The Effect Of Vegetation Restoration On The Accumulation And Stability Of Soil Organic Carbon Pool In The Mid-subtropical Of China

Global climate change is the focus in the public and scientific community,and the hotspot in the research field of contemporary ecology.Soil organic carbon?SOC?is the largest reservoir of carbon?C?in terrestrial ecosystems.Changes in SOC pool were important factors affecting the C budgets of terrestrial ecosystems and the balance of global C cycle.Thus,to increase the accumulation and stabilization of SOC and to reduce CO₂ emissions from soil to atmosphere is an important part of the global climate change research.However,the impact and mechanism of vegetation restoration on SOC accumulation and stabilization are not fully understood.By using the method of space-for-time substitution,four types of vegetation communities?Loropetalum chinense-Vaccinium bracteatum+Rhododendron mariesii scrub-grass-land?LVR?,Loropetalum chinense+Cunninghamia lanceolata+Quercus fabri shrubbery?LCQ?,Pinus massoniana+Lithocarpus glaber+Loropetalum chinense coniferous-broad leaved mixed forest?PLL?and Lithocarpus glaber+Cleyerajaponica+Cyclobalanopsis glauca evergreen broad-leaved forest?LAG??were selected for constituting a secondary forest successional sequence in a hilly region of eastern Hunan Province,China.Permanent plots were established in each community.Soil samples were collected.Laboratory incubations of SOC mineralization and a three-component first-order kinetic model were used to study the dynamic change and stabilization of SOC pool during vegetation restoration in subtropical area of China.Our objectives were:?1?to reveal the SOC accumulation and its synergistic changes with the vegetation-soil along vegetation restoration;?2?provide scientific basis for improving the C sequestration potential in forest soil and managing the soil C pool scientifically.The main results were summarized as following:1.Along vegetation restoration,SOC content(C_soc)and SOC density(D_soc)increased dramatically.The C_soc was the highest in LAG,which was 12.45,9.31 and 4.74 g/kg higher than in LVR,LCQ and PLL in 0—40 cm soil depth,increasing by 248.53%,113.14%and 58.46%,respectively.The increments of D_soc in LAG at 0—40 cm soil depth were 67.12,46.13 and 32.47 tC/hm²,and increased by 181.96%,79.73%and 45.64%compared to D_soc in LVR,LCQ and PLL,respectively.Correlation analysis showed that C_soc and D_soc were strongly and positively correlated with species diversity index,community total biomass,aboveground biomass,root biomass?FB?,existing biomass in litter layer,nitrogen?N?,phosphorus?P?content in litter layer,soil total P?TP?,soil available P?AP?,soil C/N ratio(except C_soc),soil C/P ratio,soil N/P ratio and soil clay percentage?<0.002 mm?,but significantly and negatively correlated with C/N in litter layer(except D_soc),C/P in litter layer,soil pH and soil bulk density?BD?,suggesting that the differences in C_soc and D_soc under different vegetation stages were related to both vegetation and soil properties.The results of principal component analysis and stepwise regression analysis revealed that soil C/P,pH,soil clay percentage?<0.002 mm?(except C_soc)and C/P in litter layer were the dominant factors affecting C_soc and D_soc during vegetation restoration.Among them,soil C/P ratio ranked first.These results indicated that the differences in soil C/P ratio,pH,soil clay percentage?<0.002 mm?and C/P in litter layer were responsible for the changes in SOC during vegetation restoration2.SOC mineralization rate displayed similar trends under the four restoration stages:during the early incubation period,SOC mineralization rate was higher and decreased fast,and then the rate decreased slowly and tended to stabilize during the middle and late period.The dynamics of SOC mineralization rate was well fitted with the reciprocal equation?R²=0.875,P<0.01?.SOC mineralization rate and SOC cumulative mineralization?C_m?increased significantly with the vegetation restoration with the LAG having both the highest SOC mineralization rate and the highest C_m.In 0—40 cm soil layer,C_m in LAG was 359.06%—716.31%,112.38%—232.61%,94.4%-105.74%higher than that in LVR,LCQ,PLL soils,respectively.C_m/SOC were 2.13%-4.99%,3.42%-4.18%,4.05%-4.64%,and 4.02%—5.64%in 0—10,10—20,20—30,30-40 cm soil layers,respectively.However,there was no significant difference in C_m/SOC among the different restoration stages.Among different vegetation restoration stages,soil total nitrogen?TN?content and RB were the key factors affecting C_m.Soil TN content and RB together explained 97.8%variation in C_m,of which soil TN content explained 96.9%.However,soil C/N was the dominant factor affecting C_m/SOC and it alone explained 49.4%variation.Vegetation restoration promoted SOC mineralization rate and reduced C_m in SOC in