氮磷添加对华南热带森林尾叶桉木质残体分解和养分动态的影响

Effects of nitrogen and phosphorus additions on woody debris decomposition in a secondary tropical forest of South China

木质残体(Woody debris,WD)是森林生态系统中一个重要的碳库和营养源,在全球变暖、氮(N)沉降日益严重的今天,研究N、磷(P)沉降影响WD分解的反应机制对于应对环境变化具有重要意义.以小良热带阔叶林尾叶桉(Eucalyptus urophylla)WD为研究对象,人工模拟N和P添加对尾叶桉WD分解速率及养分释放的影响.结果表明:与对照相比,N添加使WD分解速率常数k降低了11%,而P添加显著提高分解速率常数k(P=0.027),上升幅度为6%.这表明,富N缺P的热带森林,P的可利用性限制了WD分解速率,P限制有利于WD中碳的固存.N添加、P添加改变了尾叶桉WD分解过程中N元素的迁移模式,与对照相比,两种处理均使N元素迁移方式由直接释放改变为淋溶—富集—释放,而对分解过程中P元素的迁移模式有不同影响,N添加使P元素的迁移方式由直接释放改变为淋溶—富集—释放,P添加使P元素的迁移方式改变为富集—释放.分解过程中质量残留率与分解过程中C/N比、C/P比呈显著负相关(P<0.01).可见,N、P沉降的增加对WD分解速率和养分释放模式有不同的效应.

Woody debris （WD） is an important structural component of many forest ecosystems and plays a key role in carbon sequestration. Despite the large amounts of carbon stored in WD, little attention is paid to its decomposition, which is critical to mitigating climate change, including the chronically elevated atmospheric nitrogen and phosphorus deposition. This study aimed to understand the factors controlling organic matter decomposition in tropical forests. We conducted a factorial fertilization experiment in a tropical forest in South China to investigate the influence of nitrogen （N） and phosphorus （P） availability on WD decomposition using branch segments （5 cm in diameter） of Eucalyptus urophylla in plots fertilized with ＋N, ＋P, ＋NP, and controls. The results showed that nitrogen addition had negative （-11%） effect on E. urophylla WD decomposition, whereas phosphorus addition had positive （＋6%） effect. Two-way ANOVA showed that P addition statistically increased WD decomposition rate constants （P = 0.027）, indicating that P inputs stimulate WD decomposition and enhance C release. Compared to the control, in the early period of decomposition, N addition promoted N and P release by leaching, while at the middle of decomposition, N addition significantly increased the N and P content remainings （〉 100%） but decreased them （〈 100%） at the end of decomposition. We observed significant accumulation of P in P addition plots in the early period of decomposition, while at the end of decomposition, all treatments had the similar P content remaining （〈 100% for all）. Moreover, through the decomposition, WD mass remaining had a significantly negative relationship with both C：N ratio and C：P ratio. Our results indicated that decomposition of WD is primarily constrained by P availability in the tropical forest.