The impacts of large wild grazers on soil carbon persistence are strongly modulated by the environmental context

Wild animals, particularly large mammalian grazers, can influence ecosystem carbon dynamics and soil carbon sequestration. However, we lack a mechanistic understanding of how wild grazer communities influence soil carbon persistence, specifically in terms of the size of vulnerable free particulate versus more stable occluded particulate and mineral‐associated carbon pools. Resolving this is crucial for predicting how soil carbon is stored long‐term and how grazing management can contribute to soil carbon persistence. We investigated associations between a wild mammalian grazer community and soil carbon persistence in a fire‐prone savanna in South Africa's Hluhluwe‐iMfolozi Park. More specifically, we assessed how variation in grazing (areas grazed at high vs. low intensity) and woody cover (woody‐encroached vs. non‐encroached grassland areas) related to the amount of soil organic carbon (SOC) and pyrogenic organic matter (PyOM, as a proxy for pyrogenic organic carbon) in pools with varying residence times, and how environmental conditions mediated these patterns. In order of increasing residence time, we looked at the free particulate, occluded and mineral‐associated soil carbon pools. Grazing was associated with more SOC in the mineral‐associated pool in fine‐textured soils but less SOC in coarse‐textured soils. In contrast, grazing was associated with more PyOM in the mineral‐associated pool at lower rainfall and in coarser‐textured soils, but less at higher rainfall and in finer‐textured soils. Grazing was also associated with more PyOM in the free particulate pool in finer‐textured soils but less in coarser‐textured soils, and with more PyOM in the occluded pool at low fire frequency but less at high fire frequency. Woody encroachment was consistently associated with less SOC and PyOM in the mineral‐associated pool, with these associations intensifying with higher rainfall (and, for SOC, at higher fire frequency; for PyOM, in finer‐textured soils). In contrast, woody encroachment was associated with more PyOM in the free particulate and occluded pools, although the occluded PyOM associations weakened with higher rainfall and fire frequency. Woody encroachment was associated with less SOC in the free particulate and occluded pools at low rainfall and fire frequency but more SOC at high rainfall and fire frequency. Our study reveals that intense grazing by large wild grazers and woody encroachment are associated with divergent soil carbon persistence outcomes, where the direction and magnitude of these associations are strongly modulated by the environmental context. Therefore, future assessments of large wild grazer impacts on carbon cycling would benefit from considering soil carbon persistence, not just total stocks, together with the environmental context in which these animals occur. Read the free Plain Language Summary for this article on the Journal blog.