| Cupressus funebris forest, one of the major forest types in Sichuan hilly basin, has lower ecological efficiency. Its lower efficiency limits the development of the regional forest ecosystem service function largely, and reduces carbon sink capacity and economic benefits of forest land. In order to increase regional forest ecosystem service functions and sustainable use of forest resources, we proposes a new transformation technology for lower-efficiency forest. This technology is called "GMTT", means treatment of mixing with Toona sinensis in gaping area, and is a technology of hardwood’s oriented cultivation. Basing on an important role of forest ecosystem in regional carbon cycle, this thesis was viewed from was from soil carbon pool function. In this study, half-mature C.funebris forest with low efficiency was the research object. Five GMTT treatments had been created with 0,50,100,150 and 200 m2 gaping size. We full explored the main approaches and the internal running mechanisms of influence of GMTT on soil carbon pool, by determining many indices of soil carbon pool (physical composition, soluble components, microbial biomass carbon, and stability of soil organic carbon) and its ecological environment (soil temperature and humidity, soil physical and chemical properties and soil microorganisms), explored deeply the relationship between all indexes, and simulated the direct way and indirect way of influence of GMTT on soil carbon pool by doing data mining with redundancy analysis (RDA), hierarchical cluster analysis (HCA), non-metric multidimensional scaling analysis (NMDS), decision tree method analysis (DTA) and structural equation model analysis (SEM). The results could provide scientific evidence for inefficient stand transformation and carbon sink forest construction.In this study, the effects of GMTT on soil carbon pool could be broken down into three stages. The first stage was the first year after planting. This stage was a break-in period between plant and soil, and the influence of GMTT on soil carbon pool in this stage mainly came from the human disturbances in land preparation, such as logging and replanting. The second stage was after two years since planting. In this stage, the relationship between plants and soil was relatively stable, and the influence of GMTT on soil carbon pool in this stage mainly came from treatment of gaping. The third stage was the long-term of mixed forest with GMTT, the influence of GMTT on soil carbon pool in this stage mainly came from treatment of mixing with T. sinensis. According to the above research frame, this thesis explored the gaping effects of GMTT on soil carbon fractions. The conclusions of this thesis have the following 7 points.(1) In the first phase of GMTT, Land preparation behavior was a disruption to soil carbon pool, and this impact was mainly manifested in the soil depth at 0-5 cm. Total organic carbon content, total nitrogen content, carbon and nitrogen ratio, microbial biomass carbon content, the stability of organic carbon content and carbon management index of forests with GMTT were lower than the control in the first stage. These showed that land preparation increased activities of soil organic carbon stock, after reduced stability.(2) In the second phase of GMTT, Both control forest and GMTT forest, Total soil organic carbon content decreased with the increase of soil depth. However, GMTT significantly improved soil light fraction organic carbon and particulate organic carbon content. In all samples, the ratio of particulate organic carbon to total soil organic carbon was 48.73±8.82%, which was 6.83 times greater than the ratio of light fraction organic carbon to total soil organic carbon (7.13±4.22%). Moreover, soil readily oxidizable organic carbon, soil dissolved organic carbon content and soil microbial biomass carbon with GMTT were a bit lower than control. It suggested that GMTT promote the decomposition of active carbon pool, and the accumulation of inert carbon pool. Among them, GMTT reduced hexose concentration in soil solution significantly, but increased proteins concentration in soil solution. These showed that GMTT increased degradation of litters, and that organic carbon composition with simple chemistry structure could be used quickly by soil microbe. Moreover, in contrast to CK, forest soil with GMTT had lower aromatic compounds concentration in soil solution at depth of 0-5 cm, but had higher aromatic compounds concentration in soil solution at depth of 5-20 cm. It suggested that the decomposition level of 0-5 cm soil layer was lower than 5-20 cm soil layer in the early phase after treatment.(3) According to the relationships among indexes and indexes’sensitivities to GMTT, this thesis found that total water-extractable carbon concentration, and hexose, pentose and aromatic compounds concentration of water-extractable soil solution had higher sensitivity to GMTT than other indexes. And the results of decision tree analysis showed that hexose concentration, pentose concentration and their ratio were the best possible indicator in representing the early effects of GMTT on soil organic carbon.