氮沉降增加和放牧对松嫩草甸草原土壤呼吸空间异质性的影响及驱动机制

Soil respiration is the second largest terrestrial carbon flux and has a strong spatial variability due to its complex component origin (rhizospheric respiration and heterotrophic respiration). Nitrogen deposition and grazing are likely to strongly affect grassland vegetation composition, soil microbial community and soil physicochemical properties, which will cause changes in spatial variability of soil respiration, and eventually alter soil and ecosystem carbon balance. To date, the effects of nitrogen deposition and grazing on spatial variability of soil respiration are poorly understood. However, it is a prerequisite to predicting the response of carbon source/sink function of grassland ecosystem to elevated nitrogen deposition and grazing. We proposed a manipulative experiment with treatments of control, nitrogen addition, grazing and nitrogen addition plus grazing in Songnen meadow steppe, northeastern China. We will quantify spatial variability in soil respiration, plant community composition, soil microbial composition and activity, soil microclimate and soil properties and identify key factors that regulate the spatial variability in soil respiration under different treatments. Our objectives are to explore the effects of global change factors and land-use types induced grassland ecosystem composition and structure changes on the spatial variability of soil respiration and unravel the underlying ecological mechanisms. The results of the proposed study are essential for understanding the relationship between aboveground and belowground processes of grassland ecosystems, predicting how soil CO2 efflux of Songnen meadow steppe response to elevated nitrogen deposition and grazing, and providing a reasonable and scientific management strategy for meadow steppe carbon sequestration.

土壤呼吸是陆地生态系统仅次于总初级生产力的第二大碳通量，由于复杂的起源和发生过程，其展现出巨大的空间变异。氮沉降增加和放牧强烈影响草地生态系统生物和环境因子，进而影响土壤呼吸空间异质性从而改变土壤碳通量。关于氮沉降增加和放牧对土壤呼吸空间异质性影响的研究十分有限，这限制了我们预测草地碳源/汇功能对其响应的能力。拟对松嫩草甸草原进行氮沉降增加、放牧以及二者互作处理，量化不同处理条件下土壤呼吸、植被组成、土壤微生物组成和活性、土壤微气候以及土壤性质空间异质性并构建土壤呼吸空间变异的回归模型。进而探讨生物和环境因子变化对土壤呼吸空间异质性的影响，最终揭示氮沉降增加、放牧以及二者互作对土壤呼吸空间异质性的影响及驱动机制。研究结果将加深对草地生态系统地上和地下生态过程关联性的理解，有助于评估氮沉降增加和放牧干扰条件下松嫩草甸草原土壤碳通量的变化，可为制定合理的草地碳汇经营策略提供数据支撑。

为探讨放牧和氮添加是如何影响草地生态系统结构和特性，并进一步影响土壤呼吸（SR）空间异质性。在中国东北羊草草原草原上进行了对照、放牧、氮添加以及放牧和氮添加交互处理的控制实验。在4个15×15 m的样地中，分别在干旱和湿润的条件下，对75个采样点进行了SR的测定，同时测定相应的地上和地下控制变量（地上生物量，根生物量，土壤含水量，土壤养分和酶活性）。与对照相比，放牧处理降低了SR的空间依赖性。氮添加处理下未发现SR的空间结构。在施肥样地，放牧引起土壤SR空间异质性强度的降低程度低于未施肥样地。在湿润条件（4.58m）条件下，CK处理的SR自相关距离大于干旱条件下（1.61m）。湿润条件下G和NG处理的SR空间依赖程度高于干旱条件下。地上和地下变量可以解释SR空间变异的26%-59%。SR空间异质性的控制因子及其对模型的贡献依赖于处理和水分条件。这些发现有助于我们理解草地生态系统中地上和地下生态过程之间的关系以及预测草地土壤CO2排放对全球变化和土地利用的响应。