Abstract:

The nitrogen and phosphorus in forest ecosystems are closely coupled. Thus, in karst forests of southwestern China, it is crucial to elucidate the impacts of nitrogen deposition forms on the sources of soil available phosphorus for promoting the vegetation restoration of degraded karst forests under global environmental change conditions. A field study was conducted in a typical karst forest of Guilin city, Guangxi province to simulate different forms of nitrogen deposition, including no nitrogen deposition (CK), oxidized nitrogen deposition (Oxi), reduced nitrogen deposition (Red), and oxidized-reduced nitrogen deposition (RO). After simulating nitrogen deposition for 1.5 years, we continuously monitored the phosphorus inputs from litterfall, plant roots, and rainfall in different seasons. Combined with the seasonal variations of soil total and available phosphorus, we explored the effects of nitrogen deposition forms on the different sources of available phosphorus in forest soils in the karst region of southwestern China. The results showed that the Oxi and Red treatments significantly increased soil available phosphorus content, while the impact of the RO treatment on soil available phosphorus content varied by season. None of the nitrogen deposition treatments had a significant effect on soil total phosphorus content. In the CK treatment, the annual input of phosphorus was 10.64 kg∙hm⁻² from litterfall, 12.65 kg∙hm⁻² from plant roots, and 0.78 kg∙hm⁻² from rainfall. All nitrogen deposition treatments significantly increased the annual input of phosphorus from litterfall, while the Oxi and Red treatments significantly decreased the annual input of phosphorus from plant roots. The RO treatment had no significant effect on the annual input of phosphorus from plant roots. A structural equation model analysis indicated that the Oxi treatment enhanced the direct impact of the season on soil available phosphorus content, while the Red treatment strengthened the direct impact of season, litterfall phosphorus input, rainfall phosphorus input, and soil physicochemical properties on soil available phosphorus content. In conclusion, the main sources of available phosphorus in forest soils in the karst region of southwestern China are plant roots, followed by litterfall, with rainfall contributing relatively less. Different forms of nitrogen deposition alter the input of soil phosphorus from different sources and thereby soil phosphorus availability, to some extent alleviates phosphorus limitation in forest soils and promotes the vegetation restoration of degraded forest ecosystems in the karst region in southwestern China.

Key words: simulated nitrogen deposition, karst forest, soil phosphorus sources, rain-litter-root-soil, influence factor

摘要：

森林生态系统氮和磷之间具有密切的耦合关系，阐明氮沉降形态对中国西南岩溶区森林土壤有效磷不同来源的影响，对推动全球环境变化条件下岩溶区退化森林植被恢复具有重要意义。以广西桂林岩溶区典型森林为研究对象，在野外模拟不同形态氮沉降（无氮沉降 (CK)、氧化态氮沉降 (Oxi)、还原态氮沉降 (Red) 和氧化还原态氮沉降 (RO)），在模拟氮沉降1.5 a后通过连续监测各季节凋落物、植物根系和雨水磷输入量，结合土壤全磷和有效磷的季节变化特征，探究氮沉降形态对中国西南岩溶区森林土壤有效磷不同来源的影响。结果表明：Oxi和Red处理显著提高了土壤有效磷含量，而RO处理对土壤有效磷含量的影响因季节不同而存在差异，所有氮沉降处理对土壤全磷含量的影响均不显著；CK处理凋落物磷年输入量为10.64 kg∙hm⁻²，植物根系磷年输入量为12.65 kg∙hm⁻²，雨水磷年输入量为0.78 kg∙hm⁻²；所有氮沉降处理均显著增加了凋落物磷年输入量，而Oxi和Red处理均显著降低了植物根系磷年输入量，RO处理对植物根系磷年输入量没有显著影响。结构方程模型表明，Oxi处理增强了季节对土壤有效磷含量的直接影响；Red处理增强了季节、凋落物磷输入量、雨水磷输入量以及土壤理化性质对土壤有效磷含量的直接影响。综上分析，中国西南岩溶区森林土壤有效磷主要来源为植物根系，其次是凋落物，雨水对土壤有效磷的贡献较小；氮沉降形态不同程度地改变了土壤磷不同来源的输入量和磷的有效性，可在一定程度上缓解森林土壤磷限制，有利于岩溶区退化森林生态系统植被恢复。

关键词: 模拟氮沉降, 岩溶区森林, 土壤磷来源, 雨水-凋落物-根系-土壤, 影响因素