| Forest ecosystem is the largest carbon pool of terrestrial ecosystem,and vegetation restoration is an effective method to improve the carbon sequestration of forest soil,which is of great significance to mitigate global climate change and achieve the carbon peaking and carbon neutrality goals.Coastal zone protected forests is an important part of China’s national protected forests system.In the past 20 years,due to the implementation of six major forestry projects,the area of coastal zone protected forests has continued to grow.However,due to the high initial afforestation density and the lack of scientific forest management,the problem of low forest quality and lack of ecological function is very prominent.Thinning,as one of the main measures of forest management,can change litter decomposition and nutrient release,soil physical and chemical characteristics,microbial community characteristics,etc.by adjusting forest density,thus affecting the carbon sequestration function of forest soil.Therefore,it is of great significance to study the effects of thinning management measures on the sequestration and accumulation of soil carbon pool for improving the forest quality and carbon sequestration function of coastal zone protected forests.In this study,the coastal zone protected forests（Quercus acutissima Carruth.,Pinus thunbergii Parl.and mixed Quercus acutissima Carruth.and Pinus thunbergii Parl.,ie the QAC,PTP and QP,respectively）in Qingdao were taken as the research object,analyzes the spatial distribution of soil organic carbon through geostatistical methods,and calculates the decomposition rate of soil organic carbon using stable isotope technology to study the spatial distribution of soil organic carbon and its impact mechanism in coastal zone protected forests.On the basis of the above,the coastal zone protected forests in the study area was thinned with three intensities（control 0%,light 15%and heavy 30%）,and the structural equation for the effect of thinning on the soil carbon pool of coastal zone protected forests was constructed from the viewpoint on effect of thinning intensity on soil carbon pool mediated by soil microbial communities and its necromass carbon in coastal zone protected forests,using the methods of ecological stoichiometry,high-throughput sequencing and determination of microbial necromass biomarkers.The characteristics of litter decomposition and its ecological stoichiometry,soil microbial community and its necromass carbon change,soil carbon fractions and carbon pool stability of the protective forest after thinning were systematically studied,and the response mechanism of the soil carbon pool in coastal zone protected forests to thinning was revealed.The main results are as follows:（1）The spatial distribution of soil organic carbon in coastal zone protected forests was mainly controlled by the type and density of aboveground vegetation,the soil carbon sequestration capacity of QAC and QP was stronger than that of PTP.The variation range of soil organic carbon in the study area was 5.85 g kg-1 to 28.05 g kg-1,with a variation of29.07%,showing a moderate spatial variability.The soil organic carbon and the normalized difference vegetation index had a significant positive correlation,taking it as a covariate could improve the spatial prediction accuracy of soil organic carbon.The types of protected forests had a significant impact on soil organic carbon and its spatial distribution,that is,the PTP in the west of the region was distributed in a large area of extremely high value.The decomposition rate of soil organic carbon in QAC and QP was lower than that in PTP.（2）Thinning reduced the decomposition and nutrient release of the litter in the coastal zone protected forests,which was affected by the change of litter quality and canopy density,and the above results were more easily observed in the QAC.Heavy thinning significantly reduced the litter mass remaining and decomposition constant of QAC,with a reduction of 46.82%and 51.02%,respectively,compared to light thinning and control,and there was no significant difference between the PTP and QP under the two thinning intensities compared with the control.The carbon,nitrogen and phosphorus nutrient remaining of QAC were significantly increased under heavy thinning,which were 74.20%,58.92%,and 66.67%higher than the control,respectively,and thinning significantly reduced the carbon nutrient remaining of PTP.In the QAC,the canopy density was significantly positively correlated with the decomposition constant,and the canopy density was significantly negatively correlated with the remaining amount of carbon,nitrogen and phosphorus nutrients.（3）Thinning could change the structure of soil microbial community by changing the core phylum relative abundance of bacteria and fungi in coastal zone protected forests,most of the changes of soil microbial community could be explained by litter decomposition and soil physicochemical properties,and they could explain the variation of soil fungal community to a higher degree.The core phylum of fungi in QAC,PTP and QP were Basidiomycota and Ascomycota,and the core phylum of bacteria were Acidobacterota,Proteobacteria and Verrucomomicrobiota.Thinning had changed the relative abundance of the core phylum of bacteria and fungi.The degree of interpretation of litter decomposition and soil physicochemical properties on the variation of fungal and bacterial communities in QAC was 91.87%and 75.06%,respectively,88.71%and 60.78%for PTP,88.96%and68.11%for QP.