Ungulate herbivory affects grassland soil biota β‐diversity and community assembly via modifying soil properties and plant root traits

Summary Ungulate herbivory, a widespread and complex disturbance, shapes grassland biodiversity and functions primarily through three mechanisms: defoliation, trampling, and excreta return. However, the specific effects of these mechanisms on soil biodiversity and community assembly remain unclear. We conducted a 4‐yr factorial experiment in the Eurasian steppe to investigate how defoliation, trampling, and excreta return influence soil bacterial, fungal, and nematode β‐diversity and community assembly under moderate‐ and high‐density ungulate grazing scenarios. Our findings reveal that herbivores affect soil biota through multiple pathways at different grazing intensities. Specifically, selective defoliation in the moderate‐density scenario promoted stochastic community assembly of nematodes and fungi by increasing the specific root length of plant communities. Excreta return encouraged stochastic bacterial communities by carbon input, while urine‐induced acidification and elevated ammonium levels promoted environmental filtering of bacteria and nematodes. In the high‐density scenario, non‐selective defoliation and heavy trampling created harsher soil conditions, reducing bacterial and nematode β‐diversity via habitat filtering and diminishing association of soil biota with plant roots. This study explored how different components of ungulate behaviour influence soil community assembly and highlighted the crucial role of root traits in mediating soil biota responses, providing insights into the mechanisms of soil biodiversity maintenance under complex disturbances.