Data and code supporting the manuscript: Soil and microbial responses to wild ungulate trampling depend more on ecosystem type than trampling severity

1. Physical trampling is a ubiquitous activity of walking vertebrates, but is poorly understood as a mechanism impacting biogeochemical cycling in soil. Lack of detailed knowledge of soil abiotic-biotic interactions underlying trampling effects, and the primary sources of heterogeneity in these effects, limits our ability to predict ecosystem-level consequences of ongoing population changes in large animals.2. We conducted a summer field study of moose trampling effects on soil properties and nitrogen cycling using natural moose trails in boreal forest and heath barren ecosystems on the island of Newfoundland, Canada. We tested the extent to which trampling effects on soil abiotic properties, microbes, and nitrogen cycling depend on local trampling severity, and/or ecosystem type. Using structural equation models, we further tested whether trampling effects on soil nitrogen mineralization occurred via indirect interaction chains mediated by soil abiotic and microbial properties. Finally, we tested whether trampling modifies plot-scale controls on net nitrogen mineralization.3. Trampling effects on soil environment, substrate, and microbial properties depended on ecosystem type, but rarely on trampling severity. Trampling effects were of consistently greater magnitude in heath than forest. Further, trampling effects on organic matter content, pH, moisture, microbial abundance, and microbial community composition were qualitatively different between forest and heath ecosystems. Trampling severity increased the magnitude of some trampling effects in heath, but did not impact the direction of the effects, and did not significantly impact any trampling effects in forest.4. Trampling indirect effects on soil microbes were ecosystem-dependent but did not alter net N mineralization rates. In forest, trampling did not shift N mineralization rates via any indirect interaction chains, but modified relationships between N mineralization and other soil physical and microbial properties. In heath, multiple opposing interaction chains resulted in no net shift in N mineralization rates, and most relationships between diverse soil properties and N mineralization were not modified by trampling.5. Overall, our results disentangle complex abiotic-biotic interactions underlying megafauna effects on ecosystem functioning. These insights pave the way for better integration of animal consumptive and non-consumptive mechanisms into biogeochemical models, as well as prediction of trampling effects over landscapes.

1. 物理踩踏（physical trampling）是行走脊椎动物普遍存在的行为，但作为影响土壤生物地球化学循环的机制，其相关机理尚未得到充分研究。由于我们对踩踏效应背后的土壤非生物-生物相互作用（abiotic-biotic interactions），以及这些效应异质性的主要来源缺乏详细认知，难以预测大型动物种群持续变化所带来的生态系统尺度后果。2. 我们依托加拿大纽芬兰岛北方针叶林与石楠荒原生态系统中的天然驼鹿步道，开展了夏季野外研究，探究驼鹿踩踏对土壤性质与氮循环的影响。我们测试了踩踏对土壤非生物性质、微生物群落与氮循环的效应在多大程度上取决于局部踩踏强度与（或）生态系统类型。借助结构方程模型（structural equation models），我们进一步验证了踩踏对土壤氮矿化的效应是否通过由土壤非生物与微生物性质介导的间接交互链实现。最后，我们测试了踩踏是否会改变样地尺度上的净氮矿化调控机制。3. 踩踏对土壤环境、基质与微生物性质的效应取决于生态系统类型，但几乎不受踩踏强度的影响。相较于北方针叶林，石楠荒原中的踩踏效应强度始终更高。此外，踩踏对土壤有机质含量、pH值、含水量、微生物丰度以及微生物群落组成的效应，在北方针叶林与石楠荒原生态系统中存在性质层面的差异。在石楠荒原中，踩踏强度会提升部分踩踏效应的强度，但不会改变效应的作用方向，且不会对针叶林内的任何踩踏效应产生显著影响。4. 踩踏对土壤微生物的间接效应依赖于生态系统类型，但并未改变净氮矿化速率。在北方针叶林中，踩踏未通过任何间接交互链改变氮矿化速率，却调整了氮矿化与其他土壤物理性质及微生物性质之间的关联。在石楠荒原中，多条对立的交互链共同作用，使得氮矿化速率未发生净变化，且多数土壤性质与氮矿化之间的关联未受踩踏影响。5. 综上，本研究厘清了巨型动物（megafauna）影响生态系统功能背后复杂的非生物-生物相互作用机制。这些研究成果为将动物的消耗性与非消耗性作用机制更好地整合进生物地球化学模型，以及预测景观尺度上的踩踏效应提供了支撑。