氮添加对高寒草甸土壤微生物呼吸及其温度敏感性的影响

Effects of nitrogen additions on soil microbial respiration and its temperature sensitivity in a Tibetan alpine meadow

土壤氮素的可利用性是控制土壤微生物呼吸的重要因素之一,大量研究已经表明增加土壤活性氮的含量可以降低微生物呼吸,但是土壤氮输入对土壤微生物呼吸温度敏感性的影响还不清楚。以青藏高原高寒草甸为研究对象,通过野外施氮试验和室内控制试验相结合的方式,在5℃、15℃和25℃条件下对3种施氮水平的土壤(对照,0g N m^(-2)a^(-1);低氮,5g N m^(-2)a^(-1);高氮,15g N m^(-2)a^(-1))进行培养,探讨土壤微生物呼吸及其温度敏感性对不同氮添加水平的响应情况。结果表明:(1)3个温度培养下的土壤微生物呼吸速率和累积碳释放量均随施氮量的增加而显著降低(P<0.05);(2)氮添加对5℃和15℃培养条件下的微生物呼吸温度敏感性没有显著影响,但显著地增加了15℃和25℃培养条件下的微生物呼吸温度敏感性(P<0.05);(3)线性相关分析表明,土壤累积碳释放量与土壤有机碳的难降解性显著负相关(P<0.05),而15℃和25℃培养条件下的微生物呼吸温度敏感性与土壤有机碳的难降解性显著正相关(P<0.05)。结果表明,在全球气候变暖的背景下,土壤氮输入将增加预测青藏高原高寒草甸地区土壤碳排放的不确定性。

The soil C stock of the Tibetan Plateau is of global significance because of its large pool size and the potential positive feedback to climate warming. Soil microbial respiration is an important carbon （C） flux of the global C cycle, and is greatly affected by soil nitrogen （N） availability. Reactive N inputs often reduce soil microbial respiration, but how they affect the temperature sensitivity of soil microbial respiration is still poorly understood. Here, we examined the effect of N additions on soil microbial respiration and its temperature sensitivity （expressed as Q10）, and assessed the relative importance of substrate availability and the chemical composition of soil organic C in explaining the variation in soil microbial respiration and Q10. A 4-year field experiment with four simulated N deposition levels, 0, 5, 10, and 15g N m^-2 a^-1 was conducted. Soil samples were collected from three levels of nitrogen addition rate （0, 5, and 15g N m^-2 a^-1） and incubated at 5℃, 15℃ and 25℃ in the laboratory. The results showed that N additions significantly increased soil dissolved organic C and available N （P 〈 0.05）, but did not significantly affect soil pH, soil organic C, total N, available P, and total P. Moreover, N additions did not affect microbial biomass C, but significantly decreased microbial biomass N （P 〈 0.05）. The solid-state ^13C nuclear magnetic resonance showed that N additions decreased the relative content of O-alkyl C, but increased the relative contents of alkyl C, aromatic C, and carboxylic C, which resulted in higher aromaticity and aliphaticity of soil organic carbon under N addition treatments than that of the control treatment. We also found that N additions significantly decreased soil microbial respiration rate and cumulative C efflux under three temperatures （P 〈 0.05）, but significantly increased Q10 under the incubation temperature of 15℃ and 25℃ （P 〈 0.05）. Linear correlation analysis showed that microbial respiration was negatively correlated with the complexity of soil organic C, whereas Q10 under the incubation temperature of 15℃ and 25℃ was positively correlated with the complexity of soil organic C. These findings support the prediction based on kinetic theory that soil organic C, with recalcitrant molecular structure, has a high temperature sensitivity of respiration. Together, our results suggest that changes in chemical structure of soil organic C induced by N inputs may play an important role in regulating the response of soil microbial respiration to elevated temperature, and further increase the uncertainty in predicting net soil C losses in the scenario of warming on the Tibetan Plateau.