Study On Coupling Effect Of Water And Nitrogen And Efficient Utilization Mode Of Cotton Under Mulched Drip Irrigation In Salinized Fields In South Of Xinjiang

Irrigation and fertilization are the two important measures to ensure the agricultural development in arid regions.Xinjiang has become the largest cotton-producing area in China due to its unique climate suitable for cotton growth.The scarce rainfall,intense evaporation and unreasonable irrigation and fertilization strategy lead to low water and fertilizer utilization efficiency and salinization in the oasis irrigation area of southern Xinjiang,which has seriously restricted the development of the local cotton industry.Therefore,it is of great significant to study the coupling effect of water and nitrogen and efficient utilization mode of drip irrigation under film in salinized cotton in south of Xinjiang.This paper systematically studied the regulation mechanism of different water and nitrogen supply on cotton growth,the transport of water fertilizer and salt,water and nitrogen use efficiency,yield and quality through the integrated test of water and fertilizer for cotton drip irrigation.This study was conducted in southern Xinjiang from 2018 to 2019,and set four irrigation levels(W_0.6:60%ET_c,W_0.8:80%ET_c,W_1.0:100%ET_c and W_1.2:120%ET_c,where ET_c is the crop evapotranspiration)and four nitrogen rates(N₂₅₀-250 kg N/ha,N₃₀₀-300 kg N/ha,N₃₅₀-350 kg N/ha and N₄₀₀-400 kg N/ha),meanwhile,a control treatment（N₀-0 kg N/ha）was set under different irrigation levels.By studying the effects of different water and nitrogen supplies on the migration and distribution of water,fertilizer and salt in farmland,cotton growth,yield,water and nitrogen utilization efficiency,and fiber quality,effective irrigation and fertilization management strategies and scientific theoretical guidance were put forward for water saving,fertilizer saving and salt controlling,high efficiency and high quality production of saline-alkali cotton field.The main results were shown as below:1.Revealed the variation law of water-heat-salt changes in drip-irrigation cotton field under different irrigation levels;the semi-variogram model was used to analyze the effects of various irrigation amounts on soil water and salt distribution,variability,salt accumulation and desalination in mulched cotton field;and the salt tolerance threshold of cotton was determined by analyzing the relationship between irrigation amount,total dissolved solids and seed cotton yield.Soil temperature first increased and then decreased over time due to the combined effects of air temperature and cotton growth.Soil temperature under W_0.6 was significant higher than that under the other three irrigation amounts.The soil water storage first decreased and then increased over time,and reached the lowest at flowering and boll setting stage during the two growing seasons,and reached the minimum value at flowering and boll setting stage.Irrigation amount had no significant effect on the characteristics of soil moisture.The increase of irrigation amount reduced soil salt in soil profile,while increased the depth of salt accumulation below the soil surface.Soil salt content in soil profile did not change significantly after irrigation compared to that before irrigation,but the distribution of soil salt differed to that before irrigation.The semi-variogram model was used to find that the variability of soil salinity decreased with the increase of irrigation amount and soil depth.Meanwhile,the salt tolerance threshold of cotton（32 Mg/ha of the accumulation of TDS at 0～60 cm soil depth）and the optimal irrigation amount(W_1.0)were obtained,based on the relationships between irrigation amount,soil salt and seed cotton yield.2.Revealed the coupling effect of water and nitrogen on cotton growth,yield,water use efficiency and economic benefits.Increasing irrigation and nitrogen applied significantly promoted the growth and production of cotton.Seed cotton yield increased with the increase of irrigation amount,but the difference in seed cotton yield under W_1.0 and W_1.2 in 2018 and 2019 was only about0.2%.Increasing nitrogen applied had a greater promoting impact on seed cotton yield,especially under water stress conditions(W_0.6 and W_0.8).Seed cotton yield increased by16.28%,22.19%and 13.35%,13.67%under W_0.6 and W_0.8 in two years,which were significantly higher than that under W_1.0 and W_1.2.Increasing irrigation amount reduced water use efficiency and irrigation water use efficiency,which increased with the increase of