Evaluation On Soil Quality Evolution And Carbon And Nitrogen Transformation Characteristics Of Lou Soil Under Long-term Mulching And Nitrogen Application

Soil surface mulching and nitrogen（N）application are effective means to improve crop yield and water use efficiency in dry-land farming areas.However,the responses of soil quality to N fertilization under the long-term straw mulch or plastic film+straw dual mulching model are scarcely evaluated,and the characteristics of soil C and N transformation and the related microbiological mechanism remain poorly doucumented.Therefore,this study was conducted on a field experiment treated with mulching and N fertilization that established in 2003 in Lou soil region of the Loess Plateau,and the techniques,including phospholipid fatty acid（PLFA）,quantitative PCR,etc.were adopted to investigate soil quality changes,C and N functional genes,C and N distribution and C-and N-acquiring enzymes at aggregate-scale,under long-term mulching（no mulching,CT;plastic film-mulched ridges and straw mulched-furrows,RF;straw mulching,SM）and N application（no N application,N0;recommend N rate,N120;conventional N rate）treatments.The main findings obtained are as follows:（1）Compared with CT,RF and SM significantly increased soil moisture content（SMC）,available potassium（AK）,β-glucosidase（βG）,phosphatase and total PLFAs.Relative to the RF,SM significantly increased soil organic carbon（SOC）,total nitrogen（TN）,available phosphorus（AP）,AK,βG,cellobiohydrolase,β-1,4-N-acetyl-glucosaminidase（NAG）,gram-negative bacteria（G^–）and fungi by 8.2%,6.5%,18.5%,10.4%,70.2%,105.0%,78.0%,6.4%and 12.1%,respectively.Compared with N0,N120 and N240 increased soil NO₃^–-N,TN,NAG,α-glucosidase（αG）,microbial biomass C（MBC）,microbial biomass N（MBN）,and G^–by 98.0%–184.7%,7.1%–16.2%,25.0%–65.2%,19.5%–35.9%,10.6%–19.7%,28.5%–30.8%,11.0%–15.6%,respectively,but decreased soil p H,and the contents of AP and AK.（2）The soil quality index（SQI）among mulching patterns was ranked as SM（0.698）>RF（0.642）>CT（0.620）,and SQI scores among N fertilization treatments were ranked as N240（0.680）>N120（0.650）>N0（0.630）.Notably,mulching showed more beneficial effects on SQI than N application,and the SM+N240 treatment recived the highest SQI.In addition,the minimum dataset was established,which included SMC,p H,NO₃^–-N,AP,NAG,and actinomycetes,for assessing soil quality under long-term mulching and N application systems.（3）Compared with CT,ammonia-oxidizing archaea（AOA）and bacteria（AOB）in SM-treated soils were increased by 42.5%and 25.3%,respectively,while the abundance of AOA in RF treatment was decreased by 26.1%.Under the mulcing system,the abundance of AOB was increased with the increase of N application rate,and similar changing trend was observed for the potential nitrification acticity.In the conventional N application rate of 240 kg N ha^–1,the copy numbers of cbh I and GH48 genes were ranked as SM>RF≈CT and SM≈RF>CT,respectively.（4）Compared with CT,the proportion of macroaggregates in RF and SM increased by10.6%and 6.9%,respectively,and the mean weight diameter increased by 8.9%and 5.6%,respectively.Compared with N0,N application increased the contents of soil microaggregates and microaggregates within macroaggregates by 6.6%–12.2%and 4.1%–5.4%,respectively.The SOC and TN contents increased with the increase of soil particle size.Mulching increased the SOC in macroaggregates,microaggregates and microaggregates within macroaggregates by 9.4%–18.4%,5.6%–11.0%and 13.4%–22.9%,respectively,and the enhancement relating SOC was higher in SM than that in RF.Importantly,the variation of TN in soil aggregates presented similar changing patterns compared to that of SOC.Compared with N0,N120 and N240 significantly increased TN content in macroaggregates,by 4.6%and 7.5%,respectively.However,the effect of N application on SOC in aggregates was not statistically significant.（5）Mulching and N fertilization increased the activities of cellobiohydrolase,αG,NAG and _L-leucine aminopeptidase at aggregate-scale.The highest activities of hydrolase relating C-cycling and NAG were observed in silt and clay,and the highest activities of phenol oxidase and peroxidase were recorded in microaggregates,whilest MBC and MBN were the highest in macroaggregates.Mulching and N fertilization significantly increased soil microbial biomass in the macroaggregates,and the SM+N240 treatment received the highest MBC.Collectively,straw mulching combined with N application can efficiently improve soil quality,promote microbial activity,enhance aggregate stability,increase aggregate-associated C and N content and extracellular enzyme activity,and thereby is considered as an alternative option for developing sustainable dryland farming management in the Loess Plateau.