Effects Of Straw Returning And Drying-rewetting Cycles On Enzyme Activity And Nitrogen Mineralization In Brown Soils

Straw returning is an important measure to improve soil fertility in agricultural production,which profoundly affects the status and cycling of nutrients in farmland ecosystems.However,it is unknown how drying-rewetting cycle（DRC）under global climate change will affect nitrogen（N）cycling processes in soils after straw returning.DRC can rapidly alter soil physical structure,and affect soil chemical and biological properties,which in turn may impact the transformation processes and supply of soil N.This study focused on the effects of straw addition and DRC and their interaction on enzyme activity,N components including ammonium（NH₄⁺）,nitrate（NO₃^-）,inorganic N（ION）and alkali-hydrolyzable N（AHN）,and N mineralization rates in brown soils.We sampled soils from the typical brown soil areas in Liaoning and used a lab incubation experiment to explore the individual and interactive effects of straw addition and drying-rewetting on soil enzyme activity and N dynamics by setting straw addition,constant soil moisture,two drying-rewetting intensities（DRI）and eight drying-rewetting times（DRT）.The main results are as follows:（1）Without adding straw,DRI significantly affected soilβ-glucosidase activity（SβA）,but exerted no significant effects on soil urease（SUA）and sucrase activities（SSA）.After adding straw,SUA and SSA decreased with the DRI,while SβA showed a opposite pattern.The interaction effects of DRI and straw addition on soil enzyme activity were significant.Under the two DRI,SUA,SSA and SβA decreased with the DRT,but the time reaching stability was different.The dynamic pattern of soil enzyme activity with no straw addition was slightly different from one of straw addition.（2）Without adding straw,DRI significantly increased NH₄⁺and ION,but exerted no significant effects on NO₃^-and AHN.However,with straw addition,soil NO₃^-and ION significantly increased with the DRI,while NH₄⁺and AHN did not significantly change.Under the constant moisture treatment without adding straw,NH₄⁺and ION significantly decreased with the DRT and reached stability at the second cycle,while NO₃^-and AHN did not significantly change.However,after adding straw,all N components did not change with the DRT.Under both the 60%-10%and 100%-20%treatments,soil NH₄⁺,NO₃^-and ION significantly decreased with the DRT,regardless of whether straw was added;and their time reaching stability was inconsistent.Under the two DRI with no straw addition,AHN did not change with the DRT.Under the 60%-10%treatment with adding straw,AHN significantly decreased with the DRT and stabilized at the fourth cycle.Under all treatments,there were strong relationships between SUA and each N component,and the relationships were not affected by adding straw.Our results showed that straw addition may be more important for each soil N component than DRI and DRT.（3）Without adding straw,DRI significantly affected ammonification（AMR）and nitrification rates（NFR）,but exerted no significant effect on net N mineralization rates（NNMR）.However,with straw addition,DRI significantly affected AMR,NFR and NNMR.Without adding straw,AMR and NNMR decreased significantly with the DRT,and stabilized at the fourth cycle.NFR did not change with the DRT under the constant moisture condition,but showed a similar pattern to AMR under 60%-10%and 100%-20%treatments.After adding straw,DRT significantly affected AMR,NFR and NNMR under the constant moisture treatment.However,under 60%-10%and 100%-20%treatments,AMR,NFR and NNMR significantly decreased with the DRT and stabilized at the third cycle.After adding straw,soil N transformation rates had stronger association with SUA and SβA,but showed weaker association with SUA.High DRI weakened the relationship between SUA and soil N transformation rates.Our results showed that straw addition may be more important for AMR,NFR and NNMR.These results indicate that the DWC could have a significant effect on soil N cycling and N supply in brown soils and these effects may be related to straw addition and DRI.The results can provide theoretical reference and practical support for the cropland management of N fertilizers and comprehensive utilization of crop straw in brown soils of Northeastern China.