Soil Extracellular Enzyme Activities And Influencing Factors In The Stand With Different Mycorrhizal Types

Soil extracellular enzyme is a crucial indicator of microbial metabolic activity and soil quality monitoring,playing an important role in the soil carbon and nutrient cycle.Mycorrhizal fungi is the main secretor of soil extracellular enzymes,and its type succession has a great significance for enriching the theory of carbon cycle in forest ecosystem and predicting the response of ecosystem succession to global change.In order to investigate the effect of mycorrhizal types on soil extracellular enzyme activity,four stands with percentage of different mycorrhizal type（the percentage of ECM trees was<25%,25-50%,50-75%and>75%,respectively）were set up in pure Cunninghamia lanceolata stand,pure Quercus acutissima stand,and the two species mixed forest.We measured the soil extracellular enzyme activities related to carbon cycle[β-Glucosidase（BG）,β-1,4-Xylosidase（BX）,α-D-1,4-Cellobiosidase（CBH）and Peroxidase（PER）]、enzymes related to nitrogen cycle[β-N-acetylglucosaminidase（NAG）,Leucine aminopeptidase（LAP）,Protease and Urease]、enzymes related to phosphorus cycle[Acid phosphatase（AP）]and oxidase[Peroxidase（PER）]activity,analyzing the pattern of hydrolase and oxidase activities along the gradient of mycorrhizal types and influencing factors.The main results were as follows:1.The contents of ammonium nitrogen（NH₄⁺-N）,soil organic carbon（SOC）,total soil nitrogen（TN）and total soil phosphorus（TP）in the ECM-dominated forests were significantly higher than those in the AM stands,while AM-dominated forests showed higher soil moisture（SM）,potential of hydrogen（p H）,nitrate nitrogen（NO₃^--N）and the ratio of nitrate to ammonium.With the increase of ECM-associated tree species,SM and NO^3--N declined with fluctuation,p H and the ratio of nitrate to ammonium decreased gradually.Inversely,NH₄⁺-N,SOC,TN and TP contents increased gradually with the increase of ECM-associated tree species.It was significantly different in SM,p H and nutrient contents between four seasons except for TN,however,the regulations were not consistent.2.Compared with AM-dominated forests,ECM-dominated forests showed higher soil hydrolase activities（BG,BX,CBH,NAG,LAP,AP,urease and protease）.With the increase in the proportion of ECM-associated tree species in the forest,enzyme activities showed a gradual increase trend except for soil oxidase activity.PER only showed variance among different mycorrhizal types stand in summer and autumn.Soil enzymes related to carbon cycle and phosphorus cycle reached the peak in summer,and were lower in spring,autumu and winter.The seasonal dynamics of soil enzyme activities related to nitrogen cycle were inconsistent.In particular,the urease activity decreased over time.3.Soil factors and biological factors both had significant effects on soil extracellular enzyme activities,but the trends existed variation.The activities of BG,BX and CBH exhibited a positive correlation with SOC,TN and TP（P<0.01）,and a negative correlation with p H and the ratio of nitrate to ammonium significantly（P<0.01 or P<0.05）.The PER activity was more restricted by the content of SM and NH₄⁺-N.The activities of NAG,LAP,urease and protease were positively correlated with SOC and TN.There also had positive correlation between urease with NH₄⁺-N,AP with SOC and TN.All soil hydrolase enzymes were significantly positive correlated with microbial biomass（P<0.05）.However,there was no significant correlation between PER activity and microbial biomass.4.Soil total phospholipid fatty acid（PLFAs）biomass and the contents of bacteria,fungi,actinomycetes,gram-positive bacteria（G+）and gram-negative bacteria（G-）all increased with the increase of ECM-associated tree species.However,ECM-dominated forests had lower G-:G+and F:B.The biomass of total PLFA,bacteria,fungi,actinomycetes,G+and G-were significantly positively correlated with SM,SOC,TN and TP（P<0.01 or P<0.05）.5.The regression slope of the ratio of lnBG to ln（NAG+LAP）and to lnAP were significantly less than 1,indicating that AP activity was much higher than that of BG and NAG+LAP.This indicated that forest soil microorganisms were more severely restricted by phosphorus（P）.The increase of ECM-associated tree species relieved the limitation of carbon（C）and P for microorganisms.These results support the theory of resource allocation,verify that the soil extracellular enzyme activities change gradually with the percent of AM-associated and ECM-associated tree species,and confirm the applicability of the mycorrhizal-associated nutrient economy pattern.The results also strengthen the predictability of the soil physical and chemical properties as well enzyme activities in the forest with different percentage of AM-associated and ECM-associated tree species,and thus contributing to the improvement of biogeochemical cycle model.