| In September 2020,China announced to the world that it would strive to achieve "carbon peak "by 2030 and "carbon neutralization " before 2060.Against this backdrop,the carbon sequestration potential of Chinese terrestrial ecosystems has attracted unprecedented attentionand become a frontier hot topic of concern of the government,academia,and the public.This study established experimental plots in the red soil and red-yellow soil erosionprone areas of the subtropical mountainous hills in Beiluoxiao Mountain(located in Lutou State-owned Forest Farm,Xiangdong District,Hunan Province),Typical forest types were selected by taking into account the impact of forest type conversion on soil erosion,soil organic carbon(SOC),and the changes in forest ecosystem carbon storage.Typical forest types(Natural secondary mixed forests are primarily protected,natural bamboo forests are primarily thinned on a regular basis,Cunninghamia lanceolata forests are primarily intensively managed,and Grain-for-Green is based on the notion of near-natural forest management)were selected,and field experiments and laboratory analyses and model simulation were conducted to determine the dynamic distribution of SOC in different forest types,as well as external factors related to soil carbon formation,distribution,transformation,emission,and migration.Soil erosion and SOC migration in different forest types were studied based on soil 137Cs tracing technology.The erosion-deposition-carbon model(EDCM)was applied to simulate and predict the dynamic changes in soil loss,SOC migration,and forest ecosystem carbon storage from 1958 to 2050 in different forest types,revealed the interaction and mechanism of forest type conversion with soil erosion and spatial-temporal distribution of SOC.(1)Based on sampling and analysis,the carbon sequestration capacity of the forest ecosystem in the experimental area is relatively high,and the SOC,3.07 times of the plant carbon,is the most important carbon pool in the forest ecosystem.SOC accumulation,storage,and dynamic changes in different forests are significantly different.The SOC reserves in Secondary mixed forests,Cunninghamia lanceolata forests,Cyclobalanopsis glauca forest,Castanopsis eyrei forest,Natural bamboo forest,and Grain-for-Green are 15626 g C m-2,21400 g C m-2,21003 g C m-2,22537 g C m-2,20759 g C m-2,and 17636 g C m-2,respectively.The average carbon storage of each forest type is 268.31 t C hm-2,slightly higher than the national average(258.83 t C hm-2).In each forest type,the vertical distribution of SOC has a significant surface aggregation phenomenon,and the horizontal distribution is influenced by the input,storage,and transformation of organic carbon,showing significant spatial heterogeneity.Without considering other factors,forest management activities can affect organic carbon input and emissions by affecting stand growth and biomass carbon accumulation,as well as SOM decomposition.Therefore,from the perspective of increasing the forest ecosystem carbon storage,especially SOC storage,interference with stable stands should be minimized,and scientific and reasonable nurturing measures should be taken to improve the productivity of low-yielding forests and residual forests,thereby increasing the ecosystem carbon storage.(2)Through sampling and analysis,the contents of soil aggregates with high stability(>0.25mm)and low stability(<0.25mm)were measured in the secondary mixed forest,Cunninghamia lanceolata forest,and abandoned land reforestation test forest in the experimental area.The results showed that the content of high stability aggregates ranged from 67.98%to 98.40%,and that of low stability aggregates ranged from 1.13%to 19.51%.The soil aggregates were mainly distributed in the>2 mm particle size category,and the content of large aggregates and aggregates with a particle size of>2 mm decreased with increasing soil depth,while the distribution of aggregates of other particle sizes was not clearly affected by soil depth.Forest management activities tended to distribute soil aggregates of various particle sizes evenly,with the most uniform distribution of aggregates with a particle size of 2-0.5 mm found in the Cunninghamia lanceolata forest.Due to the distribution of soil aggregates,there were significant differences in aggregate stability and SOC content among different forest types.The SOC content of soil aggregates with different particle sizes in the Cunninghamia lanceolata forest was significantly higher than that in the Secondary mixed forest,with the SOC content of aggregates with a particle size of 2-0.5 mm in the Secondary mixed forest being 69.1581 g C kg-1,significantly lower than that in the Grain-for-Green(118.942g C kg-1).The stability of soil aggregates was best in the Secondary mixed forest,followed by the Cunninghamia lanceolata forest,and relatively poor in the Grain-for-Green.The impact of forest management activities on soil aggregates and SOC content was mainly reflected in aggregates with a particle size of>2mm.The content of large soil aggregates determined the stability of soil aggregates and the distribution of SOC.There was a high consistency between the distribution of soil aggregates and the distribution of SOC.The larger the content of soil aggregates,the higher the content of organic carbon in the soil.The distribution of soil aggregates and SOC were greatly affected by forest management activities,and moderate management was conducive to the formation of soil aggregates and the storage of SOC.