Soil Organic Carbon Profile Distribution And Mineralization Characteristics Of Different Forest Stands In Jinyun Mountain

Since entering the industrial era,large-scale transformation of nature has accelerated the emission of greenhouse gases such as CO₂ and broke the global carbon balance.The forest organic carbon pool is the largest organic carbon repository on land,and its weak change can affect the significant change of atmospheric CO₂ concentration,so how to realize the efficient management of the soil organic carbon bank of the forest ecosystem has become a hot spot of research.In this study,the distribution of soil organic carbon and its active components,soil aggregates and aggregate organic carbon in soil profiles of 0²0,20⁴0,40⁶0 and 60¹00 cm under different stands were studied,and the characteristics of soil organic carbon mineralization under different stands were studied by indoor culture in 5 subtropical typical stands of bamboo forest,broadleaf forest,coniferous forest,coniferous and broadleaf mixed forest and grassland in Jinyun Mountain.In order to clearly define the effect of stand types on the distribution and stability of organic carbon in forest soil and provide the basic data for the efficient management of the soil carbon bank of the subtropical forest ecosystem.The main results showed as follows:?1?On the 0¹00 cm soil profile,soil organic carbon,microbial biomass carbon and readily oxidized organic carbon content decreased with soil depth under the 5 kinds of stand.Except for coniferous forest,the content of soluble organic carbon in the other 4kinds of forests decreased with the increase of soil depth.The coniferous forest showed a"V"character that first increased and then decreased.The average organic carbon content of the soil profile is the highest in bamboo forest in 5,followed by broadleaf forest,the average organic carbon content were 16.74g/kg and 12.62g/kg,respectively,and the lowest in coniferous forest,which only were 5.98g/kg.Stand types had significant effects on soil active organic carbon fractions.Soil microbial biomass carbon content in bamboo and broad-leaved forests were 166.87mg/kg and 182.19mg/kg respectively,which were higher than other stands.The content of soluble organic carbon in grassland soil were 35.81¹41.98 mg/kg,which was significantly higher than that in other forest stands?P<0.05?,and there was no significant difference in soil soluble organic carbon between the 4 other stands.The average content of soil readily oxidize organic carbon under different stand was bamboo forest?2.65g/kg?>broadleaf forest?2.31g/kg?>grassland?1.98g/kg?>mixed forest?1.93g/kg?>coniferous forest?1.72g/kg?.?2?The proportion of soil organic carbon in different forest stands was highest in the proportion of easily oxidized organic carbon,which was between 11.92%⁴2.46%.The soil microbial entropy?SMBC/SOC?and DOC/SOC?distribution ratio of soluble organic carbon in soil?were lower than ROC/SOC,which were between 0.46%².66%and 0.08%⁰.97%.In this study,grassland was used as a control to calculate soil carbon pool management index under different stands.The carbon pool management index of 4kinds of stand decreased with the increase of soil depth.The average carbon pool management index of bamboo forest soil profile reached 122.83,which was significantly higher than that of mixed forest and coniferous forest,and the lowest is coniferous forest with an average of 98.2.The soil organic carbon storage under the 5kinds of stand was the highest in bamboo forest and grassland,reaching 186.85 t/hm²and 162.02 t/hm².Compared with the coniferous forest,these were significantly increased by 97.19%and 72.87%.?3?There are significant differences in soil aggregate distribution under different stands.>2 mm aggregates were the main components in bamboo and grassland,and their contents were between 30.73%⁵3.08%and 39.95%⁵3.5%.The contents of2-0.25 mm aggregates in broad-leaved and mixed forests were higher than other particle size aggregates,and their content were 36.27%⁴4.39%and 48.69%⁵2.44%.The2-0.25 mm and<0.053 mm aggregates were higher than other particle size aggregates in coniferous forest aggregates.At the same time,there was no obvious rule of soil aggregate composition between the layers.As far as soil aggregate stability was concerned,bamboo and grassland could effectively increase the MWD,GMD and R_0.25values of aggregates,and decreased the fractal dimension,it could be seen that the stability of soil aggregates in bamboo forest and grassland is good.With the soil layer deepened,the content of organic carbon in the soil aggregates of the other 4 kinds of forest except the coniferous forest gradually decreased,among which the bamboo forest was the highest,which was significantly higher than that of the coniferous forest and the mixed forest.As far as the particle size of different aggregates was concerned,the organic carbon of the 5 forest soil aggregates had no obvious regularity in the whole soil profile,but the organic carbon content of 2-0.25 mm and<0.053 mm aggregates was higher.There was significant difference in the relative contribution rate of organic carbon in soil aggregates under different stands,in which the contribution rate of<0.053mm aggregate organic carbon in coniferous forest was the highest,and the contribution rate of organic carbon in the>2 mm aggregate of bamboo and grassland was27.44%⁵3.47%and 38.87%⁵3.19%,respectively,while in the broad-leaved and mixed forests,the contribution of 2-0.25 mm to aggregates is the highest.?4?The change trend of soil organic carbon mineralization rate were the same under different stands,which were decreased with the increasing of incubation time.During the mineralization period,it could be divided into 3 stages:0¹0d was a fast descending stage,10³0d was slow,and after 30d,it tends to be stable.In addition,the soil mineralization rate of 5 forest types decreased with the deepening of soil layer.The type of stand and depth of soil layer have significant influence on the mineralization rate.In the early stage of culture,the mineralization rate of bamboo and broad-leaved forest in 0²0 cm soil layer is higher,but the mineralization rate of the coniferous forest in the 60¹00 cm soil layer is significantly higher than that of the other forest.In the entire incubation period,soil organic carbon cumulative mineralization in bamboo and broad-leaved forests in the 0²0 cm soil layer were the highest,which were177.66mg/kg and 120.38mg/kg respectively.With the increase of soil depth,soil organic carbon cumulative mineralization in coniferous forest were higher than other forest stands,and in the 60¹00 cm soil layer,the total mineralization amount of coniferous forest reached 46.96mg/kg.The highest mineralization intensity of 5 forest stands is coniferous forest,with a variation range of 8.98 per thousand¹0.1 per thousand,which is significantly higher than that of other stands.The correlation analysis showed that the total soil organic carbon,microbial biomass and readily oxidized organic carbon under different stands were significantly related to the cumulative mineralization of soil,and there was no significant correlation between the soluble organic carbon and the accumulated mineralization and mineralization intensity.?5?Fitting of two-pool first-order model was performed to indicate that the contents of labile SOC fraction decreased with soil depth under different stands.However,the influence of stand types on labile SOC fraction is different.In the 0²0cm soil layer,the contents of labile SOC fraction in bamboo forest was the highest?84.06 mg/kg?,while in the 60¹00 cm soil layer,the coniferous forest was the highest.In the process of mineralization,the C₀/C₆₆ and C₀/SOC of the coniferous forest is the highest,the bamboo forest is the lowest,but the highest value of k and h in the whole soil profile is the broadleaf forest and the coniferous forest.The results showed that the mineralized ability of the coniferous forest was stronger,the mineralization rate of recalcitrant organic carbon could be improved effectively,and the utilization degree of the recalcitrant organic carbon pool was high,but the soil microbial activity of the bamboo and broad-leaved forests was higher,and the carbon cycle could be promoted effectively.