object.The basin scale（Manas River Basin investigation）,rhizosphere scale（site test）and field scale（farmland drainage and salt drainage project）were combined.The spatial variation characteristics,accumulation status and migration mechanism of water and salt and nutrients in the farmland ecosystem under mulched drip irrigation in arid areas were quantitatively revealed by regional variable theory,¹⁵N isotope tracer technology,and the synergistic drip irrigation and leaching project of shaft and pipe drainage.To clarify the nutrient absorption mechanism of cotton under rhizosphere soil salt stress and the interaction effect and synergistic mechanism between soil water and salt and nutrients;to explore the improvement effect and soil salt balance law of saline soil under water conservancy improvement measures.In order to provide theoretical basis and scientific support for the efficient utilization of water and fertilizer under mulched drip irrigation and the management of water resources and environmental safety in arid areas of Xinjiang,the research conclusions of this thesis mainly include the following three aspects:（1）Watershed scale spatial pattern.In 2019-2020,the soil salinity in Manas River Basin showed strong spatial variability and weak spatial autocorrelation,and the random probability affected by soil,topography and day and night warming was small,but it would not cause the change of spatial heterogeneity of soil total nitrogen content.The strong spatial autocorrelation of soil moisture content will cause the spatial variation of soil salinity and total nitrogen content;when considering anisotropy,the spatial variability of soil total nitrogen content was mainly affected by tillage activities,soil parent material and other structural factors.The soil total nitrogen content was mainly accumulated in the plain of oasis irrigation area with 0–20 cm soil layer,for example,the annual increase of soil total nitrogen content in Shihezi irrigation area was 0.016 g kg^-1;the salinization soil area in plain area of oasis irrigation area accounts for 20.6%–25.7%of the total area,about 10426 km²,and the proportion increases with the increase of soil layer.The maximum distribution area and proportion of soil fertility grade 1（extremely high）,grade 2（high）,and grade 3（middle and upper）in Manas River Basin are in piedmont alluvial fan.（2）The coordination of water,salt and nutrients in rhizosphere scale.Under mild(4.2-8.8 g kg^-1)and severe salinization(15.5-20.0 g kg^-1)soil conditions,there was an extremely significant interaction effect between soil nutrients and salt（P<0.01）.The interaction effect between them was less disturbed by the external environment and had a significant coupling effect;When the soil salt content is within the mild salinization(<8.8 g kg^-1),the main distribution direction of nitrogen directly absorbed by cotton from fertilizer is fruit.The distribution proportion of fertilizer nitrogen to cotton fruit in 0-20 cm,20-40 cm,and 40-60 cm soil layers is 10.92%,10.31%,and 6.14%respectively;When the soil salt content exceeds mild salinization(>8.8 g kg^-1),the total cumulative proportion of fertilizer in the soil exceeds 40%.The main distribution direction of nitrogen absorbed by cotton directly from fertilizer is the stem.The nitrogen content further transported to leaves and fruits through cotton stem is limited by soil salt stress.（3）Salinization improvement and irrigation optimization.During the improvement of underground pipe and shaft drainage,the overall decline of groundwater level and soil salt content reached 2.5 m and 29.2 g kg^-1 respectively.The farther the horizontal distance from the shaft（60m）,the greater the decrease of soil salt content;The effect of deep seepage on soil salt balance is mainly reflected in the period of salt leaching,accounting for 52.2%of the total salt output;Under the condition of no leaching,the salt content of 0-200cm soil in the experimental area will reach the salinization level before improvement in 8 years.Therefore,we suggest that under the condition of irrigation amount of 4050 m³ hm^-2 and nitrogen application amount of 525 kg hm^-2,the remaining 10%of cotton irrigation amount should be reasonably allocated,and the soil leaching and drainage measures should be taken every 1-2 years during the fallow period of crops,which can effectively control the harm of soil salinization.