Soil Carbon And Nitrogen Dynamics And Enzyme Activities In Secondary Grasslands Of The Three Gorges Area

Grassland ecosystem,as an important part of terrestrial ecosystem plays a crucial role in carbon?C?and nitrogen?N?cycling.The secondary grasslands of the Three Gorges area sequester large amounts of C and N and hence offset substantial carbon dioxide emissions.However,the shifts in climate and deforestation significantly affected ecosystem functioning,and thus influenced both short-term soil organic matter?SOM?turnover and N transformations,and long-term C and N sequestration.Besides,the soil C and N dynamics are closely related to enzyme activities,which are controlled by complex interactive effects of climatic,biotic and edaphic conditions.However,the interrelations of the SOM fractions,N transformations and enzyme activities and related drivers are not fully understood in the secondary grasslands of Three Gorges area at regional scale.In this study,we conducted experiments in the typical secondary grasslands?600 km transect?of Three Gorges area.We examined the soil C and N fractions and their isotopes(?¹³C and?¹⁵N)in relation to C3 and C4 plant functional groups across climatic gradients in Three Gorges Area.We also measured the soil N transformation rates,including potential N mineralization?PMR?,nitrification?PNR?,ammonification?PAR?and denitrification?PDR?rates,and also investigated the direct and indirect effects of climates,plant inputs,and soil properties on soil N dynamics.In addition,we measured the patterns of soil enzyme activities including,?-1,4-cellobiohydrolase?CB?,?-1,4-glucosidase?BG?,?-1,4-N-acetyl-?-glucosaminidase?NAG?,_L-leucine aminopeptidase?LAP?and acid phosphatase?AP?and examined the ecoenzymatic C:N:P stoichiometry and related drivers in the study area.The main results are presented as follows:1)In the secondary grasslands of Three Gorges areas,the soil organic C?SOC?content decreased first slightly and then increased along precipitation gradients,largely due to the increase in C4 plant C inputs in the lower precipitation regions.In contrast,the soil organic N?SON?content increased with increasing N inputs from the C3 plant at higher precipitation regions.The labile C and N contents increased with increasing precipitation,whereas recalcitrant C and N did not change with precipitation.The labile C and N were correlated with plant C and N contents,as well as the mean annual precipitation,respectively.Increases in labile C and N were tightly related to enhanced plant C and N inputs influenced by precipitation,suggesting stronger sensitivity of labile pools to both plant functional groups inputs and precipitation compared to the recalcitrant pool.Moreover,the?¹³C values in recalcitrant C declined with precipitation,while the?¹⁵N values of both labile and recalcitrant N increased with increasing precipitation,further revealing that soil labile and recalcitrant C and N pools closely related to the shift in the C3 and C4 plant along precipitation gradients.Overall,the findings indicated that soil labile and recalcitrant fractions should be considered in context of precipitation under which plant inputs takes place in predicting soil C and N dynamics.2)The soil NH₄⁺-N concentration was positively correlated with SOC and microbial biomass C and N,possibly due to changes in litter inputs.The soil PNR and PDR significantly increased with increased soil pH,whereas the PMR and PAR decreased with the increasing soil pH.The PMR was positively correlated with precipitation but negatively correlated with temperature.Structural equation model analyses further revealed the soil NH₄⁺-N and pH were the primary predictors of PMR,PNR and PAR.Additionally,we found that soil NO₃^--N availability was the strongest controller over the PDR,which accounted for 43%of the variation in PDR,followed by soil pH 33%and plant N inputs26%.Our results further revealed that soil N dynamics was directly affected by soil pH,soil N availability and plant N inputs,and indirectly by climates.Overall,the results suggest the importance of soil pH,inorganic N concentrations,and substrate quality and quantity in determining N cycling rates at a regional scale.3)We observed high enzyme activity in the study area,possibly due to greater amounts of labile substrate inputs in the topsoil layer.All enzyme activities and specific enzyme activities(i.e.,activity g^-1 SOC),except for AP,were significantly related to soil pH.Soil AP,NAG and LAP activities were positively correlated with SON,whereas the BG activity was negatively correlated with recalcitrant C and positively correlated with total phosphorous?P?and microbial biomass C.Soil LAP and AP activities were positively related with recalcitrant C and N,suggesting increased enzyme production to microbial allocation to optimize nutrient acquisition of limited elements and maximize energy use.Soil CB,BG and NAG activities were negatively correlated and LAP activity positively correlated with MAT.In contrast,all enzyme activities were not significantly related with MAP.The C:N:P acquisition ratios across the gradients were positively correlated with microbial biomass C and N,MAP,and negatively correlated with soil C:N ratios,MAT and soil pH.Our results indicated that substrate quality and quantity,soil pH and MAT were the most important predictors of soil enzyme activities and stoichiometry at a regional scale in secondary grasslands of the Three Gorges area.In conclusion,the findings suggest that climates,soil edaphic properties and plant inputs are the strong controller of soil C and N dynamics and enzyme activities at regional scale.