Effects Of Nitrogen Deposition And Rainfall Changes On Soil Organic Carbon And Its Stability In Desert Steppe

Desert steppe ecosystem plays an important role in the terrestrial carbon cycle.As two key driving factors of global change,nitrogen deposition and rainfall change affect the distribution of soil aggregates,active organic carbon components and soil organic carbon mineralization through changing soil physical and chemical properties and vegetation biomass,and then change the stability of soil organic carbon and affect the soil carbon cycle.In order to explore how nitrogen deposition and rainfall change affect soil organic carbon and its stability in desert steppe,this study relied on the multi-year water and nitrogen control experiment platform,and adopted split zone design.The main zone was three water treatments:natural rainfall（CK）,rainfall enhancement 30%（W）and rainfall reduction 30%（R）.The sub-plots were treated with 0（N₀）,30(N₃₀),50(N₅₀)and 100(N₁₀₀)kg·hm^-2·a^-14nitrogen,and the effects of nitrogen deposition and rainfall on soil aggregates,soil active carbon components and soil organic carbon mineralization were studied in laboratory culture experiments.The main results are as follows:（1）Nitrogen deposition and rainfall had no significant effect on soil total organic carbon content（P>0.05）,but increased above-ground biomass.N₁₀₀ treatment significantly increased soil nitrate,ammonium and total nitrogen contents,and decreased soil p H（P<0.05）under the three water conditions.Water content had no significant effect on the activities of enzymes involved in carbon cycling（P>0.05）.Under the same water condition,nitrogen deposition significantly increased soil sucrase activity,but had no significant effect on soil cellulase,polyphenol oxidase and peroxidase.（2）Nitrogen deposition and rainfall significantly affected the distribution of soil aggregates.Rainfall increased the proportion of large aggregates（22.6%）,and decreased the proportion of micro aggregates and silt（20.1%and 12.8%）.In addition,with the increase of nitrogen deposition,the proportion of soil large aggregates increased,while the proportion of soil micro aggregates decreased,resulting in the increase of the average weight diameter of soil aggregates.Nitrogen deposition and rainfall had no significant effect on the organic carbon content of soil aggregates.The organic carbon content of microaggregates was higher than that of large aggregates,and the organic carbon content of powder clay was the lowest.（3）Nitrogen deposition and rainfall significantly affected soil active carbon components.Under three water treatment conditions,nitrogen deposition promoted the content of light organic carbon and dissolved organic carbon in soil.It inhibited the content of soil microbial biomass carbon and easily oxidized organic carbon.The increase of rainfall increased soil carbon pool activity,while nitrogen deposition inhibited soil carbon pool activity.（4）The indoor culture experiment showed that soil organic carbon mineralization was sensitive to water content,and the increase of water content significantly increased the amount and rate of soil organic carbon mineralization.Compared with natural rainfall,the cumulative mineralization of soil organic carbon in rainfed soil increased by 36.3%,while that in rainfed soil decreased by 52.1%.Soil organic carbon mineralization was significantly inhibited by N deposition under both natural rainfall and rainfall enhancement treatments,and the amount of soil organic carbon mineralization decreased with the increase of N deposition.The first-order dynamic model showed that rainfall increase significantly increased soil potential mineralized carbon pool content.In conclusion,the increase of nitrogen deposition promoted the rise of aboveground biomass,increased the proportion of soil macroaggregates,improved the stability of soil aggregates,reduced the activity of soil organic carbon pool,inhibited the mineralization of soil organic carbon,and improved the stability of soil organic carbon pool.Although the increase of rainfall improved the stability of aboveground biomass and soil aggregates,it also increased the activity of soil organic carbon pool and soil organic carbon mineralization rate,which was not conducive to the stability of soil organic carbon pool.