杉木人工林凋落物生态化学计量与土壤有效养分对长期模拟氮沉降的响应

凋落物分解的快慢和养分释放的速度决定了生态系统中土壤有效养分的供应。探讨全球变化条件下森林生态系统凋落物与土壤养分的变化规律,有利于深入认识凋落物-土壤相互作用的养分调控因素,从而揭示生态系统C、N、P循环。通过模拟氮沉降增加试验,分4个水平处理,分别为0、60、120、240 kg N hm~(-2)a~(-1)。模拟氮沉降13年后,分析了杉木人工林凋落物中不同组分(落叶、落枝、落果)生态化学计量与土壤有效养分(有效氮、碱解氮、速效磷、速效钾)的关系。结果表明:氮沉降(N1、N2和N3)显著提高了落叶和落枝的N含量,平均增幅分别为35.27%和32.21%;高水平氮沉降(N3)处理显著降低了落叶和落枝的C/N,平均降幅分别为25.95%和22.32%,但N3增加了落枝和落果N/P,平均增幅分别为38.4%和31.7%;氮沉降对凋落物各组分的C、P和C/P均影响不显著。氮沉降处理显著增加了土壤NO_3~--N和NH_4~+-N含量,均表现为N3N2N1N0,其中NO_3~--N含量更容易受氮沉降处理的影响,表现为更大的增幅。N2显著增加0—20 cm土层的碱解氮含量,N1显著降低0—20 cm土层的速效钾,但氮沉降对速效磷含量没有影响。凋落物生态化学计量与土壤有效养分之间的Pearson相关和冗余分析(RDA)表明,凋落物生态化学计量与土壤有效养分之间关系紧密,凋落物P含量(蒙特卡罗检验,P=0.018)和C/P比值(P=0.037)对土壤有效养分影响显著。凋落物中C/N比值、C/P比值与土壤有效养分呈显著负相关,其比值越高越不利于土壤有效养分的累积。

Litterfall ecological stoichiometry and soil available nutrients under long-term nitrogen deposition in a Chinese fir plantation

Nitrogen (N) deposition because of continuous anthropogenic emission of air pollutants, is one of three major drivers of global change. Elevated N deposition may lead to N saturation, soil acidification, plant nutrient imbalances, and even forest productivity decline. Nutrients released from litterfall decomposition represent a large part of the input to the soil, which has been studied extensively in forest ecosystems around the globe. However, the relationship between soil nutrient availability and litterfall remains largely unknown regarding nitrogen deposition, especially in subtropical forests in South China. In the present study, the litterfall and soil nutrients were determined in a nitrogen-loaded Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) plantation forest. A field experiment with simulated N additions at three doses, N1 (60 kg N hm^-2 a^-1), N2 (120 kg N hm^-2 a^-1), and N3 (240 kg N hm^-2 a^-1), with N0 as the control (0 kg N hm^-2 a^-1), was conducted by adding the required amount of urea dissolved in 20 L water. This field experiment was initiated in January 2004 with continuous operation and was monitored for almost 13 years. Ten 1 m×1 m litter collecting frames were randomly set up in each plot in January 2004. Litterfall samples were collected monthly and mixed together into one sample per plot. We selected litterfall samples in June 2016 and separated them into fallen leaf, branch, and fruit. In total, there were 12 plots×3 components=36 samples. Soil samples were collected randomly at the depths of 0-20 cm, 20-40 cm, and 40-60 cm in each plot in June 2016, which together (4 treatment levels×3 doses×3 soil layers) constituted 36 samples. The ecological stoichiometry of fallen leaves, branches, and fruits, and soil available nutrients, such as ammonium N (NH₄⁺-N), nitrate N (NO₃^--N), alkali-hydrolyzed N, available P, available K were measured. Our results showed that N deposition increased N content by 35.27% in fallen leaves and 32.21% in fallen branches averagely. The high level of N addition (N3) decreased the carbon to N (C/N) ratio by 25.95% and 32.21% in fallen leaves and branches, respectively, but increased the N to phosphorus (N/P) ratio in fallen branches by 38.4% and in fallen fruits by 31.7%. Nitrogen loads produced no significant effects on litterfall C content, P content, or the C/P ratio. The concentrations of soil NH₄⁺-N and NO₃^--N increased significantly with increasing N deposition levels at all three soil depths, with stronger response from NO₃^--N. The N1 treatment significantly suppressed soil available K, whereas the N2 treatment significantly promoted soil alkali-hydrolyzed N at 0-20 cm, but N deposition showed no significant effect on soil available P. Pearson''s correlation and redundancy analysis (RDA) between litterfall ecological stoichiometry and soil available nutrients indicated that litterfall P concentration (Monte Carlo (999), P=0.018) and C/P ratio (P=0.037) were the major determinants of soil available nutrients. Negative relationships exited between the litterfall C/N ratio, C/P ratio, and soil available nutrients, which suggested that higher ratios were unfavorable to the accumulation of soil available nutrients.