Data from: Nutrient limitation of woody debris decomposition in a tropical forest: contrasting effects of N and P addition

Tropical forests represent a major terrestrial store of carbon (C), a large proportion of which is contained in the soil and decaying organic matter. Woody debris plays a key role in forest C dynamics because it contains a sizeable proportion of total forest C. Understanding the factors controlling the decomposition of organic matter in general, and woody debris in particular, is hence critical to assessing changes in tropical C storage. 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 woody debris decomposition using branch segments (5 cm diameter) of four species (Acacia auriculaeformis, Aphanamixis polystachya, Schefflera octophylla, and Carallia brachiata) in plots fertilized with +N, +P, or +NP, and controls. Fertilization with +P and +NP increased decomposition rates by 5–53%, and the magnitude was species specific. Contrary to expectations, we observed no negative effect of +N addition on decay rates or mass loss of woody debris in any of the four study species. Decomposition rates of woody debris were higher in species with lower C : P ratios regardless of treatment. We observed significant accumulation of P in the woody debris of all species in plots fertilized with +P and +NP during the early stages of decomposition. N release from woody debris of Acacia (N-fixing) was greater in the +P plots towards the end of the study, whereas fertilization with +N had no impact on the patterns of nutrient release during decomposition. Synthesis: Our results indicate that decomposition of woody debris is primarily constrained by P availability in this tropical forest. However, contrary to expectations, +N addition did not exacerbate P limitation. It is conceivable that decay rates of woody debris in tropical forests can be predicted by C : P or lignin : P ratios, but additional work with more tree species is needed to determine whether the patterns we observed are more generally applicable.

热带森林是陆地碳（C）的主要储存库，其中绝大部分碳储存在土壤与腐解有机质中。木质残体在森林碳动态中发挥关键作用，因其占森林总碳库的可观比例。因此，阐明调控有机质（尤其是木质残体）分解的关键因子，对评估热带碳库变化至关重要。
本研究在中国南方的一处热带森林开展了因子式施肥实验，以4个树种（马占相思（Acacia auriculaeformis）、山楝（Aphanamixis polystachya）、鹅掌柴（Schefflera octophylla）、竹节树（Carallia brachiata））直径5厘米的枝条段为研究材料，在施加+N、+P、+NP肥的样地及对照样地中，探究氮（N）与磷（P）有效性对木质残体分解的影响。
施加+P与+NP肥可使分解速率提升5%~53%，且提升幅度因树种而异。与预期相悖的是，在4个受试树种中，施加+N肥未对木质残体的分解速率或质量损失产生任何负面影响。无论施肥处理如何，C:P比值更低的树种，其木质残体分解速率均更高。
在分解初期，施加+P与+NP肥的样地中，所有受试树种的木质残体均出现显著的磷素积累。固氮相思类树种（Acacia）的木质残体氮素释放量，在+P肥样地中于研究后期更高；而施加+N肥对分解过程中的养分释放模式无显著影响。
综合分析结果表明：本研究中热带森林的木质残体分解主要受磷素有效性限制。但与预期相悖的是，施加+N肥并未加剧磷素限制效应。据此推测，热带森林木质残体的分解速率可通过C:P或木质素:P比值进行预测，但未来仍需纳入更多树种开展研究，以验证本研究观测到的规律是否具有普适性。