(3)Based on soil 137Cs measurement and migration principle,the soil erosion modulus(SEM)of Secondary mixed forest,Cunninghamia lanceolata forest,and abandoned land reforestation experimental forest in the test area ranges from-1939.577 to 3848.464 t·km1·a-1.Soil erosion intensity was mild in most of the sample sites,and moderate in some of them.The spatial distribution of soil erosion intensity is consistent with forest type and slope position distribution.The average slope erosion rate for each forest type in the test area is 1J38.354 t·km-2·a-1,and the average sediCentation rate at the foot or bottom of the slope is 584.163 t·km-2·a-1.After the slope erosion,some sediment still outputs through runoff from the foot or bottom of the slope,and the average erosion rate at the watershed level is 754.192 t·km-2·a-1.Along with soil erosion,SOC also undergoes migration.The spatial heterogeneity of soil erosion and SOC is the result of the comprehensive effects of factors such as forest management,topography,soil texture,and.physical and chemical properties.Forest management activities have a complex impact on soil erosion,with Cunninghamia lanceolata forest showing the most severe soil erosion,followed by Secondary mixed forest,and the least soil erosion in the Grain-for-Green.In addition,there is a significant correlation between soil texture,soil aggregates,and soil erosion.The higher the content of soil aggregates,the lower the soil erosion.The impact of soil erosion on the distribution of SOC mainly occurs in the 0-40cm soil layer.Using the soil erosion and SOC migration models based on soil 137Cs migration principle and under the current management scenario,the simulation shows that the soil erosion of each forest type in the test area has been decreasing,with a 54.98%decrease in the 1970s compared to the 1960s,a 34.01%decrease in the 1980s compared to the 1970s,and the lowest point in the 21st century.In 2030,there is a weak upward trend.SOC increases over time due to the joint effects of soil erosion,forest biomass carbon input,and soil carbon emissions.The SOC of each management type generally shows an increasing trend over time,and the increase occurs in all layers,with an annual increase in SOC storage of 0.023 kg C·m-2·a-1 in the 0-100cm soil layer.(4)Through simulation and analysis using the EDCM model,it was found that the net primary productivity(NPP)of various forest communities in the experimental area has shown a fluctuating upward trend over the years,which is consistent with the trend of SOC changes.This indicates that all forest types in the experimental area are currently in a period of vigorous growth,and forest growth plays a decisive role in SOC sequestration.The forest SOC in the experimental area is divided into active,slow,and passive pools,with the passive pool having the largest storage and the active pool having the smallest.The slow pool storage of SOC for each forest type in the experimental area has shown a downward trend over time due to management activities,while the storage of the active and passive pools has decreased over time in Secondary mixed forests,Cyclobalanopsis glauca forest,Grain-for-Green,Castanopsis eyrei forest,and Cunninghamia lanceolata forests.However,the storage has shown an increasing trend in Natural bamboo forest.Different forest types also showed significant differences in soil erosion due to forest management activities.Cunninghamia lanceolata forest have been experiencing soil erosion for many years,while Secondary mixed forests showed erosion followed by sedimentation,and Grain-for-Green showed a shift from erosion to sedimentation,resulting in an average annual increase in soil thickness of 2.3 mm after the formation of Secondary mixed forests.(5)According to comprehensive research,soil erosion,changes in forest NPP,and the distribution of carbon and SOC in forest ecosystems are the result of the combined effects of natural geographical conditions and human interference.Soil aggregate particle size composition and stability are the most important natural factors affecting soil erosion and the distribution pattern of SOC,while forest management practices are the most important human factors affecting soil erosion and the distribution pattern of SOC.SOC storage is closely related to forest biomass accumulation,but has a lag compared to it.Dynamic changes in SOC occur with soil erosion,and similarly,have a lag effect.Changes in forest ecosystem carbon and SOC are more sensitive to human interference,both fluctuating with the increasing intensity of human interference measures but changes in forest SOC have an obvious lag effect on human activity.The influence of forest management practices on soil erosion and SOC storage is complex,and reducing interference with stable forest stands is beneficial to SOC storage.Returning farmland to forest is a good ecological protection measure that helps reduce soil erosion.Managing and operating the various forest types in the experimental area in accordance with current management models will contribute to achieving the goals of "carbon neutrality" and "peak carbon emissions". |
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