模拟氮沉降对两种竹林不同凋落物组分分解过程养分释放的影响

利用原位分解袋法研究了华西雨屏区苦竹(Pleioblastus amarus)和撑绿杂交竹(Bambusa pervariabilis × Dendrocala mopsi)人工林几种凋落物组分在模拟氮沉降下分解过程中养分释放状态,试验周期为2 a。氮沉降水平分别为对照(CK, 0 g · m^-2 · a^-1)、低氮(5 g · m^-2 · a^-1)、中氮(15 g · m^-2 · a^-1)和高氮(30 g · m^-2 · a^-1),每月下旬定量地对各处理施氮(NH₄NO₃)。结果表明,苦竹林和杂交竹林凋落物主要由凋落叶、凋落箨和凋落枝组成,其中凋落叶约占80%;两个竹种凋落物在分解过程中养分元素释放的种间差异主要与初始养分元素含量有关;凋落物养分元素初始含量对元素释放模式和最终净释放率的大小具有重要的决定作用;目前,这两种竹林生态系统土壤氮输入主要以大气氮沉降(8.24 g · m^-2 · a^-1)为主,同时凋落物氮输入(苦竹和杂交竹林分别为1.93,5.07 g · m^-2 · a^-1)也是一个重要途径;模拟氮沉降对苦竹凋落物碳、磷、钾、钙元素和杂交竹凋落物碳、氮、磷、钾、钙、镁元素释放的抑制作用较弱,处理与对照之间元素总释放率差异一般小于10%;氮沉降显著抑制了苦竹林凋落物氮元素释放,减小幅度为19.0%-27.2%,但由于氮沉降增加对土壤肥力的直接改良作用,氮沉降的增加并不会因为凋落物分解速率的降低造成植物生长所需养分供应的减少;从短期来看,在氮沉降继续增加的情况下,该地区这类竹林生态系统的碳吸存能力仍可能会因为N沉降对植物生长的促进作用而增加。

Effect of simulated nitrogen deposition on nutrient release in decomposition of several litter fractions of two bamboo species

Increased nitrogen (N) deposition can increase net primary productivity (NPP) in many terrestrial ecosystems. However, unless there are comparable changes in litter decomposition, increases in productivity will most likely be unsustainable. Bamboo forest is one of the most important and fast-growing forest types in the world. To investigate the effect of increased N deposition on nutrient release in decomposition of litter of bamboo species, we conducted a two-year field experiment of simulated nitrogen deposition in a Pleioblastus amarus and a Bambusa pervariabilis × Dendrocala mopsi plantation, Rainy Area of West China. The levels of nitrogen application were control (CK, 0 g · m^-2 · a^-1), low nitrogen (5 g · m^-2 · a^-1), medium nitrogen (15 g · m^-2 · a^-1) and high nitrogen (30 g · m^-2 · a^-1). A field experiment using the litterbag method was implemented on the decomposition of litter fractions of the two bamboo species. At the end of each month, NH₄NO₃ was added into N-treated plots. The total litterfall were composed of leaf litter, sheath litter, and twig litter, and leaf litter contributed about 80% of total litterfall in both plantations. The initial nutrient element concentration of litter determined the nutrient release pattern and the amount of the final net element release ratio. At present, the atmospheric N deposition (8.24 g · m^-2 · a^-1) was the major N input of soil in the two plantations, and the N input through litterfall in P. amarus and B. pervariabilis × D. mopsi plantations were 1.93 and 5.07 g · m^-2 · a^-1, respectively. The release of C, phosphorus (P), Potassium (K), Calcium (Ca) of P. amarus litter and C, N, P, K, Ca, and Magnesium (Mg) of B. pervariabilis × D. mopsi litter were gently inhibited (the difference between control and treatments of total element release ratio < 10%) by simulated N deposition. Simulated N deposition significantly slowed down the release of N in decomposition of P. amarus litter and the rate of reduction was 19.0%-27.2%. However, the soil nutrient supply for the growth of plant was not cut down because of the direct and indirect fertility effect of N deposition on the soil. On the whole, the continuous increasing N deposition may strengthen the C sequestration in the young plantation ecosystems in Rainy Area of West China, for the enormous potential C sequestration ability in these ecosystems through fast growth of plant.