氮沉降下杉木(Cunninghamia lanceolata)人工林凋落叶分解过程中C、N元素动态变化

Carbon and nitrogen dynamics of decomposing foliar litter in a Chinese fir(Cunninghamia lanceolata) plantation exposed to simulated nitrogen deposition

在12年生的杉木(Cunninghamia lanceolata)人工林中开展模拟氮沉降试验,分N0(对照)、N1、N2、N3等4种处理,N沉降量依次为0、60、120、240kgNhm-2a-1,每处理重复3次。通过2a的监测和分析发现,经N0、N1、N2、N3处理后,凋落物分解过程中C平均含量分别为46.47%、46.35%、46.79%、46.6%,各处理之间无显著差异,但均随着分解时间的增加呈下降趋势。氮沉降明显增加了凋落物中的N含量,且随着沉降水平的增加而增加。各处理凋落物C的分解系数依次为0.739、0.744、0.936、0.708,周转期为4.26a、4.26a、3.46a、4.41a;而N的分解系数分别为0.458、0.543、0.776、0.565,周转期为6.26a、5.44a、3.91a、5.20a。N1处理表现出促进N释放的作用,但对C释放影响不明显;N2处理促进了凋落物C、N元素的释放,而N3处理则表现出一定的抑制作用。氮沉降处理也明显降低了凋落物的C/N比,N1、N2、N3处理使C/N比分别比N0下降8.59%、14.20%和17.54%。

To investigate the response of subtropical forests to increased nitrogen deposition, a field experiment was conducted in a 12-year-old Chinese fir （Cunninghamia lanceolata） plantation exposed to four N treatments designated as NO （control） , N1, N2 and N3 at the doses of 0, 60, 120 kg N hm^-2 a^-1 and 240 kg N hm^-2 a^-1, respectively. Each treatment comprised three replicate plots of 20 m×20 m which were sprayed with urea on the forest floor at the beginning of each month, lasting from January 2005 to December 2006. Litter bags were randomly placed on the forest floor and collected bimonthly, and litter was analyzed for mass remaining, carbon and nitrogen content. Based on two years of litterbag collection and analyses, the average carbon contents in the decomposing litter collected bimonthly were estimated at 46.47%, 46.35%, 46.79% and 46.6%, respectively, for NO, N1, N2, and N3, showing no significant difference between treatments but decreasing with time consistently. The concentrations of nitrogen in leaf litter were found to increase significantly with the doses of nitrogen additions. For the above four treatments, the decomposition coefficients determined by Olson equation were respectively 0.739, 0.744, 0.936 and 0.708, with turnover time of 4.26, 4.26 , 3.46 and 4.41 years, for C element ; the decomposition coefficients were 0. 458, 0. 543, 0. 776 and 0. 565, with turnover time of 6.26, 5.44 , 3.91 and 5.20 years for N element. N1 treatment stimulated N release from the litter, but produced no effects on C release; N2 accelerated release of both C and N, but N3 exhibited inhibitory effects to some extent. The N1, N2 and N3 treatments decreased C/N ratios in leaf litter by 8.59% , 14.20% and 17.54% , respectively.