Data from: Towards a mechanistic understanding of the effect that different species of large grazers have on grassland soil N availability

1. Herbivore grazing has major effects on soil nutrient dynamics in a variety of grassland ecosystems. Previous studies have examined how large herbivores as a group affect nutrient cycling, but little information is available on how assemblages of different herbivore species may influence nutrient cycling, and whether herbivore assemblage effects are influenced by plant community characteristics (e.g., composition, diversity) of the grazed grassland. 2. We conducted a five-year, replicated grazing experiment to test the effects of different large herbivore assemblages (cattle grazing, sheep grazing, combined cattle and sheep grazing, no grazing) under moderate grazing intensity on soil nitrogen (N) mineralization rate in two types of grassland communities (high forbs/high diversity and low forbs/low diversity) in meadow steppe habitat of northeast China. Moreover, we examined two distinctly different pathways that herbivores could influence soil N availability: directly through urine and dung deposition and indirectly by shifting grassland species composition (i.e., the grass: forb ratio), thereby the quality of plant litter available to soil decomposers. 3. We found that grazer effects on soil N availability (indexed with anion and cation adsorption strips) depended on herbivore assemblage, and the herbivore assemblage effects varied in the two types of grasslands. In one type of grassland characterized by low diversity, grazing by each of the herbivore assemblages enhanced soil N availability compared to the ungrazed plots, and mixed species (cattle and sheep) grazing had a greater effect than single species grazing. In high diversity grassland, single species herbivore grazing significantly increased soil N availability, but mixed grazing had no effect. 4. Mixed linear modelling revealed that soil N availability was facilitated primarily by excreta additions to the soil and secondarily by the abundance of grasses. 5. Synthesis. Grazers increased soil N availability directly by adding readily accessible N in urine and dung to the soil. Herbivores indirectly influenced soil N availability by altering the plant composition (grass: forb cover). Both mechanisms contributed to the variation in how different herbivore assemblages affected soil N availability in the two grassland types.1. 草食动物放牧对多种草原生态系统的土壤养分动态具有显著影响。既往研究多关注大型草食动物整体对养分循环的调控作用，但目前仍缺乏关于不同草食物种类群组合如何影响养分循环，以及草食动物类群（herbivore assemblage）效应是否受放牧草原植物群落特征（如群落组成、多样性）调控的相关研究数据。 2. 本研究在中国东北草甸草原生境中开展了为期5年的重复放牧控制实验，设置中度放牧强度下的4种大型草食动物类群处理：单牛放牧、单羊放牧、牛羊混合放牧、无放牧对照，旨在探究不同类群处理对两种草原植物群落（高杂类草/高多样性群落、低杂类草/低多样性群落）土壤氮（N）矿化速率的影响。此外，本研究同时解析了草食动物调控土壤氮有效性的两种截然不同的途径：一是通过尿液与粪便沉积直接向土壤输入养分，二是通过改变草原植物群落组成（即禾草与杂类草的比例），进而改变土壤分解者可利用的植物残体质量。 3. 研究结果显示，以阴阳离子吸附袋为指标的土壤氮有效性，其草食动物调控效应取决于草食动物类群组合，且该类群效应在两种草原群落中表现出显著差异。在低多样性草原群落中，各草食动物类群处理的土壤氮有效性均显著高于无放牧对照，且牛羊混合放牧的调控效果优于单物种放牧；而在高多样性草原群落中，单物种放牧可显著提升土壤氮有效性，但混合放牧未产生显著影响。 4. 混合线性模型（mixed linear modelling）分析显示，土壤氮有效性主要受土壤排泄物输入的促进，其次受禾草丰度的影响。 5. 综合分析表明：草食动物一方面通过向土壤输入尿液与粪便中易获取的氮直接提升土壤氮有效性，另一方面通过改变植物群落组成（禾草与杂类草的盖度比）产生间接调控作用。上述两种机制共同解释了不同草食动物类群组合对两种草原类型土壤氮有效性的调控差异。