our study,which might be related to the changes in RB and soil nutrient during vegetation restoration.3.There was no significant seasonal change in the soils of LVR,LCQ,PLL,or the 10—20 cm soil layer of LAG.In the 0—10 cm soil layer,SOC mineralization rate had no evidently seasonal change in LCQ,whereas those in the other three restoration stages were generally higher in spring than in summer and autumn.In the 10—20 cm soil layer,the SOC mineralization rate was higher in spring and summer than in autumn and winter in LVR,LCQ,and PLL,but not in LAG.Seasonal dynamics of C_m were different whether in the same soil layer of different vegetation restoration stages or in different soil layers of the same vegetation restoration stage.The seasonal variations in soil potentially mineralizable C?C0?content were basically consistent with those in C_m in the corresponding soil layer in all four vegetation restoration stages.The ratios of Co/SOC ranged from 4.24%to 9.17%and 2.64%to 10.07%in the 0—10 and 10—20 cm soil layer,respectively,and they were higher in spring than in summer and winter.Path analysis showed that the direct and main influencing factors for SOC maximum mineralization rate?C_max?were SOC and soil pH value,for C_m for were SOC and soil pH value,for Co were soil-TN and SOC,and for k were soil C/N and SOC.In addition,soil C/N had a strong negative effect on k.The residual path coefficients of C_max,C_m and Co were small,while the residual path coefficients of k was large,impling that vegetation factors,soil factors had a great influence on C_max,C_m and C0,and other factors had been neglected in this analysis that have a strong influence on k.These results suggested that the seasonal dynamics of SOC mineralization in different vegetation restoration stages were influenced by the comprehensive effects of soil factors.4.SOC,the active SOC?C_a?,the slow SOC?C_s?,and the resistant SOC?C_r?increased with the vegetation restoration,and the increment varied with SOC sub-pools.The smallest increment occurred in C_a?0.02-0.32 g/kg?,while the relatively larger increments observed in C_s?4.54-17.36 g/kg?or C_r?2.70-17.68 g/kg?.C_a had the lowest proportion to SOC among the three SOC sub-pools,while the relatively stable pools?e.g C_s or C_r?tended to have higher proportion to SOC.C_r/SOC showed an increase trend during restoration.These results indicated that vegetation restoration not only increases the amount of SOC,C_a,C_s,and C_r,but also promotes the stability of the SOC pool.FB was the main driver that controls SOC sub-pools in the process of vegetation restoration.FB explained 80.7%,66.7%and 79.2%variation in C_a,C_s and C_r,respectively.In addition,the C/N of litter had more crucial roles in C_a and C_s than did the other factors,while the soil clay percentage?<0.002 mm?contributed secondly to C_r.This illustrated that the FB,litter quality,soil texture are critical for regulating SOC pool and improving SOC stability during vegetation restoration.5.SOC mineralization rate displayed similar trends under different incubation temperatures,and higher incubation temperature sustained in a relatively higher value for a longer time,and decreased greatly.SOC mineralization rates at the four restoration stages in each soil layer showed significantly and exponentially rising trends with the incubation temperature increased from 15? to 45?.Exponential model could well simulate the relationship between SOC mineralization rate and incubation temperature?R² = 0.4841-0.6725,P<0.01?.A rise of incubation temperature could promote SOC mineralization rate and C_m and it had a significant interaction with vegetation restoration and soil depth.More specifically,the influences of incubation temperature on SOC mineralization rate and C_m strengthened with vegetation restoration,while weakened with the increasing soil depth.The temperature sensitivity of SOC mineralization(Q₁₀)ranged from 1.45 to 1.65 at different soil layers.Q₁₀ showed a general trend of decline with vegetation restoration and soil layers deepened.In LVR and LCQ,the differences of Q₁₀ among the different soil layer were significant.These results indicated that vegetation restoration would be helpful to reduce the sensitivity of SOC mineralization to warming,and enhance SOC stability to a certain degree.The increasing soil depth also reduced the temperature sensitivity of SOC.Qio was strongly and positively correlated with soil TN and microbial biomass C?Be?content,but significantly and negatively correlated with soil BD,TP and microbial biomass P?B_P?content.Therefore,reasonable management strategies should be adopted to promote the vegetation restoration,improve the soil fertility,enhance the nutrient supply capacities,reduce the temperature sensitivity of soil C pool and the soil C release,and strengthen the soil C sink capacity.