Defoliation decreases soil aggregate stability by reducing plant carbon inputs and changing soil microbial communities

Soil aggregation and its stability are fundamental in ensuring soil function and ecosystem service provision, which can be greatly altered by ungulate disturbance, particularly the indirect effects from biomass removal by defoliation and direct effects from hoof trampling, but the mechanisms underlying the effect of defoliation and trampling on soil aggregation remains largely unexplored. Here, we conducted a 3-year manipulative experiment to investigate the effects of defoliation and trampling of ungulates and their interaction on soil aggregate stability in a semiarid grassland of Inner Mongolia, China. We found that defoliation rather than trampling dominantly affected soil aggregate stability. Defoliation disintegrated macroaggregates, reducing aggregate stability (-5 %), mainly attributed to the direct influence of a decline in plant inputs (aboveground biomass: -32 %) and the indirect effects of soil microbial biomass (-5 %). Moreover, compared to no defoliation plots, defoliation significantly decreased soil fungal diversity and altered fungal communities, particularly decreasing the relative abundance of Glomeraceae fungal taxa favorable to the formation of soil aggregates. Trampling only slightly attenuates the involvement of microorganisms (indirectly, -7 %) in aggregate formation by increasing soil compaction (directly, -4 %), and the positive effects from accelerating litter-derived C inputs due to trampling disturbance may counteract these limited negative effects. Our results highlight the negative effect of biomass removal by ungulates on soil aggregation, and suggest that the role of plant inputs and soil microbial properties on soil function and their correlation with soil aggregate stability needs to be explored under a diversity of grazing management strategies to refine management to improve soil conservation, C persistence and restore grasslands.