杉木人工林凋落物分解对氮沉降的响应

凋落物分解是陆地生态系统养分循环的关键过程，是全球碳（C）收支的一个重要主要组成部分，正受到全球大气氮（N）沉降的深刻影响。探讨大气氮沉降条件下森林凋落物的分解，有利于揭示森林生态系统C平衡和养分循环对全球变化的响应。选择福建沙县官庄林场1992年栽种的杉木（Cunninghamia lanceolata）人工林为研究对象，自2004年开始野外模拟氮沉降试验，至今12年。氮沉降处理分4个水平，N0、N1、N2和N3分别为0、60、120、240 kg N hm^-2 a^-1。2015年12月开展分解袋试验，对经过氮沉降处理12年的凋落物（叶、枝、果）进行模拟原位分解，每3个月收回一次分解袋样品，为期2年，同时测定凋落物干物质残留量及其C、N和磷（P）含量。结果表明，经2年分解后，氮沉降条件下凋落物叶、枝和果的干物质残留率平均值分别为27.68%、47.02%和43.18%，说明分解速率大小依次为叶 > 果 > 枝。凋落物叶、枝和果的分解系数平均为0.588、0.389和0.455，周转期（分解95%年限）分别为4-5年、6-8年和5-7年。低-中氮处理（N1和N2）均促进凋落物叶、枝和果的分解，以N1的效果更明显，而N3起到抑制作用。N1处理的凋落物叶、枝和果的周转期分别为：4.50年、6.09年和5.85年，N2处理的分别为4.95年、8.16年和6.19年。模拟氮沉降在一定程度上增加了凋落物叶、枝和果分解过程中的N和P含量，但降低了C含量。凋落物叶、枝和果分解过程中C元素呈现释放-富集-释放模式，N和P元素呈现释放与富集交替，除枝的N元素外，其他均表现为释放量大于富集量。

Litter decomposition in a Chinese fir plantation in response to nitrogen deposition

Litter decomposition is a key process of nutrient cycling in terrestrial ecosystems and a major component of the global carbon budget, which is presently being affected by the global atmospheric nitrogen deposition. Studying the litter decomposition and nutrient release patterns might be of great importance for better understanding the carbon balance and nutrient cycling of forest ecosystems in response to global change. Many studies on the litter composition have focused on fallen leaves. However, very little is known about the composition of fallen branches and fruits. To investigate the response of nutrient (C, N, and P) dynamics in fallen leaves, branches, and fruits decomposition to elevated N deposition, a field experiment was conducted in a Chinese fir (Cunninghamia lanceolata) plantation, located at the Guanzhuang National Forestry Farm, in the subtropical region of China. Simulated N deposition treatments were designed as N0 (0 kg hm^-2 a^-1, control), N1 (60 kg hm^-2 a^-1, low-N), N2 (120 kg hm^-2 a^-1, medium-N), and N3 (240 kg hm^-2 a^-1, high-N), with three replicates in each treatment. Starting from January 2004, each treatment was sprayed with CO(NH₂)₂ on the forest floor at the beginning of each month, and this spraying is being continued till date. After 12 years of N-application, the decomposition of fallen leaves, branches, and fruits were studied by using the litterbag method. The fresh litter samples were collected in December 2015 and sorted into three litter fractions:leaves, branches, and fruits from each plot. Decomposing litter in the litterbags was collected at 3-month intervals for 2 years from each plot, and then the mass loss and C, N, and P contents were determined. The results showed that the percentage of dry matter remaining of fallen leaves, branches, and fruits was on an average 27.68%, 47.02%, and 43.18%, respectively, for all the treatments, after 2 years of decomposition. The decomposition rate of litterfall fraction decreased in the order leaf > fruit > branch. The decomposition coefficients (K values) of fallen leaves, branches, and fruits were 0.588, 0.389, and 0.455 on average. The turnover period (time to 95% decomposition) of litter leaves, branches, and fruits was 4-5 years, 6-8 years, and 5-7 years, respectively. Low-and medium-N treatments (N1 and N2) had a significant, positive effect on the litter decomposition rate, with N1 showing a greater impact. The turnover period (time to 95% decomposition) was 4.50, 6.09, and 5.85 years for fallen leaf, branch, and fruit, respectively, under N1 treatment, and 4.95, 8.16, and 6.19 years, respectively, under N2. Simulated N deposition increased the N and P contents in the litterfall fractions, but decreased the C content. During the decomposition process, litter C showed release-enrichment-release pattern, and N and P showed alternately release and enrichment patterns, except for the N content in the branches. The released amounts of litter C, N, and P content were greater than the enrichment. In summary, low-and medium-N deposition promote litter decomposition, with stronger effects for the low-N treatment. The litter C, N, and P exhibited the pattern of nutrient release under long-term N deposition.