nitrogen rate.Economic benefit first increased and then decreased with the application of water and nitrogen,and the highest economic benefit was obtained under W_1.0N₃₅₀.3.Clarified the response of cotton nitrogen uptake,nitrogen nutrition status and soil nitrogen residue to different water and nitrogen supplies,and the improved critical nitrogen dilution curve model and nitrogen nutrition index were used to effectively diagnose cotton nitrogen nutrition status.Increasing nitrogen application enhanced the nitrogen concentration and accumulation in cotton.Increasing irrigation amount decreased the nitrogen concentration,but increased the nitrogen accumulation by increasing biomass.The highest total plant nitrogen accumulation was obtained under W_1.2N₄₀₀,which was 223.88 kg/ha in 2018 and 242.77kg/ha in 2019.There was a significant effect of water and nitrogen supplies on nitrogen use efficiency（a NUE,NRE,p NUE,PFPN）.The improved critical nitrogen dilution model was used to diagnosis the nitrogen nutrition status of cotton,combined the results of nitrogen nutrition index under different treatments,the optimal nitrogen nutrition status was found under W_1.0N₃₀₀,followed by W_1.0N₃₅₀.₃;-N content in soil layer increased with the increase of nitrogen rate.Increasing irrigation amount reduced₃;-N content in the surface soil,while improved that in the deeper soil.4.Explored the coupling effect of water and nitrogen on cotton fiber quality.There was a positive correlation between irrigation amount and fiber length,fiber strength,fiber quality index（FQI）and textile parameter,but a negative correlation with micronaire and short-fiber index.Fiber length,fiber strength,FQI and textile parameter first increased and then decreased with increasing nitrogen rate.Irrigation amount and nitrogen rate had no significant effect on maturity index.There was significant correlation among fiber quality indexes.5.Comprehensive evaluated the coupling effect of water and nitrogen on seed cotton yield,water and nitrogen use efficiency,economic benefit and fiber quality indexes,combined with multivariate nonlinear regression and spatial analysis to achieve multi-objective optimization for efficient and high-quality cotton production under mulched drip irrigation.A variety of evaluation methods were used to evaluate seed cotton yield,water and nitrogen use efficiency,economic benefits and fiber quality,and the evaluated results showed that ranking results were different,but there was a strong correlation between the evaluation results.The overall difference combination model was used to comprehensively evaluate the evaluation values obtained by each evaluation method to obtain the optimal treatment.Meanwhile,we proposed three irrigation and nitrogen application intervals based on multivariate nonlinear regression and spatial analysis to meet different production requirements:The irrigation and nitrogen application mode for cotton water-saving and high-efficiency production,the corresponding irrigation and nitrogen application intervals was irrigation amount of 275 mm～304 mm and nitrogen rate of 290 kg/ha～440 kg/ha;The irrigation and nitrogen application mode for cotton high-production and high-quality production,the corresponding irrigation and nitrogen application intervals was irrigation amount of 356 mm～450 mm and nitrogen rate of 335 kg/ha～395 kg/ha;and the comprehensive model of water and nitrogen management for high-production,water-saving,quality-improvement and efficiency-improvement of cotton,the corresponding irrigation and nitrogen application intervals was irrigation amount of 344 mm～380 mm and nitrogen application rate of 335 kg/ha～395 kg/ha.6.Optimized cotton crop coefficient in southern Xinjiang using soil water balance method.The water consumption at flowering and boll setting stages accounted approximately for 60%of the seasonal water use of cotton under various irrigation amounts,meanwhile,the average daily water consumption at flowering and boll setting stages was significantly higher that that at other gorwing stages.Therefore,flowering and boll setting can be considered as the pivotal water requirement period of cotton.The average locally developed K_c(K_c-local)values over two seasons were 0.38,1.30 and 1.09 for initial,mid-season,and end-season stages,respectively.A second-order polynomial relationship between cotton K_c and t/T（the days after emergence/the days of total cotton growth period）was established,which can provide a reference for the determination of local cotton K_c and irrigation management.