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. 草食动物的采食活动对多种草地生态系统的土壤养分动态（soil nutrient dynamics）具有显著调控作用。既往研究多围绕大型草食动物整体类群对养分循环（nutrient cycling）的影响展开，但关于不同草食物种类群组合（herbivore assemblage）如何影响养分循环，以及草食动物类群的效应是否受被采食草地的植物群落特征（如群落组成、物种多样性）调控，目前仍缺乏相关研究数据。2. 本研究在中国东北草甸草原（meadow steppe）生境中，设置了为期5年的重复放牧控制实验，以检验中度放牧强度下，4种大型草食动物采食组合（单牛放牧、单羊放牧、牛羊混合放牧、无放牧）对两种草地植物群落（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（soil nitrogen mineralization rate）的影响。此外，本研究还探究了草食动物调控土壤氮有效性的两条截然不同的路径：一是通过尿液与粪便的沉积直接作用于土壤，二是通过改变草地植物群落组成（即禾草与杂类草比例，grass: forb ratio），进而改变可供土壤分解者利用的植物残体质量。3. 研究结果显示，草食动物对土壤氮有效性的调控效应（以阴阳离子吸附条（anion and cation adsorption strips）为检测指标）取决于草食动物类群组合，且该类群组合的效应在两种草地群落中存在显著差异。在低多样性草地群落中，各草食动物采食组合均较无放牧对照提升了土壤氮有效性，其中牛羊混合放牧的促进效果显著优于单物种类群放牧；而在高多样性草地群落中，单物种类群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（excreta）输入的促进，其次受禾草丰度的影响。5. 综合分析表明，草食动物通过直接向土壤添加尿液与粪便中的易获取态氮，提升了土壤氮有效性；同时通过改变植物群落组成（禾草与杂类草盖度比，grass: forb cover）间接调控土壤氮有效性。这两种作用机制共同解释了不同草食动物类群组合在两种草地类型中对土壤氮有效性的调控差异。