Study On Soil Ecological Carrying Capacity Of Dry And Barren Mountains In The Yimeng Mountainous Area

The area of plantation in China is the broadest in the world,which is up to a quarter of the new green area from 2000 to 2017 in the world.However,with the continuous development of afforestation project,the reduction of suitable afforestation space and the difficulty of using many of dry and barren mountains have gradually become prominent.On account of the poor site environment,low ecological carrying capacity,poor stand quality and a series of problems,most of the dry and barren mountains have not been effectively utilized.Forest is the main terrestrial ecosystem and plays an important role in the global carbon cycle.Improving the C fixation capacity of dry and barren mountain forest and the ecological carrying capacity of forest soil,that play an important role in improving the C pool reserves of forest ecosystem and the service function of soil ecosystem in China.At present,the research on the dry and barren mountainous areas mainly focused on soil nutrients,and water conditions in the Yimeng Mountains.There are many studies on soil physical and chemical properties,while insufficient studies on the activities of extracellular enzymes,microbial metabolism and soil ecological carrying capacity in soil and litter of different communities.At the same time,the role of litter as a carrier of nutrient transformation in explaining soil carrying capacity was ignored.In this study,Quercus acutissima forest,Pinus thunbergii forest,Quercus thunbergii and Pinus thunbergii mixed forest,shrub and herb as the research object,the soil moisture,nutrients content and extracellular enzyme activity in different seasons,the cellulose,nutrient content and extracellular enzyme activity of litter in different seasons and different decomposition stages were analyzed to determine the soil moisture of different communities.The stoichiometry of litter at different decomposition stages,to explore the nutrient limitation and influencing factors of microbial metabolism in soils of different communities and litter at different stages of decomposition,and to clarified the effect of litter properties on soil nutrients.At the same time,high-throughput sequencing of soil and litter microorganisms in the growing and non-growing seasons was performed to clarified the changes in the diversity and composition of fungi and bacteria in different communities of soil and different decomposition stages of litter,and to analyzed microbial composition and soil and litter properties.Under natural conditions,the indicators reflecting the properties of soil and litter are greatly affected by seasons,which will have an impact on the evaluation of soil ecological carrying capacity,resulting in their evaluation values also changing with the seasons.In order to reduce the impact of seasonal changes and obtain the evaluation value of soil ecological carrying capacity on a time scale of years,this paper uses the index values of soil and litter properties in different seasons within a year to comprehensively evaluate the soil ecological carrying capacity of dry and barren mountainous areas in the Yimeng Mountains.The TOPSIS model and the entropy weight method were used to evaluate the soil carrying capacity and litter layer carrying capacity of different communities,respectively.Combined with soil and litter indicators,the soil ecological carrying capacity of different communities was comprehensively evaluated,and the appropriate thresholds for each index of different communities were discussed.(1)The soil of different communities had a large proportion of sand and gravel,the water storage performance was poor,and the water deficit was serious in autumn.The soil moisture content in the Quercus acutissima forest and mixed forest was lower than that in the shrub and herb of summer,autumn and winter.(2)The content of NO₃^--N in the dry and barren mountain soil was relatively low,and the content of NO₃^--N in the soil in summer was significantly lower than that in other seasons in the Yimeng Mountains,which was not conducive to the growth of forest trees.The soil NO₃^--N content was lower in different seasons and different communities,and the highest NO₃^--N content in shrub in spring was only 19.55 mg·kg^-1.In addition,the summer as the growing season,plants have a large demand for nutrients,but the NO₃^--N content of different communities in summer was significantly lower than that of other seasons.The contents were93.81%,94.47%,79.43%,87.06%and 90.04%were lower than their corresponding seasons with the highest contents,which will affect the growth of trees.(3)The microbial metabolism of soil and litter in different communities was limited by P.After the extracellular enzyme activity was logarithmically transformed,the vector angles obtained by calculation can be used to quantitatively that indicating the metabolic limitation of microorganisms.If the vector angle was less than 45°,microbial metabolism is limited by N,and greater than 45°,microbial metabolism is limited by P.The vector angles calculated by logarithmic transformation of soil extracellular enzyme activities in different communities and different seasons were analyzed.The variation range of vector angles in different seasons of Quercus acutissima forest was 48.39°-54.37°,and the variation range of vector angles in different seasons of Pinus thunbergii forest was 48.26°-54.31°,the variation range of the vector angle in different seasons of mixed forest was 