西沙永兴岛抗风桐与海岸桐群落凋落叶分解及中型土壤动物的贡献

我国南海诸岛主要是珊瑚岛。植物凋落物分解是生态系统元素循环的关键环节，但目前关于南海珊瑚岛生态系统凋落物分解的研究还是空白。以我国西沙群岛的优势树种抗风桐（Pisonia grandis）和海岸桐（Guettarda speciosa）为研究对象，采用凋落物袋法，分别于分解期间的第3、6、9、13和15个月取样，探究中型土壤动物对两种植物群落中凋落物分解过程中质量损失和养分释放的影响。结果表明：与没有中型土壤动物存在的情况（0.1 mm凋落物袋）相比，分解开始后的6个月内，中型土壤动物存在（2 mm凋落物袋）使抗风桐和海岸桐凋落叶分解速率分别提高了12.3%和4.8%（P<0.05）；分解6-15个月期间，中型土壤动物存在使抗风桐和海岸桐凋落叶分解速率分别提高了33.0%和12.3%（P<0.05）。中型土壤动物排除显著影响了不同分解阶段凋落叶总碳（Total carbon，TC）、总氮（Total nitrogen，TN）、纤维素、木质素和半纤维素的残留率变化。中型土壤动物群落组成受土壤温度显著影响（P<0.05），它们对凋落叶分解的贡献可能主要受优势类群如真螨目和寄螨目的影响。相较海岸桐，抗风桐凋落叶的分解周期更短，中型土壤动物对其的贡献更大；选用抗风桐作为南海珊瑚岛退化植被恢复或新建的先锋种对促进生态系统元素循环更有利。

Leaf litter decomposition and soil mesofauna contribution in Pisonia grandis and Guettarda speciosa plant communities in the Yongxing Island of South China Sea

Coral island is the major terrestrial ecosystem type in South China Sea. Litter decomposition is a key biogeochemical process that drives ecosystem nutrient cycling, but has not been studied in these coral islands. We set up three quadrats in each of the two plant communities dominated by Pisonia grandis and Guettarda speciosa in the Yongxing Island of South China Sea. Decomposition litterbags were deployed in the quadrats in July 2018 to quantify the litter turnover rate and nutrient release pattern. The decomposition experiment lasted for 15 months till October 2019 and we sampled 5 times within the period to weigh remaining litter mass and to analyze the chemical constitutes of the decomposing litter. The analyzed chemical elements included total carbon (TC), total nitrogen (TN), total phosphorus (TP), lignin, cellulose, and semi-cellulose. To examine the role of soil mesofauna with body size of 0.2-2.0 mm in the litter mass loss and nutrient release from decomposing P. grandis and G. speciosa leaf litter, litterbags with two different mesh sizes (0.1 mm and 2.0 mm) were used. The relative mass low difference between these two mesh-size litterbags was used to quantify the contribution of mesofauna. During the earlier 6 months of decomposition, the presence of soil mesofauna increased the leaf litter decomposition rate of P. grandis significantly by 12.3% and that of G. speciose by 4.8% (P<0.05). During the latter 6 to 15 months of decomposition, the effects exerted by soil mesofauna seemed to increase, with decomposition rate being increased significantly by 33.0% and 12.3% for the two litter species (P<0.05). The presence of soil mesofauna accelerated the release rate of total carbon, total nitrogen, cellulose, lignin, hemicellulose from the decomposing litters. Change in soil mesofauna community structure was mainly affected by soil temperature. However, the contribution of soil mesofauna to litter decomposition was not correlated to the changes in mesofauna richness and abundance. We argue that the measured mesofaunas contribution to litter decomposition was most likely affected by the dominant groups such as Acariformes and Parasiformes, but the photodegradation of litter and litterbags of the differed mesh-size might also played a role. The exact contribution of these factors/mechanisms ought to be explored in further studies. Based on the results of this study, we suggest that using P. grandis, in comparison of G. speciosa, as pioneer species in the establishment and restoration of degraded vegetation on the coral islands of South China Sea could better promote the ecosystem nutrient cycling.