桂西北喀斯特区原生林与次生林凋落叶降解和养分释放

凋落叶降解及养分释放研究对喀斯特生态脆弱区森林生态系统的恢复与重建具有重要指导意义。本文选取桂西北喀斯特区3种原生林与3种次生林进行比较，研究其凋落叶降解与降解过程中的营养元素释放规律以及降解速率的影响因子。结果表明，原生林凋落叶的降解速率略大于次生林。C、N、K元素在前180天释放速率较快，随后趋于稳定。次生林凋落叶总P含量在降解初始阶段呈净积累，随后净释放，而原生林的凋落叶在降解360天后仍呈现P素净积累。相关分析表明，凋落叶降解速率与凋落叶初始总N、木质素含量及木质素:N比值呈负相关，与C:N比呈正相关。综合比较发现，次生林圆叶乌桕(Sapium rotundifolium Hemsl)凋落叶的降解速率与养分释放速率较快，是喀斯特退化土地及植被恢复过程中潜在的优势种和建群种。

Litter decomposition and nutrient release in typical secondary and primary forests in karst region, Northwest of Guangxi

Karst ecosystem has been shown to be extremely vulnerable under severe water and soil erosion due to improper land uses and human activities. The karst area of Southwestern China covers a land of 550,000 km2 but 105,000 km2 of that is suffering from serious rocky desertification. Recently, national and provincial afforestation programs have been developed to protect and/or restore the degraded karst lands. Litter decomposition is the major pathway to provide organic and inorganic elements for the nutrient cycling processes and control nutrient return to the forest ecosystem. So, leaf litter decomposition and nutrient release characteristics play an important and leading role in the forest ecosystem restoration and reconstruction in the fragile karst region. In view of litter fall as the important component of forest ecosystem and the shortage study about karst area in southwest China, confronting the restoration programs. Litter bag decomposition experiments were set up at six sites to compare the litter decomposition differentiation under different karst forest ecosystems from Dec. 2008 until Nov. 2009. In this study, three typical primary forest communities (Dye-tree (Platycarya longipes Wu), Eurycorymbus (Eurycorymbus cavaleriei Hand., Wing-hackberry (Pteroceltis tatarinowii Maxim)) and three typical secondary forest ones (Round-leaved Tallow-tree (Sapium rotundifolium Hemsl), Chinese Alangium (Alangium chinense (Lour.)Harms), Negundo Chaste-tree (Vitex negundo Linn)) were chosen to elucidate the decomposition characteristics of leaf litter, release pattern of nutrient elements over the course of leaf litter decomposition, decomposition rate and related impact factors in Karst areas of northwest Guangxi of China. The result showed that the decay rates of leaf litter at primary forests were slightly faster than that of secondary forest. Release rates of carbon (C), nitrogen (N) and Potassium (K) were generally higher within initial 180 days and then tended to become stable. The content of total phosphorus (TP) in leaf litter in secondary forest communities displayed a status of net accumulation at the initial stage of decomposition, and subsequently turned to be replaced by a net release. However, the TP content of leaf litter in primary forest communities was remained in net accumulation status till 360 days after litter presence. Correlation analysis showed that decomposition rate of leaf litter was negatively correlated with total initial N content, lignin content and lignin to nitrogen ratio in leaf litter, whereas was positively correlated with leaf C/ N ratio. Comprehensive and comparative analysis suggested that leaf litter decomposition rate and nutrient element release rate within the round-leaved tallow tree (Sapium rotundifolium Hemsl) community were totally greater in secondary forests than those of primary forest communities. With regard to species choices for the re-establishment of nutrient cycling in disturbed forest ecosystems, species with high nutrient recycling capacity (e.g. species with fast litter decomposition rate) may be favored. In this study, it can be argued that the tallow tree could act as a potential constructive and dominant species in the restoration of degraded Karst land and vegetation. So our study results can provide some support and refer for national and provincial afforestation programs in Karst areas of northwest Guangxi of China.