Effects Of Fargesia Denudata Density On Soil Carbon, Nitrogen Pool Beneath Subalpine Forest Of Western Sichuan

Fargesia bamboo is not only a staple food for the giant panda, but also a dominant synusia under subalpine coniferous forest. Fargesia bamboo is perennial monocarps, which once flowered will cause subsequent death and food shortage for the giant panda and the ecological banlance of subalpine coniferous forest. Current studies about Fargesia bamboo mainly focus on the above ground part of Fargesia bamboo, and little on the relationship of the bamboo growth and soil available nutrients. As a clonal plant, fasciate growth of Fargesia bamboo demands much more soil nutrient during growing period. Under given condition, we assume that the carry capacity of soil will soon reaches its limit as the density of bamboo increase. Therefore,we selected three bamboo (Fargesia denudata) communities densities (D1: 80±4 culms. m^-2, D2: 140±7 culms. m^-2 and D3: 220±11 culms. m^-2) under same subalpine coniferous forest with similar altitude, slope aspect and edaphic type in Wanglang National Nature Reserve. The input and output character of soil nutrient and status of soil nutrient pool in different regeneration stage of bamboo were investigated by the methods of combining field investigation and lab experiment. Result were as follows: (1) Individual and total nutrient absorption amount of bamboo increased with density from May to October 2003, N and P element increased significantly in studded elements. The nutrient absorbing course of bamboo initially increases with time and comes to its peak in July, then it droped in August and September. After that drop, the absorbing rate gradually increases. The nutrient amount of individual element in the dry matter of increment is in the order of K>N>Ca>P>Mg. (2) During whole growing season, total nutrient and individual element return amount increase with bamboo density. For the nutrient returned, Ca and N are the largest and P and Mg are the least. K has a moderate input amount. Alhough total nutrient increased with density, the proportion of elements studied changes as Ca and Mg increase and N, P and K decrease. (3) Net soil nutrient input increased with the increase of density in the process of rainfall from May to October 2003. The major input of nutrient comes from throughfall and only little from stemflow. The sequence of input amount of elements were K>Ca>N>Mg>P. K and Ca are the major portion which consist of 79.34-82.52% of net input. P is the least of input element which occupied only 0.79-1.08% of net input. (4) Carbon stock in each layer increases with density. The distribution pattern is similar along bamboo density sequence. Carbon stock reaches its maximum in the surface layer (0-10cm) and then decrease apparently with depth of soil. The soil carbon stock in different density of bamboo mainly concentrated in the rooting area (0-30cm). (5) Total and available N, P, Ca, Mg concentration decreased with the soil depth (0³0 cm) except for K which was uniformly distributed in whole soil profile. The dynamic range of available nutrient in soil surface is greater than layers below and similar pattern true for soil supporting bamboo density from high to low. The seasonal dynamic of available nutrient in soil surface firstly decline gradually and reach the minimum in July and then rise gradually till the level of the beginning of growing season in the end of growing season. Among the available nutrient in soil layer (0-30cm), P is the lowest in stock and N the largest. The distribution pattern of nutrient stock was different from concentration in soil layers, which of nitrogen stock was variable for density. While P, K, Ca and Mg stock were higher in L1 than L2 and L3. Although the total nutrient amount increasewith bamboo density, the amount of N and K do not have significant difference among different bamboo density while the amount of P decreases with increased bamboo density. (6) The net output of soil nutrient increase with the increased bamboo density in the whole growing season with 8.90 g.m-2 for D1, 17.20 g.m-2 for D2 and 30.70 g.m-2 for D3. Potassium is the maximum output element with 2.93 g.m-2 for D1, 5.78 g.m-2 for D2 and 11.84 g.m-2 for D3, while magnesium is least, with 0.32 g.m-2 for D1, 0.56 g.m-2 for D2 and 1.01 g.m-2 for D3. The sequence of output strength for soil nutrient is K>N>Ca>P>Mg in three plots. The main part of nutrient input to soil comes from throughfall, it consists of 74.0%, 64.9% and 62.2% for D1, D2 and D3, respectively. Other input sources include littlefall and stemflow. Nutrient output from soil are mostly fed into bamboo, for which 94.1%, 96.7% and 98.7% in plot D1, D2 and D3, respectively. In this study, we find that the calcium and magnesium content in soil will increase with bamboo density, while nitrogen and potassium will keep balance and phosphorus decrease. Phosphorus might be one of the main factors limiting the growing and regeneration of bamboo community. Therefore, the results supported the nutrient theory to explain why bamboo flowered.