Effects Of Understory Vegetation Removal And Thinning On Soil Organic Carbon And The Related Processes In Pinus Massoniana Plantations

Increasing carbon sequestration in terrestrial ecosystems through scientific forest management has been widely accepted by the international community.However,the carbon storage is very large in soil,even if forest management has a small change in soil carbon,it will also have a great impact on the net carbon accumulation of the whole forest ecosystem.Understory vegetation removal and thinning treatments,as a tool for controlling the composition of tree species,are the major forest management technique used in plantations,and have a complex effect on soil carbon storage and carbon process.Scientific understanding of the mechanism of forest soil carbon pool maintenance and the characteristics of carbon sequestration process,the determination of reasonable forest management measures,and the increase of carbon sinks in forest ecosystems are the focus of current research at home and abroad.The Pinus massoniana plantations are the main afforestation species used in southern China,which are the largest area in the Three Gorges Reservoir area.However,the soil carbon stocks are relatively low there due to the impact of human activities coupled with the complex terrain in the region.In recent years,for the purpose of forest management activities,the focus of sustainable Pinus massoniana management has been to rebuild forest structure and optimize soil utilization.A aerially seeded Pinus massoniana plantation was chosen as subject,and considering regular management method four harvest types was designed:i.e.,assigned to one of four treatments:all shrubs harvested and residual materials removed?SR?,low-intensity thinning?LIT;mean basal area reduction of 15%?,high-intensity thinning?HIT;mean basal area reduction of 70%?and intact forest?CK?.In this study,we studied that the effect of treatment on soil carbon storage and carbon chemical structure,the effect of treatment on soil physical and chemical properties,microbial community structure and how they affect soil carbon content,and the effect of treatment on litter decomposition process by using elemental analysis,solid-state ¹³C nuclear magnetic resonance?NMR?spectroscopy,and the aggregate classification method,and litter or root decomposition bags methods.The results can be illuminated soil organic carbon and the related processes of Pinus massoniana Plantations as affected by understory vegetation removal and thinning treatments.The main results are as follows:1.In different treatments,the percentage of aggregates was the highest in the range of250-2000?m ranged from 46.78%to 53.87%,the fraction of silt?<53?m?was the least ranged from 2.99%to 5.91%.Soil organic carbon mainly distributed in large aggregates,accounting for about 80%of total soil organic carbon,and the content of organic carbon in soil aggregates was significantly different among different treatments.In water-stable aggregates,the response of large aggregates to silvicultural measures was more sensitive.In the SR and thinning treatments?LIT and HIT?,the amount of organic carbon binding in soil aggregates was significantly reduced comparing with the control treatment.2.The contents of total organic carbon,water-soluble organic carbon and easily oxidized organic carbon decreased with the increasing soil depth.Compared with the control treatment,the HIT treatment resulted in a significant increase in water-soluble organic carbon and easily oxidized organic carbon in 0-10 cm soil layer.3.Soil total nitrogen,total phosphorus,calcium content and moisture content all decreased with the deepening of soil layer while the soil pH showed the opposite trend.Compared with the control treatment,in the top soil layer of 0-10 cm and 10-20 cm layer,the thinning treatments significantly reduced soil total nitrogen content.4.Compared with the CK treatment,the cellulase and amylase activities in the soil were significantly decreased after thinning treatments,and the invertase activities in the soil were significantly increased under LIT and SR treatments.The fungal PLFAs was the highest(4.67nmol g^-1)in the soil treated with HIT,which was significantly higher than that in control treatment and increased the amount of fungal PLFAs,and the F/B of the soil in the sample was significantly higher than that of the fungal and bacteria PLFAs and it is more sensitive than total changes in microbial communities,fungi and bacteria.The result of correlation analysis showed that soil bacteria and fungi PLFAs were significantly correlated with soil nitrogen,nitrate nitrogen and water content,while enzyme activity was closely related to soil nitrogen and nitrate nitrogen.5.The HIT treatment significantly reduced the chemical stability of soil organic carbon.The composition of soil microbial community was the key to regulate soil organic carbon decomposition rate.After understory vegetation removal and thinning treatments,the composition of soil microbial community was affected by changing soil moisture and nitrogen content,which affected the decomposition of soil organic carbon and the content of soil organic carbon.6.In the four treatments,the decomposition coefficient of litter leaves?in situ?ranged from 0.511 to 0.567 year^-1,which increased the decomposition rate of litter leaves significantly after LIT and HIT treatments.The decomposition coefficient of fine roots?not in situin situ?was in the order of CK>SR>LIT>HIT.The thinning treatments significantly reduced the decomposition coefficient of fine roots.The carbon,nitrogen,and calcium contents of litter and fine roots tend to accumulate during decomposition while the C/N ratio was decreased pattern.Compared with the control treatment,the enzyme activities related to soil organic carbon cycle and litter decomposition rate were changed by the SR,LIT and HIT treatments.The content of 0-10 cm organic carbon and the binding amount of organic carbon in soil aggregates were significantly decreased in the SR,LIT and HIT treatments.HIT treatment resulted in a significant increase in water dissolved organic carbon and easily oxidized organic carbon,and decreased the chemical stability of soil organic carbon.The results of path analysis showed that the difference of soil organic carbon content was caused by the change of soil moisture and nitrogen content after treatment,which affected the composition and activity of microbial community,and resulted in the change of soil organic carbon content and active carbon content.