43.83°-58.38°,the variation range of vector angle in different seasons of shrub was 53.51°-58.86°,and the variation range of vector angle of herb in different seasons is 54.88°-61.07°.Except that the vector angle of mixed forest in winter was 43.83°,all of which were greater than 45°,indicating that soil microbial metabolism in different communities was limited by P.The vector angles calculated by logarithmic transformation of the activity of litter extracellular enzymes in different communities and seasons were analyzed.The variation range of the vector angle in different seasons of Quercus acutissima forest was 46.15°-66.31°,and the variation range of vector angle in different seasons of Pinus thunbergii forest was 48.44°-67.82°,the variation range of vector angle in different seasons was 44.83°-62.78°for mixed forest,46.60°-57.03°for shrub in different seasons,and 45.64°-64.93°for herb in different seasons.Except that the vector angle of the decomposed layer of the mixed forest litter in winter was 44.83°,all of which were greater than 45°,indicating that the microbial metabolism of litter in different communities is limited by P.Therefore,the metabolism of microorganisms was limited and cannot fully exert their functions,which is not conducive to the decomposition of litter,the transformation of soil nutrients,and the material cycle between soil and litter.(4)The undecomposed layer of Pinus thunbergii forest litter has higher C content and lower P content and decomposition rate than others.In autumn,the cellulose content of litter in different communities and different decomposition stages showed a downward trend with the deepening of decomposition degree,and the cellulose content in the undecomposed layer of Quercus acutissima forest(12.56%),Pinus thunbergii forest(19.77%)and mixed forest(13.25%)was significantly higher than other seasons;the cellulose content in the semi-decomposed layer of litter of different communities in summer was significantly lower than that in other seasons.The C content in the litter of different communities in spring,summer and winter showed a decreasing trend with the deepening of the decomposition degree,and the C content in autumn decreased and then increased.In spring,summer and winter,it showed a decreasing trend with the deepening of the decomposition degree,and the undecomposed layer of Pinus thunbergii forest litter was relatively high at 129.51,99.15,and131.83,that indicating that the decomposition rate was low,which was significantly different from other seasons.(5)The chemical properties and enzyme activities of litter were closely related to soil microbial metabolic limitation.The BG activity of litter from different communities was positively correlated with its microbial limited by C,and the LAP enzyme activity was negatively correlated with its microbial limited by P.Different communities and different decomposition stages of litter microbial metabolic limitation were affected by different factors.The effects are different of litter enzyme activity,stoichiometry,and nutrients on microbial metabolic limitation.(6)The soil microbial diversity in the non-growing season was better than that in the growing season.The soil bacteria Chao1 index,Shannon index and Observed species index in the Pinus thunbergii forest in the growing season were significantly higher than those in the Quercus acutissima forest(24.30%,6.62%,25.57%)and the mixed forest(23.13%,5.54%,21.93%);in the non-growing season,the shrub soil Chao1 index,Shannon index and Observed species index were significantly higher than those of the herb by 28.48%,7.09%and 27.19%.The Chao1 index,Shannon index and Simpson index of soil fungi were the highest in herb in the growing season,that relatively fewer in Quercus acutissima forest and shrub,and higher in Vitex negundo shrub,and relatively lower in herb and mixed forest in non-growing season.(7)The evaluation of soil ecological carrying capacity of different communities was all in the middle and lower grades.Selecting TOPSIS model and entropy weight method to comprehensively evaluate soil ecological carrying capacity of different communities.The evaluation results of soil ecological carrying capacity of different communities showed that the Pinus thunbergii forest had the highest carrying capacity,and the Quercus acutissima forest and mixed forest were low at 0.37 and 0.38,which belonged to the lower level of the carrying capacity evaluation.Through the research on the soil ecological carrying capacity and its evaluation indicators of different communities in the dry and barren mountainous area of Yimeng Mountains,it is found that the Quercus acutissima forest,Pinus thunbergii forest and mixed forest have more sand and silt content,low water storage performance,and accelerated nutrient loss,and affect the availability of soil nutrients.At the same time,litter input has an impact on soil microbial metabolism and nutrient cycling,which in turn affect soil ecological carrying capacity.The soil moisture and nutrient contents of the Quercus acutissima forest and mixed forest were low,and they belonged to a lower level in the evaluation of soil ecological carrying capacity.Therefore,reasonable interventions should be carried out on the Quercus acutissima forest and shrub,and comprehensive technical means such as optimizing the soil and litter environment should be used to improve the soil ecological carrying capacity,and promote the growth of the Quercus acutissima forest and mixed forest in the management in the Yimeng mountain.