| Fine root decomposition plays an important and irreplaceable role in mass cycle and energy flow of the forest ecosystem, and is controlled by integrated biotic and abiotic factors. Due to the severe weathering of the soil in the subtropical regions, N is relative surplus, while the phosphorus (P) availability is low, and under the background of global climate change, the N/S deposition has intensified potential P limitation of the ecological system. So, P may be the more important limiting factor in fine root decomposition in the region. We conducted fine root decomposition in the mid-subtropical forest by using the method of litterbag in Castanopsis natural forest in Sanming C.kawakamii Nature Reserve. Fine root of castanopsis carlesii, Cunninghamia and its mixture were chosen as the research object. With various P addition treatment, extracellular enzymes activity were measured in the process of decomposition, to better understand the mechanism of fine root decomposition in mid-subtropical forest ecosystem. The major results of this study were summarized as follows:(1) In 2 years of decomposition, the fine roots of Castanopsis carlesii and Cunninghamia was fast at first and slow at the later stage. Fine roots dry mass loss rate of were showed as MP (90.16%)> LP (90.09%)> HP (88.28%)> CT (87.51%) for Castanopsis carlesii, and MP (87.76%)>LP (87.46%)> HP (84.11%)>CT (80.39%) for Cunninghamia. P addition increased dry mass loss rate of fine roots, but the decomposition rate did not accelerate with higher levels of adding P. The decomposition coefficient (K) were MP (0.3811)>LP (0.3771)>HP (0.3655)>CT (0.3549) for Castanopsis carlesii, and MP (0.3048)>LP (0.2565)>HP (0.2436)>CT (0.2212) for Cunninghamial. The effect of P addition on the Cunninghamia root decomposition rates differed significantly. This suggests that adding P increased the overall fine root decomposition rate, and played a positive role in fine root decomposition, while the enhanced effect is stronger for Cunninghamia fine root which is lower P availability. But there showed no phenomenon of the decomposition acceleration with P fertilizer levels increased in this study.(2) In 2 years of decomposition, mixed fine roots decomposed faster in the early stage and slower in the later stage, the dry mass loss rate were as followed:MP (84.65%)>HP (81.29%)>LP (81.65%)>CT (69.14%), indicating that P addition enhanced its dry mass loss rate and decomposition constant K was MP (0.3021)>HP (0.2692)>LP (0.2519)>CT (0.1260). Mixed fine root decomposition rates were significantly increased with P addition. The value of mass remaining in all treatment at 180 days is lower than the predicted values, showing that the fine root decomposition rate was increased at the beginning, and then inhibited in 180-720 days as the measured value higher than the predicted. While in 360-720 days, the measured value and the predicted value was not significantly (P>0.05) different between various treatments. It was demonstrated that the addition of P promoted the decomposition of mixed fine roots.(3) In the two years of decomposition period, the N content of Castanopsis carlesii, Cunninghamia and its mixed fine root showed a trend of rise first and then descend. The highest values was in 270-360 days. On the whole, there was a rising trend, showed a pattern of accumulation. P addition increased the N content of fine root, which varied with time among different treatments. The change of P content was the same as the N, but reached the highest value in 360 days, showed a decreasing trend generally. The release pattern was accumulation-release. P addition could increase N content of the fine root. The lignin concentration of Castanopsis carlesii, Cunninghamia and its mixed fine roots showed a growth trend along with the decomposition of time, but cellulose concentration was slightly lower, and P addition decreased the lignin and cellulose concentration. Fine root decomposition rates showed a significant negative correlation with initial C/N and lignin concentrations (P<0.05). This indicates that the differences of the quality of fine root was one of the main factors affecting their decomposition.(4) In 2 a decomposition stage, the enzymes activity of fine roots increased and then decreased, such as cellulose hydrolysis(CBH), acid phosphatase(AP), P-glucosidase(BG), ?-n-acetyl-glucosidase(NAG), while peroxidase(PhOx) and phenol oxidase (PhOx) activity showed the tendency of rising. Adding P enhanced extracellular enmyze and oxidase enmyze activity, decreased AP activity and different P levels were vary significantly, and have different effects on different enzyme. Fine root decomposition rate presented a significantly negative correlation with AP activity, while showed a significant positive correlation with PerOx and PhOx activity (P<0.01), and there had significant quadratic function with hydrolase enzyme, but showed significant exponential relationship (P< 0.05) with oxidase enzyme. So in all, AP?PerOx and PhOx is the main enzymes of fine root decompositions. |
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