Effects Of Adding Straw On Enzyme Activity And Nitrogen Dynamics In Brown Soils Under The Freeze-thaw Conditions

Northeast China is an important grain production base in China,located at mid to high latitudes,with significant seasonal freeze-thaw phenomena.Recently the intensification of abnormal precipitation and frequent freeze-thaw in winter caused by global climate change may affect soil ecological processes,especially soil nitrogen（N）transformation processes.Currently,straw returning is an important measure to improve soil fertility,affecting soil environments and soil N fertility levels,but little is known about how straw returning affects the soil N transformation processes under freeze-thaw cycles.Based on the topsoil of typical brown soil areas,we used a laboratory incubation experiment to explore the effects of straw addition（add or not add）,initial soil water content（20%、60%、100%）,freeze-thaw cycle times（1、3、5、7、9、12、15、18、21、25）on soil enzyme activity,components and dynamics of N.This study will unravel the relationship between soil N dynamics and enzyme activity under freeze-thaw conditions with straw.The main results are as follows:（1）Freeze-thaw,straw addition,and water treatments their interaction showed significant effects on soil urease and sucrase activities（P<0.05）.Relative to the control（without freeze-thaw cycles）,freeze-thaw cycles significantly reduced soil sucrase activity by 5%,but showed no significant effects on soil urease activity.Relative to the control（without adding straw）,straw addition significantly increased soil sucrase and urease activities by 88%and 67%,respectively.Water treatments significantly affected soil enzyme activities under the freeze-thaw conditions.Relative to the 20%water treatment,soil sucrase and urease activities were significantly higher than those under 60%and 100%water treatments;while there was no significant difference in enzyme activity between 60%and 100%water treatments.Soil urease activity significantly decreased with the increase of freeze-thaw cycling times,and soil urease activity is the highest at the first freeze-thaw cycle,while soil sucrase activity showed an overall increasing trend.（2）Freeze-thaw,straw addition,and water treatments and their interaction showed significant effects on soil N components（P<0.05）.Relative to the control（without freeze-thaw cycles）,freeze-thaw cycles significantly increased soil ammonium N（NH₄⁺）,inorganic N（TIN）,alkaline N（AN）and easily hydrolyzable organic nitrogen（EHON）by 98%,33%,15%and 96%,respectively.However,freeze-thaw cycles showed no significant effects on soil nitrogen（NO₃^-）.Relative to the control（without adding straw）,straw addition under the freeze-thaw cycle significantly decreased soil NH₄⁺and TIN,and increased NO₃^-and EHON,but exerted no significant effects on soil AN.Water treatments significantly affected soil N components under freeze-thaw cycles.Relative to the 20%water treatment,soil NH₄⁺and TIN were significantly lower than those under 60%and 100%water treatments;soil NO₃^-,AN and EHON were significantly higher than those under 60%and 100%water treatments.However,there was no significant difference in N components between 60%and 100%water treatments.The correlation relationships between soil N components and soil properties varied greatly under different soil water treatments.The results of variance partitioning showed that soil water treatment and straw addition,as well as their interaction effects,explained a large variance in soil N components.（3）Freeze-thaw,straw addition,and water treatments and their interaction showed significant effects on siol N mineralization rates（P<0.05）.Relative to the control（without freeze-thaw cycles）,freeze-thaw cycles significantly increased soil ammonification（AMR）and net nitrogen mineralization（NNMR）rates by 81%and86%,respectively.Relative to the control（without adding straw）,straw addition significantly decreased soil AMR and NNMR under the freeze-thaw cycles,but significantly increased nitrification rates（NR）.Water treatments significantly affected soil N mineralization rates during the freeze-thaw cycles.Relative to the 20%water treatment,soil AMR and NNMR were significantly lower than those under 60%and100%water treatments;soil NR significantly higher than those under 60%and 100%water treatments;there was no significant difference in N mineralization rates between 60%and 100%water treatments.The freeze-thaw cycling times significantly affected the dynamics of soil N.At the first freeze-thaw cycle,the soil AMR and NNMR were the highest,then significantly decreased,and lastly got to a balance.The results of variance partitioning showed that the freeze-thaw cycle times explained the highest variance in the AMR and NNMR.The interactions of straw,soil moisture,and freeze-thaw cycling times explained the largest part of the variance in the NR.These results indicate that the freeze-thaw cycle could have a significant effect on cycling and supply of brown soil N and these effects may related to straw addition and soil water content.The results could provide theoretical reference and practical support for the cropland management of N fertilizers and comprehensive utilization of crop straw in brown soils of Northeatern China.