| Forest litter decomposition is an important step in terrestrial ecosystems soil organic matter, mineralization and formation carbon balance. The quantity and qualityof litter also play important role in soil carbon storgage and nutrient cycle. Forest soil carbon pools have40%of the global soil carbon pool, global annual litter returned by decomposition rate to soil organic carbon is about50Gt. The contribute of litter decomposition (including of the root litter) in total annual carbon flux was70%with about68Pg C a-1(Pg=1015g), so small changes in forest ecosystems will affect the process of soil-atmosphere carbon exchange and atmosphere CO2concentration.Three Gorges Reservoir Area locates in the middle and upper reaches of the Yangtze River, which is a key ecological environment protection area. Pinus massoniana forest is one of important vegetations of the Three Gorges Reservoir Area, studes on the relationship between litterfall, substrate quality, decomposition rate and nutrient return to soil nutrients would help to investigate the possible effects of climate change to soil caron storage and nutrients cycling and provide a scientific basis of suitability, and also contribute to further understanding forest floor dynamic assessment and litter-soil biochemistry continuum and provide a reference for sub-tropical regions sustainable forest management and increasing the exchange potential.Our object was studied on decomposition couse of younger stand with20years old (YS)、 middle-aged stand with30years old (MS) and older stand with46years old (OS) Pinus massoniana mixed litter and litter leaf by litter weight loss method from June2010to December2011, research and principal component analysis the main influence factors including climate (soil temperature and water content), substrate quality, soil fauna and nitrogen deposition and so on. The study would illustrate the influence and contribution of the litter quality and quantity imput to soil carbon pools. The results showed that: 1. The litterfall in20years old (YS),30years old (MS) and46years old (OS) Pinus massoniana stands were3.38,4.69and5.60t·ha-1·a-1, respectively. The monthly dynamic of total litterfall was similar with litter leaves in three Pinus massoniana stands, both were single peak mode, which was most in November, least was in February. The order of litterfall in each organs:floweres and fruits<barks<branches<debries<leaves. The proportion difference of litter organs was great, the proportion of litter leaves was most for the variation. The reproductive organs (flower and fruits) were highest with2.7%in OS, lowest with0.6%in MS. The time of flowers fall in MS was in February while YS and OS in May. Litterfall was inversely proportional with air temperature, and related but not significantly with rainfall and wind speed.2. The litter leaves decomposition rate with dry mass remain rate after540d were63.5%in YS,59.8%in MS and65.5%in OS. The decomposition mode in three stands was similar. The litter decomposition rate constant was0.29,0.33and0.30in YS, MS and OS, respectively. Predicted decomposition50%,95%in three stands were2.4a,10.36a;2.11a,9.15a;2.35a,10.07a. The mixed litter decomposition rate after360d were faster3.33%than mixed litter in YS, while the decomposition rate of mixed litter were faster10.38%,4.67%than the litter leaves in MS and OS, respectively.3. Litter decomposition rate was positively correlated but non-linear with soil temperature and water content instead by a polynomial function with soil water content/soil temperature ratio (SWC/ST). Litter decomposition rate was relatively fast when SWC/ST ratio was0.5-2.0, which was relatively slow when SWC/ST ratio was0.5-2.0. Litter decomposition rate in three stands was positive correlation with nitrogen content and N/P ratio, negative correlation with C/N ratio, and negative quadratic function with phosphorus content and Mn content, positive quadratic function with C/P ratio. Soil fauna obvious increased litter decomposition rate, micro-pore size<middle-pore size<large-pore size. Litter dry mass remain rate was positive correlation with polyphenoloxidase. The influence order of factors of litter decomposition was soil fauna<C, C/N ratio and C/P ratio<polyphenoloxidase and cellulase activities<soil water content/soil temperature ratio and soil water content<N and N/P ratio according to the principal component analysis. The indirect influence of soil temperature to litter decomposition rate was controlled by the carbon release rate instead of initial carbon content, and the litter decomposition rate was inversely proportional to initial carbon content by soil temperature.4. The influence of simulated nigrogen deposition to the litter decomposition rate was bigger in late stage than in early stage. Compared with control (CK), low nitrogen (LN) improved the decomposition rate while higher