Characteristics Of Nitrogen Transport And Leaching In Winter Irrigation Freeze-Thaw Cotton Field During Fallow Period And Its Cotton Stalk Carbon Prevention And Control Technology

Objective:Nitrogen residue is the main factor of soil water and nitrogen leaching loss.Nitrogen will move to the deep soil with water.Excessive or unreasonable nitrogen application will increase the residual nitrogen in the soil and increase the risk of nitrogen leaching during winter irrigation.In this study,the water distribution,nitrogen distribution and nitrogen leaching amount of soil profile were analyzed and discussed,and the characteristics of soil water and nitrogen transport loss and the resistance effect of cotton stalk carbon application on nitrogen leaching were revealed,which provided a basic basis for reducing soil nitrogen leaching loss and realizing cotton stalk carbon resource utilization.Methods:In this study,through soil column test and winter irrigation freeze-thaw test,three factors were set.The irrigation quota was 4200,5100 and 6000 m³·hm^-2,which were denoted as W1,W2 and W3,respectively,and the application rates of cotton stalk charcoal（biochar accounts for the mass of dry soil in the tillage layer）were set at 0%,1%,2%and 4%,which were designated as C0,C1,C2 and C4.The residual nitrogen（nitrate nitrogen 1,1.3CK,1.5CK,ammonium nitrogen 1,5.1CK,5.9CK）is recorded as N1,N2 and N3.The application rates of nitrogen in winter irrigation freeze-thaw experiment were CK,300 m³·hm^-2 and 450m³·hm^-2.Results:（1）At the same soil depth,the content of nitrate nitrogen is W1>W2>W3;With the increase of soil depth,nitrate nitrogen content increased gradually,ammonium nitrogen increased first and then decreased,and ammonium nitrogen mainly accumulated in the soil layer of 20～40 cm.With the increase of irrigation quota,the leaching amount of nitrate nitrogen and ammonium nitrogen gradually increased,and the leaching amount of ammonium nitrogen was less than nitrate nitrogen.The irrigation-drainage ratio reached 0.27 when the irrigation amount was 4200 m³·hm^-2,Reducing irrigation quota can reduce leakage and nitrogen leaching loss.（2）Nitrogen residue was the main factor of soil nitrogen leaching.The higher the nitrogen residue level,the more soil nitrogen leaching.The leaching amount of N3 under W1 treatment was 65.30%higher than that under N1 treatment.Under the condition of ensuring the optimal irrigation decision,reducing the residual amount of soil nitrogen can inhibit and control the leaching loss of soil nitrogen.（3）It was found that freeze-thaw action would change soil moisture distribution,and soil moisture content showed a downward trend in the stable freezing period,but increased first and then decreased in the melting period.Freeze-thaw action greatly weakened soil leaching resistance to nitrogen,resulting in increased leaching of nitrate and ammonium nitrogen in soil.The leaching amounts of nitrate nitrogen and ammonium nitrogen in W1 treatment were 64.2 and 17.65 kg·hm^-2,respectively.（4）Cotton stalk charcoal promoted water infiltration,and the promotion effect weakened with the increase of the dosage.The initial infiltration rate of C1,C2 and C4 treatments increased by 6.5%,22.8%and 32.0%respectively compared with C0 treatment,and the average infiltration rate increased by 6.9%,16.8%and 21.4%respectively.The irrigation-drainage ratios of treatments W1,W2 and W3 are 0.27,0.38and 0.46,respectively.The soil water holding capacity accounts for 63.6%,53.3%and 46.6%of the total irrigation amount,and the evaporation accounts for 9.1%,8.5%and 7.8%of the total irrigation amount,respectively.（5）The addition of cotton stalk carbon has a certain fixation effect on nitrogen in soil,especially in the surface layer.The greater the proportion of cotton stalk carbon,the better the fixation effect.The addition of4%cotton stalk carbon has a significant effect,and the nitrate content of C4 treatment is increased by 18.52%compared with C0 treatment.The addition of cotton stalk charcoal can increase soil moisture content,improve soil water-holding capacity and reduce the maximum difference of soil temperature during freezing and thawing period in winter,thus reducing soil nitrogen leaching loss.C4 treatment has the best effect of preventing and controlling nitrogen leaching,improving nitrogen retention rate and reducing environmental pollution.Conclusion:In this experiment,the increase of irrigation quota will aggravate nitrogen leaching loss,while the decrease of irrigation quota can reduce water leakage and nitrogen leaching loss.Under the condition of ensuring the best irrigation decision,reasonably reducing nitrogen residue can reduce soil nitrogen leaching loss,and the treatment with 4%cotton stalk carbon has a better effect of preventing and controlling nitrogen leaching.