（4）Thinning increased the content of soil fungal and bacterial necromass carbon in coastal zone protected forests.The proportion of soil fungal and bacterial necromass carbon in soil organic carbon increased with the increase of thinning intensity.The variation range of soil fungal necromass carbon s was 3.06 g kg-1 to 12.11 g kg-1,the variation range of soil bacterial necromass carbon was 1.30 g kg-1 to 2.90 g kg-1.The change of fungal and bacterial necromass carbon was affected by the core phylum relative abundance of bacteria and fungi,respectively.Thinning increased the content of soil fungal and bacterial necromass carbon and its proportion in soil organic carbon in QAC and PTP,while thinning had a relatively small impact in QP.The ratio of fungal necromass carbon to soil organic carbon in QAC and PTP was negatively correlated with Basidiomycota.In addition,the ratio of fungal necromass carbon to soil organic carbon in QAC was positively correlated with the Ascomycota.The content of bacterial necromass carbon and its proportion in soil organic carbon were negatively correlated with Proteobacteria,and positively correlated with Verrucomimicrobiota.（5）Thinning improved the stability of soil carbon pool in coastal zone protected forests,the change of carbon pool stability was affected by the change of bacterial and fungal necromass carbon,and was related to the mineral-associated mechanism of soil organic carbon.The variation range of soil particulate organic carbon in coastal zone protected forests was 4.91 g kg-1 to 11.73 g kg-1,the variation range of mineral-associated organic carbon was 9.14 g kg-1 to 15.83 g kg-1,and the variation range of readily oxidizable organic carbon was 1.79 g kg-1 to 4.87 g kg-1.Thinning resulted in a significant reduction of soil readily oxidizable organic carbon and an increase in the stability of carbon pool in the QAC and QP.The content of soil fungal and bacterial necromass carbon in QAC and PTP was positively correlated with soil organic carbon and mineral-associated organic carbon,and negatively correlated with readily oxidizable organic carbon.The relationship between proportion of soil microbial（fungal and bacterial）necromass carbon in soil organic carbon and carbon fractions were different among the three types of protected forests,QAC and PTP were mainly affected by fungal necromass carbon,while QP was mainly affected by bacterial necromass carbon.（6）Thinning had different effects on the soil carbon pool in the three types of protected forests,but it was ultimately beneficial to the accumulation and stability of the soil carbon pool of the coastal zone protected forests.Different thinning intensities in QAC can improve soil organic carbon and its stability,which was achieved by reducing nutrient release from litter,changing microbial community structure,increasing the content of soil microbial necromass carbon and enhancing the mineral-associated mechanism of SOC.Different thinning intensities in PTP could improve the stability of SOC,which was achieved by increasing the carbon release of litter and reducing the nitrogen release,changing the structure of microbial community,and increasing the content of fungal necromass carbon.Different thinning intensities in QP could improve the stability of soil organic carbon,which was achieved by reducing the decomposition rate of litter,changing the structure of microbial community,and increasing the content of bacterial necromass carbon.In general,thinning was conducive to the accumulation and stability of soil carbon pool of coastal zone protected forests in the short term.However,further observation was needed for the long-term effect.From the perspective of improving soil carbon sequestration function,heavy thinning（30%）had the best effect.There was a mechanism that the effect of thinning intensity on soil carbon pool mediated by soil microbial communities and its necromass carbon in coastal zone protected forests,and changes in soil carbon pool were more easily observed in QAC and QP.Therefore,it was recommended to incorporate soil microbial necromass carbon into the research framework of the soil carbon pool of coastal zone protected forests.Distinguishing between bacterial and fungal necromass carbon was beneficial for deepening the understanding of the mechanism of the impact of thinning on soil carbon pool,and the soil carbon fixation function of QAC and QP under thinning conditions was more likely to be improved in the short term.In the management of coastal zone protected forests,attention should be paid to taking reasonable tree species allocation and selecting appropriate thinning intensity to improve the carbon sequestration capacity of soil.The research results could provide theoretical basis and scientific support for the improvement of forest quality and carbon sequestration function in coastal zone protected forests. |
