Changes Of Nitrogen Transformation Rates In Cotton Fields Under Long-term Continuous Cropping And Straw Return

Xinjiang is one of the most important cotton planting areas in China.However,with the increase of cotton cultivation area and cropping years,the phenomenon of long-term and large-scale continuous cropping is common.In order to alleviate the soil problems caused by long-term continuous cropping of cotton,after cotton harvesting,Xinjiang Corps Farm smashed all the straw into the field,and gradually formed a cotton Organic Fertilizer Cultivation Mode of cotton continuous cropping and straw returning to the field.In this experiment,¹⁵N isotope dilution method was used to investigate the change of soil nitrogen transformation rates in cotton fields with different continuous cropping years under straw returning,revealing the relationship between soil nitrogen transformation rates and soil properties,and clarifying the nitrogen transformation rules and principles of cotton field soil for evaluation and measurement.The ecological effect of cotton stalk returning to the field provides a scientific basis for providing theoretical support for the more rational use of straw resources in the Xinjiang cotton region.The main results are as follows:Straw returning can increase the content of total nitrogen and organic matter,increase the value of soil conductivity,and reduce the content of ammonium nitrogen,nitrate nitrogen and soil pH value.With the increase of years of continuous cropping in cotton field with straw returning,the contents of total nitrogen and organic matter in soil showed an upward trend;the content of ammonium nitrogen in soil increased first and then decreased with continuous cropping years,and reached the maximum in 22 years;soil nitrate nitrogen content increased first,then decreased and finally increased with continuous cropping years,reaching its maximum in 17 years and lowest in 22 years;soil pH value decreased first,then increased and then decreased with continuous cropping years.the soil pH value reached the highest value in 17 years of continuous cropping and the lowest in 27 years of continuous cropping;the electrical conductivity of soil increased gradually with successive cropping years,with the lowest value in 7 years and the highest value in 27 years.Compared with straw-returning cotton field,straw returning significantly increased soil nitrogen gross mineralization rates,gross NH₄⁺and NO₃^-immobilization rates and inhibited gross nitrification rates,net mineralization rates and net nitrification rates.Under the straw returning field,soil nitrogen gross mineralization rates,gross NH₄⁺and NO₃^-immobilization rates,gross nitrification rates all showed a gradual increase with the continuous cropping years;the net mineralization and nitrification rates of soil decreased with the continuous cropping years in cotton fields.In the treatment of straw non-returning,the gross nitrogen mineralization and nitrification rates,gross NH₄⁺immobilization rates and net nitrification rates increased gradually with the continuous cropping period;while the gross NO₃^-immobilization rates showed a downward trend with the increase of continuous cropping years;The net mineralization rates of soil showed a downward and upward trend with continuous cropping years.Under the straw returning treatment,the results of correlation analysis between the continuous cropping years of cotton fields and the rate indicators of various nitrogen conversion processes showed that the continuous cropping time was extremely significantly positively correlated with the gross nitrification rates,gross NH₄⁺immobilization rates and gross mineralization rates;and was extremely significantly negatively correlated with net mineralization rates,significantly positively correlated with gross NO₃^-immobilization rates,but no significant correlation with the net nitrification rates.The above results indicate that the mineralization,nitrification and fixation of nitrogen are more intense with the increase of the continuous cropping time of straw returning cotton fields,the active mineralization and nitrification increase the soil's supply of inorganic nitrogen,while active nitrogen fixation enhances the soil's fixation to inorganic nitrogen and,to some extent,reduces nitrogen volatilization and leaching losses.The results of correlation analysis between soil basic properties and soil nitrogen transformation rate indicators in cotton field showed that there was a significant correlation between some soil physical and chemical properties and nitrogen transformation rate,indicating that the change of soil physical and chemical properties caused by long-term continuous cropping and straw returning could affect the transformation of soil nitrogen to a certain extent.