氮沉降对中亚热带米槠天然林微生物生物量及酶活性的影响

Effects of nitrogen deposition on soil microbial biomass and enzyme activities in Castanopsis carlesii natural forests in subtropical regions

氮沉降对土壤微生物的扰动可能会影响土壤的养分循环,然而关于中亚热带天然林土壤微生物及酶活性对氮沉降的响应鲜有报道。通过3 a的氮沉降模拟实验,研究中亚热带米槠天然林土壤的理化性质、土壤微生物量及土壤酶活性的响应。结果表明:氮沉降并未引起土壤的有机碳和总氮显著性变化;高氮(80 kg N hm^(-2)a^(-1))处理下,土壤p H下降,出现酸化现象;低氮(40 kg N hm^(-2)a^(-1))处理促进淋溶层(A层)中土壤纤维素分解酶(β-葡萄糖苷酶和纤维素水解酶)和木质素分解酶(多酚氧化酶和过氧化物酶)活性升高,同时促进土壤微生物生物量碳、氮的积累。冗余分析(RDA)表示,可溶性有机碳(DOC)是驱动A层土壤酶活性的重要环境因子;而在淀积层(B层),这4种酶活性并未发生显著性差异。施氮处理后,A、B层中土壤的酸性磷酸酶活性增加(P<0.05)。研究表明:低水平氮沉降增加了土壤微生物生物量碳氮含量以及土壤有机碳分解相关酶活性,从而加速了土壤碳周转;这为未来氮沉降增长背景下,探索中亚热带天然林土壤碳源汇问题提供了依据。

Anthropogenic nitrogen （N） enrichment is a concern worldwide, as it affects almost every aspect of ecosystem function and composition, particularly soil microbial communities that control soil organic matter （SOC） and nutrient turnover. Although many long-term nitrogen addition experiments have been conducted, there are still no conclusions about how microorganisms respond to nitrogen deposition, especially in mid-subtropical natural forests. Assays for soil microbial biomass and enzyme activity have become a common methodology for studying soil function in response to global environmental changes and disturbances. Between 2012 and 2015, we stimulated nitrogen deposition in a Castanopsis carlesii natural forest in Sanming Fujian Province in subtropical China. Soils were collected from four control plots （CT）, four low-nitrogen addition plots （LN）, and four high-nitrogen addition plots （HN）. We studied the physicochemical properties of the soil; soil microbial C, N, and P biomasses; and potential activity of six enzymes in response to 3 years of nitrogen deposition. The results showed that nitrogen deposition had little impact on SOC and total nitrogen content. The HN treatment significantly decreased the pH and acidified the soil. Cellulose hydrolysis enzymes （β-glucosidase （βG） and cellobiohydrolase （CBH）） and lignin oxidases enzymes （phenol oxidase （PHO） and peroxidase （PEO）） showed a significant response to the LN treatment in the A horizon. Similarly, the LN treatment promoted microbial biomass carbon （MBC） and nitrogen （MBN） in the A horizon. Redundancy analysis （RDA） showed that dissolved organic carbon （DOC） greatly accounted for the variation in soil enzyme activities in the A horizon. However, it did not reveal any significant differences in cellulose hydrolysis and lignin oxidases in the B horizon. Moreover, nitrogen deposition promoted acid phosphatase activities in the two soil horizons. In summary, the LN treatment accelerated soil carbon turnover, promoting soil carbon mineralization. This may provide a theoretical basis for assessing whether mid-subtropical natural forest soil will become a carbon sink or source in an N-deposition background.