(4) In the second phase of GMTT, GMTT impacted soil physical and chemical properties and soil microbial structure and function, but did not significant influence on soil temperature and humidity. On soil physical and chemical properties, total nitrogen content and available potassium content in 0-5 cm soil depth, available phosphorus content in 5-40 cm soil layer and total potassium content in 0-40 cm soil layer were significantly reduced, total phosphorus content in 5-40 cm soil layer was significantly increased, available potassium content in 20-40 cm soil depth was slightly reduced, and alkaline hydrolysis nitrogen content in 0-40 cm soil depth was slightly increased by GMTT. On the structure of microorganisms, population of bacterial in 20-40 cm soil layer and population of actinomycetes in 0-40 cm was restrained by GMTT, and fungi population in 0-5 cm soil layer was modified by GMTT. On the function of microorganisms, there was no significant influences on microbial functional intensity by GMTT, but was apparent effects on microbial functional diversity by GMTT. Among them, activities of diastase, β-glucoside enzyme, urease, invertase, polyphenol oxidase, peroxidase and catalase responded GMTT significantly. However, activities of acid phosphatase, alkaline phosphatase, protease and cellulose responded GMTT non-significantly.(5) The constraint percentage of total inertia of dataset of soil carbon pool, explained with dataset of soil temperature and humidity, soil physical and chemical properties, soil nitrogen cycle, soil microbial structure and function, and soil respiration alone, was 5.6%,55%,51%, 91% and 3.4% respectively. Among them, the percentage explained by dataset of soil physical and chemical properties and dataset of soil nitrogen cycle constraints to dataset of soil organic carbon pool were mostly contained in the percentage explained by dataset of soil microbial structure and function. The dataset of populations of the three major microbes could constraint 32.89% of total inertia of dataset of soil carbon pool, just was 53.20% and 48.23% of constraint percentage by dataset of soil microbial biomass and dataset of soil enzyme activity respectively. Among them,58.04% of constraint percentage by dataset of populations of the three major microbes was shared with the constraint percentage by dataset of soil microbial biomass and dataset of soil enzyme activity respectively, and only 0.85% of constraint percentage by dataset of populations of the three major microbes was unique. In contrast to GMTT forest, there were higher total nitrogen and available potassium content, lower total phosphorus content in 0-5cm soil layer of the soil physical and chemical environment of non-water-insoluble active carbon pool. And there were larger actinomycetes population, higher activities of diastase, β-glucoside enzyme, and invertase, lower activities of urease, protease and acid phosphatase in 0-5cm soil layer of the soil physical and chemical environment of non-water-insoluble active carbon pool. Moreover, Soil actinomycetes played the most comprehensive role in the microbial function, it was closely related to soil microbial biomass and function. Soil bacteria was just closely related to the biochemical process with soil microbial biomass carbon, and soil fungi was just closely related to the biochemical process with soil microbial biomass nitrogen. And these biochemical processes had significant correlation with microbial functional intensity, but not with microbial functional diversity.(6) The result of SEM showed that, the earlier influences of GMTT on soil carbon pool had two ways:direct and indirect. Direct way was that GMTT changed the Water-soluble active carbon pool directly. Indirect way was GMTT changed the Water-soluble active carbon pool through regulating soil microbial population and soil nitrogen cycle. Moreover, GMTT mainly changed microbial respiration of soil respiration indirectly, through effecting soil nitrogen cycle, soil water-soluble active carbon pool, and microorganism population and soil enzyme activity.(7) After C. funebris had been changed to T. sinensis, soil organic carbon density was increased. Under control forest, soil organic carbon density of 0-40 cm soil layer was 90.73 Mg·hm-2, and from top to bottom, the soil organic carbon density was 30.21,47.54 and 12.98 Mg·hm-2 in soil depth at 0-10,10-30 and 30-40 cm respectively. After Cupressus funebris had been changed to Toona sinensis, the increment (INEQSOCD) was positively related to area size of Toona sinensis (XCA). That basic meet the formula, INEQSOCD=XCA* 1.9 (Mg·hm-2).In conclusion, the biodegradation ability of forest soil carbon pool increased with the increase of gap size of GMTT in earlier. Together the results hinted that Different treatments level had different influence sphere on soil carbon mineralization. Plant residues decomposed firstly with lower treatment level, soil microbial carbon was broken secondly with increased treatment levels, and inert organic carbon was mineralized finally with larger increased treatment levels. Therefore, the three indicators, the ratio of the microbial biomass carbon and water soluble organic carbon, the ratio of the water soluble of hexose and pentose and water-soluble aromatic compounds content, would be considered in evaluating the earlier effect of GMTT on soil carbon pool. |
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