Optimized Regulation Of Water-Salt Transport And Greenhouse Gas Emissions In Salinized Farmland Under Biochar Addition

The Hetao Irrigation District in Inner Mongolia is an important commodity grain and oil production base in the country and the autonomous region.Due to the special geographical environment and natural climatic conditions,the soil salinization problem in this area is serious,resulting in insufficient water supply for crop roots,serious soil compaction,and low soil nutrient content.The problem seriously affects crop yield,income and sustainable development of local agriculture.Therefore,reasonable improvement of saline-alkali soil is the primary task of sustainable agricultural development in the region.In this paper,the spring maize field in Hetao Irrigation District was taken as the research object,and a two-year field positioning experiment was carried out from 2021 to 2022.Five treatments were set up:no fertilizer（CK）,single nitrogen fertilizer（BC0）,nitrogen fertilizer combined with 7.5 t·hm^-2 biochar（BC7.5）,nitrogen fertilizer combined with 15 t·hm^-2 biochar（BC15）and nitrogen fertilizer combined with 22.5 t·hm^-2 biochar（BC22.5）.The effects of biochar addition on soil water and salt transport,soil temperature,soil carbon and nitrogen,crop growth and greenhouse gas emissions were studied.Based on the evaluation results of principal component analysis,TOPSIS method,fuzzy membership method and grey correlation method,the effect of biochar application rate on farmland soil was evaluated by means of fuzzy Borda combination evaluation model,and the recommended biochar application rate suitable for saline soil in Hetao Irrigation District was selected.The main conclusions are as follows:（1）The effect of biochar on soil environment mainly depends on the application amount of biochar.Compared with the control group,the application of biochar could increase the water content of 0-120 cm soil profile by 8.73%～16.06%and the water storage by 8.84%～15.19%,but excessive biochar(22.5 t·hm^-2)would lead to the decrease of soil water content and water storage.Compared with the control group,biochar significantly increased the salt content of 0-120 cm soil profile by 5.5%～19.17%and the salt storage by 8.94%～23.00%.With the increase of biochar application,the accumulation of soil salt increased.The application of biochar could increase the 0-10 cm soil temperature by 0.71%～21.36%,and the two-year test results of soil temperature were as follows:BC22.5>BC15>BC7.5>BC0>CK.（2）The application of biochar could significantly increase the content of nitrate nitrogen in soil profile,but with the increase of biochar application,the content of nitrate nitrogen in0-120 cm soil profile increased first and then decreased.Applying biochar can increase the total nitrogen content of 7.14%～27.22%in 0-40 cm soil layer and increase the proportion of total nitrogen content in 0-20 cm soil layer,effectively reducing nitrogen leaching.Biochar could significantly increase the organic matter content of 0-40 cm soil by 4.36%～22.88%and was positively correlated with the amount of biochar.（3）The application of biochar can significantly increase the plant height,leaf area index and dry matter accumulation of spring maize.Biochar can increase the number of grains per row and 100-grain weight of spring maize,increase the yield by 0.57%～7.77%and the water use efficiency by 0.39%～12.14%while reducing the water consumption of maize.The application of biochar could increase the nitrogen accumulation,nitrogen uptake efficiency and nitrogen use efficiency（NUE）of stems,leaves and ears at maturity stage,but reduce nitrogen use efficiency（NPE）and nitrogen harvest index（HIN）.（4）The application of biochar could significantly reduce the N₂O emission peak,N₂O emission coefficient and cumulative emission by 55.06%～62.78%,and promote soil absorption of CH₄.Biochar could reduce soil CO₂ cumulative emissions by 6.72%-21.41%,but excessive application of biochar(22.5 t·hm^-2)would increase CO₂ cumulative emissions.Moisture and temperature were positively correlated with greenhouse gas emissions,while soil conductivity,nitrate nitrogen content,total nitrogen content and organic matter content were negatively correlated with greenhouse gas emissions.Single application of nitrogen fertilizer can significantly increase the global warming potential and greenhouse gas emission intensity,but nitrogen fertilizer combined with biochar can significantly reduce the global warming potential and greenhouse gas emission intensity.（5）Through principal component analysis,TOPSIS method,fuzzy membership method and grey correlation degree method combined with soil temperature,soil moisture content,soil conductivity,soil nitrate nitrogen storage,soil organic carbon,soil total nitrogen,CO₂cumulative emissions,global warming potential,greenhouse gas emission intensity,yield,water use efficiency and nitrogen use efficiency,the effect of biochar on farmland soil was comprehensively evaluated.The results of principal component analysis and fuzzy membership method were the same,but different from the other two evaluation methods.Therefore,a fuzzy Borda combination evaluation model was established based on the above four evaluation results.The results showed that 15 t·ha^-1 biochar was the recommended amount of biochar for farmland soil in Hetao Irrigation District.