Ecological Stoichiometry Of Soil-Microbial-Extracellular Enzyme Of Three Forest Types In The Subtropical Area Of China

Ecological stoichiometry is a hot spot in ecology nowadays.Studying the ecological stoichiometry of soil-microbial-extracellular enzyme will help to understand the problems of the restrictive nutrient elements and homeostasis of the ecosystem.At present,the study on ecological stoichiometry is mainly focusing on the ecological stoichiometric characteristics of soil carbon(C),nitrogen(N),and phosphorus(P),while there are few studies on the ecological stoichiometric characteristics of microbial biomass and extracellular enzyme.Particularly,the study on the ecological stoichiometric characteristics of soil microbial biomass and extracellular enzyme in central subtropical forest has not been reported yet.In the subtropical hilly area of Hunan Province,the study selected three typical forest types that were adjacent to each other and had basically the same site conditions:Pinus massoniana-Lithocarpus glaber coniferous-broad leaved mixed forest(PLF);Choerospondias axillaris-Litsea pungens deciduous broad-leaved forest(CAF);Lithocarpus glaber-Cleyera japonica broad-leaved forest(LGF).We set up fixed sample plots,collected soil samples,determined soil physical and chemical properties,soil microbial biomass,β-1,4-glucosidase(BG),β-cellobiosidase(CBH),β-1,4-N-acetylglucosaminidase(NAG)and Acid phosphatase activity(ACP),studied the ecological stoichiometric characteristics of soil-microbial-extracellular enzyme in different forest types,analyzed the relationship between soil stoichiometric ratios and microbial biomass stoichiometric ratios.Combined with the ecological stoichiometry of extracellular enzyme,we explored the nutrient limitation of soil microbial communities and the potential mechanism behind it in the subtropical forest.The study aimed to provide scientific basis for subtropical forest protection and sustainable management.The main results are as follows:(1)Soil physical and chemical properties,microbial biomass and extracellular enzyme activities were significantly affected by forest types.The soil microbial biomass and extracellular enzyme activities were affected by different biological and soil factors.There was no significant difference in soil bulk density(BD)among different forest types in the 0-15 cm soil layer,but in the 15-30 cm soil layer,the BD of CAF was significantly greater than the other two forest types.The soil of three forest types was mainly composed by the silt,followed by sand and clay.Soil pH in CAF was significantly higher than the other two forest types.The ammonium nitrogen content in every forest type was generally greater than the nitrate nitrogen content.The dissolved organic carbon content,soil total phosphorus(TP)content and available phosphorus content in each soil layer were not significantly different among different forest types.In each soil layer,the contents of soil organic carbon(SOC)and soil total nitrogen(TN)were both expressed as:CAF>LGF>PLF.The soil microbial biomass(SMB)in each soil layer was expressed as:CAF>LGF>PLF.The extracellular enzyme activity was expressed as:PLF>CAF>LGF.Soil microbial biomass carbon(SMBC),Soil microbial biomass nitrogen(SMBN),Soil microbial biomass phosphorus(SMBP)and 4 kinds of extracellular enzyme activities were affected by different biological and soil factors.Among them,the TP content in soil and the TN content in litter layer were the most important factors of SMBC and SMBP,respectively,The organic carbon content of fine root and silt content were the most important factors of BG and CBH.The SOC was the most important factor of NAG and ACP,respectively.(2)The influences of forest types on the ecological stoichiometry of soil and microbial biomass were complex.The forest types and soil layers had no significant effect on SOC/TN.In each soil layer,SOC/TP of CAF was significantly higher than that of PLF and LGF,SOC/TP between different soil layers was not significantly different;In the 0-15 cm soil layer,TN/TP of CAF was significantly higher than that of PLF and LGF,while in the 15-30 cm soil layer,TN/TP was not significantly different among the three forest types.In the 0-15 cm soil layer,SMBC/SMBN had no significant difference among the three forest types,while in the 15-30 cm soil layer,SMBC/SMBN of PLF was significantly higher than the other two forest types.In each soil layer,SMBC/SMBP was expressed as:PLF>LGF>CAF,but there was no significant difference in SMBC/SMBP between the two soil layers.In each soil layer,SMBC/SMBN had good homeostasis characteristics,while SMBC/SMBP had no homeostasis characteristics.SMBC/SMBP was obviously affected by SOC/TP.Relative C content:Fine root(FR),Litter layer(L)>Soil>SMB;Relative N content:FR,L>Soil,SMB;Relative P content:Soil,SMB>FR,L.(3)The soil microbial community of 3 different forest types generally had the P limitation,while the soil microbial community of LGF had more serious P limitation.The soil microbial community of CAF had more serious C limitation.The Partial Least Squares Path Modeling(PLS-PM)explained the variation of 17%and 12%for‘C Limitation’and‘P Limitation’,respectively.The‘Soil C Resource’had a significant positive impact on the‘C Limitation’,but the‘Soil P Resource’had a significant negative impact on the‘P Limitation’.FRB had a significant positive impact on the‘Soil C Resource’ and the‘Soil P Resource’.The PLS-PM showed that FRB directly affected the nutrient limitation of the soil microbial community by indirectly affecting the condition of soil resources.There was P limitation in the microbial communities of three typical forest types,and LGF had more serious P limitation.In order to realize the conservation and sustainable management of subtropical forest resources,coniferous and deciduous broad-leaved tree species should be added appropriately in the evergreen broad-leaved forest to alleviate the P limitation of the soil.