Effects Of Nitrogen Application Rates On Soil Nitrogen Transformation And Cotton Plant Nitrogen Uptake And Utilization In Cotton Fields

The experiment was conducted in the cotton experimental field of Shandong Agricultural University in Chiping District,Shandong Province,in 2022.The tested variety was Liaomian15.Under the condition of returning straw to the field,six nitrogen application levels of 0,150,180,210,240,and 270 kg hm^-2were set,each nitrogen application level was repeated four times,with a randomized block design between replicates.Urea（containing 46.0%pure N）was used as nitrogen fertilizer,with a base to topdressing ratio of 1:1.The amount of phosphorus and potassium fertilizer used was P₂O₅90 kg hm^-2and K₂O 105 kg hm^-2,respectively.The rest of the management was the same as in general cotton fields.By studying the effects of nitrogen application rate on the conversion rate,enzyme activity,and related microbial functional gene abundance during the soil nitrogen transformation process under the condition of returning cotton straw to the field in the northwest of Shandong Province,as well as the relationship between the soil nitrogen transformation process and the uptake and utilization of nitrogen in the cotton shoot,the optimal nitrogen application rate was obtained,providing theoretical basis and technical support for the weight loss and production promotion in the northwest of Shandong Province.The main research results are as follows:1.Effect of nitrogen application rate on soil physical and chemical propertiesUnder the condition of returning straw to the field,compared with N0,nitrogen application significantly increased the proportion of large aggregates（>2 mm）and small aggregates（0.25～2 mm）in N180,N210,N240,and N270 soils,as well as the total porosity of the soil,and both increased first and then decreased with the increase in nitrogen application,with N210 increasing the most.Nitrogen application significantly reduced the proportion of N210 and N240 microaggregates（0.053～0.25 mm）to silty clay particles（<0.053 mm）and soil bulk density,and both decreased first and then increased with the increase of nitrogen application,with the largest decrease in N210.Compared with N0,N240 and N270significantly increased soil organic carbon,total nitrogen,and nitrate nitrogen content,while nitrogen application did not significantly affect ammonium nitrogen content.2.Effect of nitrogen application rate on soil nitrogen conversion rateOverall,after adding NH₄NO₃solution,the ammonium nitrogen content of the six treatments gradually decreased and the nitrate nitrogen content gradually increased as the incubation time progressed.The higher the nitrogen application rate,the faster the decrease or increase.In terms of net ammonification rate,N0,N180,N210,N240,and N270 reached their peak in 1 day,N150 reached their peak in 1d to 2d,and in terms of net nitrification rate,N180,N210,N240,and N270 reached their peak in 1 day,while N0 and N150 reached their peak in1d to 2d.After reaching the peak value,the net ammonification rate and net nitrification rate of each nitrogen application treatment gradually decreased as the incubation time progressed.There was no significant difference in the average net mineralization rate among the six treatments,and the overall trend was that the higher the nitrogen application rate,the lower the net mineralization rate.In addition,according to the range of incubation time,the net ammonification rate and net nitrification rate increased with the increase of nitrogen application rate from 2h to 1d;After 1d to 2d,the net ammonification rate increased with the increase of nitrogen application,but the net nitrification rate first increased and then decreased with the increase of nitrogen application,reaching the maximum value at N180;From a comprehensive perspective,the net ammonification rate and net nitrification rate of the low nitrogen application treatment were higher than those of the high nitrogen application treatment,and the gap became larger as the incubation time progressed,ultimately manifested as the higher the nitrogen application rate,the lower the net ammonification rate and net nitrification rate.To some extent,it indicates that the microbial activity of high nitrogen application treatment is higher than that of low nitrogen application treatment.3.Effects of nitrogen application rates on the abundance of enzymes and related functional genes during soil nitrogen transformationIn soil biological nitrogen fixation,nitrogen application significantly decreased soil nitrogenase activity compared to N0.N180,N210,N240,and N270 significantly reduced the abundance of nif H gene,and the greater the amount of nitrogen application,the more significant the reduction effect.During soil nitrification,compared with N0,nitrogen application significantly increased the activity of hydroxylamine reductase,and the effect became more significant with the increase of nitrogen application rate;N180,N210,N240,and N270 significantly increased AOB gene abundance,with N210 increasing the largest amount by 236.53%.During soil denitrification,compared with N0,nitrogen application significantly increased the activity of nitrate reductase,with a trend of first increasing and then decreasing with the increase of nitrogen application,with N210 increasing the largest amount,increasing by 234.97%;N180,N210,and N240 significantly decreased the activity of nitrite reductase,with N240 decreasing by 45.73%,followed by N210;N180,N240,and N270 significantly increased the nos Z gene abundance by 17.92%,21.75%,and 19.37%,respectively.4.Effect of nitrogen application rate on dry matter accumulation and distribution and nitrogen accumulation in cotton shootUnder the condition of returning straw to the field,with the advancement of the growth process,the aboveground vegetative organs,reproductive organs,total dry matter accumulation,and nitrogen accumulation of cotton under the six treatments showed an upward trend.Compared with N0,N180,N210,N240,and N270 significantly increased the final accumulation of total dry matter in the shoot and the final accumulation of dry matter in the reproductive organs,with the largest increase in N270,with increases of 77.58%and66.38%,respectively;N210,N240,and N270 significantly increased the final accumulation of dry matter in the vegetative organs above ground,with N270 increasing the maximum amount by 90.25%.With the growth process,the proportion of dry matter allocated to reproductive organs gradually increased,but there was no significant difference between the six treatments in terms of final nutrition and the proportion of dry matter allocated to reproductive organs.Compared with N0,N210,N240,and N270 significantly increased the final nitrogen accumulation in the shoot,with N270 increasing by 85.32%.5.Effect of nitrogen application rate on cotton yield and component factorsUnder the condition of returning straw to the field,compared with N0,nitrogen application significantly increased the number of bolls per unit area,single boll weight,and lint yield of cotton,and showed a trend of first increasing and then decreasing with the increase of nitrogen application rate.The maximum increase was achieved at the nitrogen application rate of 210 kg hm^-2,with the increase amounts of 37.43%,11.54%,and 52.52%,respectively.Nitrogen application had no significant effect on lint percentage.6.Effect of nitrogen application on nitrogen utilization efficiency of cottonUnder the condition of returning straw to the field,the agronomic utilization rate,contribution rate,and physiological utilization rate of nitrogen fertilizer all showed a trend of first increasing and then decreasing with the increase of nitrogen application rate,with a maximum value at 210 kg hm^-2nitrogen application rate.The partial productivity of nitrogen fertilizer gradually decreased with the increase of nitrogen application rate,with N240 and N270 significantly lower than N150.