Grazing is the dominant land use across the world, and large mammalian herbivores exert strong influence over biogeochemical cycles. Grazing ecosystems feature C-rich soils, even though herbivores consume a major fraction of plant production to reduce detrital input to soil. Yet, counter-intuitively, moderate grazing can promote net soil-C storage in many ecosystems compared to grazer-exclusion. We address this enigmatic influence of grazers on soil-C and test their indirect effect on proximate drivers of decomposition: microbial extracellular enzyme activity. We used a replicated long-term grazer-exclusion experiment to measure responses in above- and belowground plant biomass, soil-C stock, microbial biomass, labile/recalcitrant C pools and three enzymes relevant to the C-cycle: peroxidase—which initiates decomposition of recalcitrant matter, alongside beta-glucosidase and cellobiohydrolase—which act further downstream on more labile fractions. Consistent with other ecosystems, upto 12 years of herbivore exclusion did not increase soil-C in the fenced plots despite higher plant biomass and higher potential detrital C-inputs. Grazer-exclusion did not alter microbial biomass; peroxidase increased threefold and beta-glucosidase was doubled; cellobiohydrolase was unaffected. Grazer-exclusion also led to twofold increase in recalcitrant-C and in microbial respiration, but it did not influence labile-C. Structural equation models supported the hypothesis that grazing favours soil-C via its indirect effect on peroxidase, but they did not support that the effects can run in the opposite direction where soil-C affects enzymes. Grazer-mediated shifts in how microbes deploy enzymes emerge as a plausible mechanism that affects soil-C. These linkages may be important to maintain soil-C sequestration in drylands which support large mammalian herbivores.

放牧是世界上主要的土地利用方式，大型哺乳动物食草动物对生物地球化学循环产生重大影响。放牧生态系统以富含碳的土壤为特色，尽管食草动物消耗了大部分植物产量以减少对土壤的碎屑输入。然而，与直觉相反，与排除放牧者相比，适度放牧可以促进许多生态系统中土壤碳的净储存。我们解决了食草动物对土壤碳的这种神秘影响，并测试了它们对分解的直接驱动因素的间接影响：微生物细胞外酶活性。我们使用了重复的长期 grazer-exclusion 实验来测量地上和地下植物生物量、土壤碳储量、微生物生物量、不稳定/顽固碳库和三种与碳循环相关的酶：过氧化物酶——启动分解的反应顽固的事物，与 β-葡萄糖苷酶和纤维二糖水解酶一起作用于下游更不稳定的部分。与其他生态系统一致，尽管植物生物量和潜在碎屑 C 输入量更高，但长达 12 年的草食动物排斥并未增加围栏地块中的土壤碳。Grazer-exclusion 不改变微生物生物量；过氧化物酶增加三倍，β-葡萄糖苷酶增加一倍；纤维二糖水解酶不受影响。Grazer-exclusion 也导致顽固-C 和微生物呼吸的两倍增加，但它不影响不稳定-C。结构方程模型支持放牧通过对过氧化物酶的间接影响有利于土壤碳的假设，但他们不支持这种影响可以在土壤碳影响酶的相反方向上运行。Grazer 介导的微生物如何部署酶的变化成为影响土壤 C 的一种合理机制。这些联系对于维持支持大型哺乳动物食草动物的旱地土壤碳封存可能很重要。