| Forest harvesting is a major method of forest management, and has complex effect on soil carbon pool and carbon sequestration. Understanding the characteristics of soil carbon pool and main carbon sequestrating process to harvesting are vital to make proper forest management methods and to improve carbon sink level in forest ecosystem, and is the focus at home and abroad. The Three Gorges Reservoir Area that locating in midstream of Yangtze River is the key ecological and environmental protection area in China. As the main vegetation type of Three Gorges Reservoir Area, Pinus massoniana has large distribution area but low quantity due to extensive management, and that might be chief reason for low carbon density of forests in this area. So it is necessary to start a management experiment in Pinus massoniana stand and to observe soil carbon storage and main carbon sequestrating processes after management, which can provide basic data for exploring possible influences of climate change in soil carbon pool and nutrient cycle, and can provide important guiding and practical implications for Pinus massoniana management.A aerially seeded Pinus massoniana forest of similar site condition in Jiulingtou forest farm of Zigui county, Hubei province was chosen as subject, and considering regular management method four harvest types was designed: i.e., shrubs-cutting(harvesting shrubs, and moving all residuals), harvest 1(harvest intensity 15% without moving harvest residuals), harvest 2(harvest intensity 70% without moving harvest residuals) and control, soil respiration and its components was observed continuously immediately after harvesting for 1 year by using LI-8100 infrared gas analysis in situ, the traditional physical and chemical methods was used to analyze seasonal variation of soil organic carbon and relative environmental factors of Pinus massoniana stand with different harvesting methods, and PLFA and substrate method was selected to observe responses of soil microbial community structure and soil enzyme to different treatments. The results showed that:(1)Total soil respiration rates of control, shrubs-cutting, harvest 1, harvest 2 were 2.18±0.05, 1.82±0.07, 2.37±0.07, 2.86±0.1μmol CO2m-2s-1 respectively during investigation period. Shrubs-cutting reduced total soil respiration rates significantly, and total soil respiration rates of harvesting(harvest 1 and harvest 2) increased positively with harvesting intensity. Only shrubs-cutting reduced annual soil respiration amount by 26.24% significantly of three disturbing treatments, and harvest showed no effect on it. The annual soil respiration amount of control, shrubs-cutting, harvest 1, harvest 2 were 2.82±0.17, 2.08±0.09, 2.87±0.19, 2.97±0.12 kg CO2m-2 separately. The change of soil temperature and soil organic carbon content were main regulation factors for total soil respiration.(2) Trenching method was used to isolate soil respiration component RR(autotrophic respiration) and RM(heterotrophic respiration), results showed that contribution rate of RR to total soil respiration was between 35.75% to 40.44%, and 59.65%~64.25% for RM. Compared with control, shrubs-cutting reduced RR and RM simultaneously, and variation rage was 17%、18.9%; Harvest 1 reduced RR but increased RM, and harvest 1 had no influence in RR but increased RM significantly. Meanwhile, shrubs-cutting reduced RR and RM annual respiration amount significantly, and harvest1 and harvest 2 showed no apparent effect. Fine root biomass and soil temperature were main factors for the change of RR, and soil temperature is key part for regulating RM, which could explain 36.7%~88% variation of RM.(3) Soil respiration rates and it components reached maximum in vigorous growth period of all treatments, soil temperature and soil moisture Two-Factor Model could interpret total soil respiration and its components variation very well. Q10 value of control, shrubs-cutting, harvest 1, harvest 2 were 2.18±1.09, 1.65±0.07, 2.20±0.09, 2.36±0.09 respectively. Compared with control(2.16±0.19), shrubs-cutting(2.29±0.23) and harvest 2(2.18±0.15) improved effects of soil temperature on the variation of RR during the investigation time, and harvest 1(1.60±0.13) showed opposite results. Temperature sensitivity of RM of harvest 1(2.41±0.14) was higher than control(2.18±0.13), but both shrubs-cutting(1.71±0.09) and harvest 2(1.99±0.08) reduced RM temperature sensitivity.(4) Soil microbial biomass and community structure of soil top layer(0~10cm) in different period after treatments(i.e. three months after harvesting; twelve months after harvesting) of each treatments were analyzed, the results showed that: microbial biomass of trenched and untrenched plots had no significant differences; gram positive bacteria biomass of shrubs-cutting was higher than control in twelve months after harvesting; and biomass of fungi and arbuscular mycorrhizal fungi(AMF) in harvest 2 of trenched plots was higher than control; there was no significant difference of microbial community structure in all treatments in three months after harvesting, and at the later investigation shrubs-cutting and harvest 1 showed quite different from control and harvest 2. Redundancy analysis(RDA) indicated that soil temperature was the main reason for the change of soil microbial community structure, and vegetation composition was another key regulation factor.(5) Different treatments had various effects on soil enzymes activity: in three months after harvesting, there was no obvious change law for soil enzymes activity. And later(twelve months after treatments), compared with control, harvesting methods(harvest 1 and harvest 2) increased activity of ?-Glucosidase but reduced activity of N-acetyl-glucosaminidase and leucine-amino-peptidase, so as to oxidase activities(phenol oxidase and peroxidase); phenol oxidase activity of shrubs-cutting was higher than control, and soil enzymes activity left were lower than control. Soil moisture, soil organic carbon content and soil microbial biomass carbon were major environmental factors that influenced soil enzymes activity significantly, and gram positive bacteria group was the chief microbial group influenced soil specific enzymes activity. Each treatments changed soil microbial community structure and related enzymes activity by regulating environmental factors, which then showed different effects on soil carbon emission, however, the regulation mechanism needed to be analyzed further.Generally speaking, after one year observation and analysis of soil respiration and related microbial factors under different harvesting method, we found that shubs-cutting could reduced soil respiration rates and annual respiration amount significantly, changed microbial community structure and lowerd all soil enzymes activety except peroxidase activity. Soil respiration rates and annual respiration amount of Harvesting(harvest 1 and harvest 2) were higher than control, and increased with harvest intensity; only harvest 1 had changed microbial community structure signicicantly, both harvesting method had reduced soil enzyme activity that related to carbon cycle. The variation of vegetation composition, soil temperature and soil moisture under different harvest method were main regulation factors for the change of soil respiration and microbial properties. However, results of short period observation could not acount for the effect of harvest on soil carbon pool, carbon dynamics and related factors needed to be monitored further to explor incluence of harvest disturbances on soil carbon pool. |
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