Effects Of Nitrogen Reduction Combined With Organic Fertilizer Application On Soil Nitrogen Availability And Cotton Growth

【Objective】Based on the current production background of“continuous cropping+total cotton stalk returning+high nitrogen fertilizer input”in Xinjiang oasis,the effects of nitrogen fertilizer reduction and organic fertilizer substitution on soil nitrogen transformation,occurrence characteristics,nitrogen availability and cotton growth were explored,in order to improve the nitrogen use efficiency of cotton field,reduce the environmental risk of nitrogen and promote the quality and efficiency of oasis cotton field ecosystem,and provide scientific basis for enriching the fertilization theory of soil nitrogen effective supply and the application of chemical fertilizer reduction measures in Xinjiang oasis cotton field.【Method】The experiment was carried out in 2021 and 2022.Eight treatments were set up,including no fertilizer（CK）,conventional nitrogen application（CF）,reduced nitrogen application（CF-8%,CF-16%,CF-24%）with 8%gradient,constant total nitrogen reduction and equal amount of organic fertilizer（8%OF,16%OF,24%OF）.The effects of different treatments on soil physical and chemical properties,inorganic nitrogen content,active organic nitrogen components,nitrogen invertase activity and nitrogen conversion rate,agronomic traits,yield structure and nitrogen use efficiency of cotton were studied.【Results and Conclusions】1.Nitrogen reduction combined with organic fertilizer can improve the soil microenvironment of cotton field and reduce the risk of NO₃^--N leaching in deep soil.The soil p H increased and the electrical conductivity（EC value）decreased under organic fertilizer replacement treatment.The organic matter content in 0-20 cm increased by 23.45%and 31.75%on average compared with CF under 16%and 24%organic fertilizer treatments in two years,and the accumulation of NO₃^--N in deep soil decreased significantly.Based on the results of partial correlation analysis,it was found that the changes of soil p H,EC value and organic matter（SOM）had significant effects on the contents of total nitrogen（TN）,NH₄⁺-N and NO₃^--N.2.The increase of available carbon and nitrogen sources SOM,NH₄⁺-N and NO₃^--N content and nitrogen invertase activity in soil increased the content and distribution proportion of soil active organic nitrogen components,and the improvement of soil physical and chemical properties p H,EC value,SOM and the increase of nitrogen invertase activity further promoted the conversion process of active organic nitrogen to soil NH₄⁺-N and NO₃^--N,and the soil nitrogen supply potential increased.With the increase of the proportion of organic fertilizer application,TN,particulate organic nitrogen（PON）,soluble organic nitrogen（DON）,microbial biomass nitrogen（MBN）content,PON/TN,DON/TN distribution ratio increased first and then decreased,with 16%OF treatment The best performance,light group organic nitrogen（LFON）,LFON/TN,MBN/TN gradually increased,high amount of organic fertilizer application is more conducive to the storage of soil nitrogen in the form of LFON and MBN in the soil.3.Nitrogen reduction combined with organic fertilizer can increase soil nitrogen invertase activity and net nitrogen mineralization rate,reduce soil net nitrification rate in short-term culture,improve soil nitrogen availability and reduce nitrogen loss risk.Compared with CF,the net nitrogen mineralization rate of 16%OF treatment increased by 14.86%and 27.83%in cotton bud stage and flower and boll stage,and the net nitrification rate in short-term culture decreased with the increase of organic fertilizer application ratio.Based on the results of partial correlation analysis,it was found that the substitution of organic fertilizer mainly promoted the improvement of cotton nitrogen use efficiency and seed cotton yield by reducing the net nitrification rate of soil and reducing the content of NO₃^--N and TN in deep soil layer（20-60 cm）.4.Under the current fertilization level of cotton field,the growth and yield of cotton were not significantly inhibited by reducing nitrogen fertilizer by 0%～24%.Compared with conventional fertilization treatment,nitrogen reduction combined with organic fertilizer can improve cotton yield and nitrogen use efficiency.There was no significant effect on plant height,number of fruit branches,spatial distribution of cotton bolls,number of bolls per plant and single boll weight by reducing nitrogen fertilizer by 0%～24%.Nitrogen reduction combined with organic fertilizer can promote cotton growth,increase photosynthetic rate,maintain a higher proportion of photosynthetic sources during the boll opening period,and optimize the spatial distribution of cotton bolls.Cotton yield and nitrogen use efficiency increased first and then decreased with the increase of organic fertilizer substitution ratio.The effect of 16%OF treatment was the best.The number of bolls per plant,single boll weight and seed cotton yield increased by 20.03%,5.10%and 17.16%on average compared with CF for two years.Nitrogen agronomic efficiency（NAE）,nitrogen partial factor productivity（NPFP）,nitrogen contribution rate（NCR）increased by 71.66%,17.15%and 68.04%compared with chemical fertilizer treatment.Through the regression equation model analysis,nitrogen reduction of 9.9%～14.1%and organic fertilizer replacement of 16.6%～17.6%of chemical fertilizer,cotton seed cotton yield and nitrogen use efficiency at the same time to achieve the best results.In summary,under the“continuous cropping+cotton stalk returning+high nitrogen fertilizer input”farming system of Xinjiang oasis cotton,the reduction of inorganic nitrogen by 9.9%～14.1%can achieve the effect of reducing nitrogen and stabilizing yield;nitrogen reduction combined with organic fertilizer promoted cotton growth by improving soil microenvironment,increasing soil nitrogen metabolism enzyme activity and soil nitrogen conversion rate,and enhancing soil nitrogen availability in cotton fields.Among them,16%OF treatment was the best substitution ratio.