Effects Of Thinning And Mixing On Soil Fertility Of Larix Principis-rupprechtii Plantations In Saihanba Mechanical Forest Farm

In the context of“sustainable development”,it is of great practical significance to maintain soil fertility,prevent soil degradation,and evaluate the impacts of management measures on soil properties.In responses to the needs of restoring soil fertility,improving ecosystem stability,and maintaining functional sustainability of plantations,this study took the Larix principis-rupprechtii plantations in Saihanba Mechanical Forest Farm in Hebei Province as the research object.We set standard plots with different thinning intensities(control,CK;low thinning,LT;moderate thinning,MT;high thinning,HT)and different mixing modes(pure Larix principis-rupprechtii stands,LP;mixed Larix principis-rupprechtii and Betula platyphylla stands,BL;mixed Larix principis-rupprechtii and Pinus sylvestris var.mongolica stands,ML).This study was to analyze the influence of thinning and mixing on the characteristics of soil fertility,reveal the effects of soil environment,microbes,and enzyme activities on soil nutrients,and explore the effects of understory vegetation and litter on soil properties.Simultaneously,the Soil Quality Index(SQI)method was used to comprehensively evaluate soil fertility under different thinning intensities and mixing modes.The results show that:(1)After thinning,soil temperature(ST),soil water content(SWC),and electrical conductivity(EC)increased,while p H decreased.Thinning,especially MT,significantly increased the contents of soil total C(STC),total N(STN),total P(STP),dissolved organic C(DOC),and available P(SAP)and the activities of invertase(INV),urease(NRE),?-1,4-glucosidase(BG),?-1,4-N-acetylglucosaminidase+leucine aminopeptidase(NAG+LAP),and acid phosphatase(AP).MT and HT significantly increased soil microbial abundance,microbial biomass C(MBC),and microbial biomass P(MBP).The microbial biomass N(MBN)was increased significantly in MT stands.There were no significant differences in soil dissolved organic N(DON)content and polyphenol oxidase(PPO)activity among different thinning intensities.The ST,SWC,MBC,MBP,INV,PPO,BG and NAG+LAP were important factors affecting the variations in soil nutrients in different thinned plantations,and the explanation rates of soil microorganisms and enzymes to soil nutrient variations were higher than that of soil environment.Thinning increased the coverage,aboveground biomass,and diversity of understory vegetation,the biomass and nutrient storage in litter layer,the annual production and nutrient return of litter,and the rates of litter decomposition and nutrient release,thereby improving soil physicochemical and biological properties.Based on analyses of the ecoenzymatic stoichiometry,this study found that the metabolism of soil microbes was limited by N in L.principis-rupprechtii plantations in study region.The N limitation in MT and HT was higher than that in CK and LT.The N limitation was increased in winter.(2)Compared with LP,the BL increased ST,SWC and EC,and decreased p H.The BL significantly increased soil nutrient contents,soil microbial abundance,microbial biomass,and enzyme activities.The ST,SWC,and soil nutrient contents were slightly lower in ML than those in LP.There were little differences in soil microbial abundance,microbial biomass,and enzyme activities between ML and LP.The ST,SWC,MBP,INV,BG and AP were important factors for the variations in soil nutrients in pure forests and mixed forests.In addition,the variations in soil nutrients in pure forests and mixed forests were better explained by soil enzymes and soil environment than by soil microorganisms.Compared with LP,the BL significantly increased the coverage,aboveground biomass,and species richness of understory vegetation,raised the N contents and N storage in litter layer,decreased C:N ratio in litter layer,and accelerated litter decomposition and nutrient release;while the ML was on the contrary.The differences in coverage,aboveground biomass,Shannon-Wiener diversity index of understory vegetation,and litter decomposition constant among the different mixing modes were closely related to changes in soil properties.Based on analyses of the ecoenzymatic stoichiometry,the metabolism of soil microbes was limited by N in pure forests and mixed forests in study region.The N limitation in BL was lower than that in LP and BL.(3)The SQI method was used to evaluate the soil fertility under different thinning intensities and different mixing modes.We selected the minimum data set(MDS)from 24 soil properties.The DON,MBP,BG,and p H were identified as the MDS in different thinned plantations.The NAG+LAP,STP,PPO,and p H were retained as the MDS indicators in pure and mixed forests.The Weighted Additive Soil Quality Index(SQI_w)was superior to the Nemoro Soil Quality Index(SQI_n)due to the larger range and coefficient of variance.The SQI increased first and then decreased with the increase of thinning intensity,and the highest values of SQI occurred in MT stands.The litter decomposition constant,Shannon-Wiener index of understory vegetation,and coverage of understory vegetation were important factors affecting SQI_w in different thinned plantations.The SQI in BL was higher than that in LP and ML.There were no significant differences in SQI between LP and ML.The aboveground biomass of understory vegetation,litter decomposition constant,and coverage of understory vegetation were important factors affecting SQI_w variability in pure forests and mixed forests.The SQI method could provide a practical,quantitative tool for evaluating soil fertility in forests,and provide references for other tree species or other regions.Overall,thinning can significantly improve the status of understory vegetation and litter,change soil environment,promote nutrient cycling,and enhance soil fertility.Among the four thinning treatments,in terms of soil fertility,MT is recommended in 28a Larix principis-rupprechtii plantations.Compared with the LP,the BL can promote the development of understory vegetation,change the composition of litter,accelerate litter decomposition,improve stand environment,promote nutrient cycling,and increase soil fertility.While the ML had no obvious effects on improving soil fertility.