nitrogen (HN) slowed down decomposition rate after540d exposure. The influence of nigrogen deposition to carbon release was most in MS, followed by YS, least in OS, the influence of nigrogen deposition to the amount of nitrogen net retention and phosphorus net release was great. The response of soil fauna to nitrogen deposition was obvious. Analysis of variance results showed that forest age, processing time and N deposition on decomposition were significant.5. Soil organic matter and total nitrogen contents both were MS<YS<OS, while soil ammonium nitrogen was MS<OS<YS, nitrate nitrogen was OS<YS<MS. Soil pH value was obvious seasonal dynamic with unregular, opposite in bulk density. Soil carbon density dynamic in Pinus massoniana stands were different. In vertical, soil carbon density was decreased with soil depth. Soil carbon storage in0-20cm on different-aged Pinus massoniana including four seanson dynamic were MS<YS<OS, and seasonal dynamics difference of soil carbon dynamics was more and more obvious with stand age. The contribution of different soil layer in the same stand to the soil carbon storage was different, which was greatest in0-5cm.6. K, Ca, Mg nutrient contents by soil surface litter return were increased with stand age. Soil nutrient content was high with relatively high litter substrate quality, opposite, soil nutrient poor accompanied relatively low litter quality. Soil nutrient content was negative correlation with litter decomposition rate, litter decomposition rate from poor soil nutrient contents was faster than in soil rich nutrients. The higher was C/N ratio, C/P ratio in soil surface nutrient, retaining the faster was litter N content, the release sooner was P content. The influence order of litter index to soil nutrients and carbon pools were nutrients return<littfall<substrate quality (C/N ratio, lignin/N ratio<litter decomposition rate and carbon release rate.Carbon contents in litter layer were increasing with stand age, carbon pools of litter layers and soil in YS, MS and OS were1.85.39.52,2.62,26.12,3.22.66.69t·ha-1, respectively. The percentage of litter layers in soil carbon pools were4.68%,10.03%and4.83%, the contribution of litter layers to soil carbon pools were obvious bigger in MS than in YS and OS.7. The litter decomposition rate and substrate quality of coniferous stand, broad-leaved stand and mixed stand were obvious different. The order of litter decomposition rate in three forest types was Pinus massoniana stand-<Quercus variabilis stand<mixed stand. Litter substrate quality were significant difference, initial litter carbon content was mixed stand<Quercus variabilis stand<Pinus massoniana stand, carbon release rate after450d was fastest in mixed stand and lowest in Quercus variabilis stand, while nitrogen content was most in Quercus variabilis, least was in Pinus massoniana stand, the amout of nitrogen retention after450d were mixed stand (44.5%)<Quercus variabilis (44.9%)<Pinus massoniana (66.7%). Relationship between of litter substrate quality and decomposition rate kept consistent, litter decomposition rate in three types stands was positive correlation with nitrogen content and N/P ratio, negative correlation with C/N ratio, and negative quadratic function with phosphorus content and Mn content, positive quadratic function with C/P ratio.0-5cm soil organic matter content in three types stands were Pinus massoniana stand<mixed stand<Quercus variabilis stand. Soil carbon storage0-20cm in Quercus variabilis stand was higher than in Pinus massoniana and mixed stands except in November, and the contribution of0-20cm soil carbon pools in three types stands were0-5cm soil layer.In conclusion, litterfall, substrate quality, and annual nutrients return and decomposition rate together decided to soil carbon pools in Pinus massoniana stands. Annual litterfall and return contents were increasing with stand age, while substrate quality carbon was no significant difference, decomposition rate and carbon release rate were highest in MS, lowest in YS and soil carbon pools was lowest in MS, highest in OS. Soil carbon pools and nutrients contents dynamics in three Pinus massoniana stands were significantly different with litterfall, substrate quality and nutrients returns and opposite trend with littter decomposition rate, which indicated that the influence of decomposition rate to soil carbon pools was more important than annual litterfall and subatrate quality. The stands with poor soil nutrients contents would promote litter decomposition rate for relieving the restrictions of plant growth. Pinus massoniana stands with poor soil nutrients accompanied relatively poor carbon sequestration potential and would have greater potential for increasing carbon sink through the adjustment and optimization of forest age and mixed structure. |
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