Latitudinal Variation And Driving Factors Of Soil Extracellular Enzymes And Stoichiometry In Different Tropical And Subtropical Forest Types

Soil microorganisms are the core driving force of C,N and P biogeochemical cycling.Soil enzymes reflect the physiological activitys of soil microorganisms and play an important role in soil nutrient supply and the performance of ecosystem service function.Vegetation type profoundly shapes the composition and physiological activity of soil microorganisms,and is the energy and nutrient source of soil microbial physiological metabolism.The changes of plant community structure affect the metabolism of microorganisms and the energy flow in soil.At present,empirical studies on enzyme stoichiometry have been mainly focusing on small spatial scales.we are unclear of the spatial variation and driving factors of soil extracellular enzyme activity at geographical scale.Here,we investigated extracellular enzymes and their stoichiometry,as well as related soil and microbial properties from four vegetation types（evergreen broad-leaved forest,deciduous broad-leaved forest,coniferous and broad-leaved mixed forest and coniferous forest）in tropical（Jianfengling,Wuzhishan）and subtropical mountains（Jiulian Mountain,Daiyun mountain,Wuyi Mountain,Qizimei Mountain,Xingdou Mountain,Yaoluoping Mountain and Shennongjia Mountain）along a latitudinal gradient in the southeastern regions of our country.This study can provide informative data for biogeochemical cycling model and vegetation geographic model,and enrich the biogeochemical cycle database on a large regional scale.The main conclusions are as follows:（1）Vegetation type had significant effects on microbial biomass carbon,nitrogen and phosphorus and extracellular enzyme activities,showing different spatial patterns in the four vegetation types.Microbial biomass carbon（MBC）varied in an order of coniferous and broad-leaved mixed forest>deciduous broad-leaved forest>evergreen broad-leaved forest>coniferous forest.Correspondingly,C:P_EEA and N:P_EEA from the four vegetations were 0.62,0.69,0.61,0.71,and 0.52,0.68,0.57and 0.58 respectively.C:P_EEA was significantly lower than C:N_EEA,and N:P_EEA was lower than 1.with increasing latitude,With the increase of latitude,the activities of carbon and phosphorus acquisition enzymes in deciduous broad-leaved forest and coniferous forest decreased linearly,and the activities of soil phosphorus acquisition enzymes in coniferous broad-leaved mixed forest decreased linearly,while the stoichiometric ratio of soil enzymes did not change significantly with the increase of latitude.In conclusion,vegetation types significantly affected the latitude pattern of soil enzyme activity in tropical and subtropical mountain forests,but had no significant effect on the latitude pattern of soil enzyme stoichiometry.（2）The averaged BG enzyme across the whole latitude gradient were 0.60,0.35,0.48 and 1.04 nmol h^-1g^-1SOC in evergreen broad-leaved forest,coniferous forest,deciduous broad-leaved mixed forest and coniferous broad-leaved mixed forest respectively;Corerspondingly,the averaged NAG+LAP was 0.49,0.30,0.57and 0.62nmol h^-1g^-1SOC;the averaged AP enzyme was 3.58,2.19,3.59 and 3.16 nmol h^-1g^-1SOC.In terms of enzyme stoichiometry,the averaged C:N_EEA was 1.19,1.04,1.11and 1.26;the averaged C:P_EEA was 0.62,0.70,0.59 and 0.72;and the averaged N:P_EEAwas 0.53,0.69,0.56 and 0.59.The key factors driving extracellular enzymes and their stoichiometry differed in terms of vegetation type.TN,C:Nsoil and dissolved organic carbon（DOC）were the main factors driving the variation of soil extracellular enzymes and their stoichiometry in evergreen broad-leaved forest;N:Psoil,p H and annual average temperature were the main factors responsible for the variation of soil extracellular enzymes and their stoichiometry in deciduous broad-leaved mixed forest;DOC,C:Nsoil and dissolved organic nitrogen（DN）are identified to drive the variation of soil extracellular enzymes and their stoichiometry in coniferous and broad-leaved mixed forest;Soil water content（SWC）,electrical conductivity（EC）and TN were attributed to the variation of soil extracellular enzymes and their stoichiometry in coniferous forests.（3）The stoichiometric ratios of C:N:P_EES in top soil（0-20cm）were 1:0.69:1.52,1:0.96:1.60,1:0.96:1.43 and 1:0.95:1.22 in tropical,subtropical,middle subtropical and north subtropical region,respectively,and it was 1:0.89:2.35,1:0.75:1.76,1:0.84:1.41and 1:0.81:1.82 in subsoil（20-40cm）.These values all deviated from 1:1:1.With increasing latitude,the vector length decreased linearly with increasing latitude in deciduous broad-leaved forest.Carbon use efficiency increased in evergreen broad-leaved forest,deciduous broad-leaved mixed forest,coniferous broad-leaved mixed forest and coniferous forest.TER_C:N and TER_C:P showed no obvious pattern with increasing latitude.These results suggest that microorganisms in tropical and subtropical forests are generally P limited,while the magnitude of N limitation is associated with climate zone and soil depth.（4）The four enzymes associated with C,N and P cycling showed different patterns with latitude and soil depth.In most sites,soil extracellular enzyme activity and its stoichiometry decreased with increase in soil depth.However,the specific enzyme activity is higher in subsoil（20-40cm）than topsoil（0-20cm）,such as that from evergreen broad-leaved forest and coniferous broad-leaved mixed forest in Jianfengling area,BG enzyme,NAG+LAP enzyme and AP enzyme from deciduous broad-leaved forest in Yaoluoping.Soil TN,DN and MAT were the main factors affecting enzyme activity and its stoichiometric ratio in top soil,while soil SWC,TN and MAT are attributed to soil enzyme activity and its stoichiometric ratio in subsoil.