Effects of grazer exclusion on aboveground and soil biogeochemical properties

The removal of livestock grazers from historically grazed grasslands is widely proposed as a key strategy for the enhancement of soil organic carbon (SOC) for climate mitigation. Yet, accurate assessments of how grazer exclusion impacts SOC pools of differing stability are lacking, with most studies focusing on total SOC rather than the distribution of SOC within fast and more stable, slow-cycling pools. Here, we used 12 historically grazed grassland sites along an 800 km south–north gradient across the United Kingdom to test how particulate (POC) and mineral-associated organic carbon (MAOC) pools were linked to long-term (>10 y) exclusion of large domesticated grazers. We found that grazer exclusion was associated with relatively higher fast-cycling C pools, including plant and litter C, and to a lesser extent POC, but lower more stable, slow-cycling MAOC pools compared to grazed controls. Grazer exclusion was also associated with a marked shift in vegetation composition, with greater cover of ericoid mycorrhizal (ErM) shrubs over arbuscular mycorrhizal (AM) graminoids. This vegetation shift likely played a dual role in regulating SOC, contributing to higher POC via both the input of recalcitrant litter and by the enhancement of soil moisture and lower MAOC due to priming and decreased mineral protection of SOC. Our findings provide evidence that while the exclusion of grazers tends to favor fast-cycling C pools, it coincides with lower SOC persistence, potentially increasing the vulnerability of grassland SOC stocks to future climate change.将放牧家畜从历史放牧草原中移除，被广泛提议为提升土壤有机碳（soil organic carbon, SOC）以实现气候减缓的关键策略。然而，目前尚缺乏针对放牧排除如何影响不同稳定性SOC库的精准评估，多数研究仅关注总SOC含量，而非SOC在快速循环与更稳定的慢速循环库中的分布格局。本研究依托横跨英国800千米南北梯度的12个历史放牧草原站点，探究颗粒态有机碳（particulate organic carbon, POC）与矿物结合态有机碳（mineral-associated organic carbon, MAOC）库与大型家养牲畜长期（>10年）放牧排除的关联。研究发现，与放牧对照组相比，放牧排除与较高的快速循环碳库（包括植物与凋落物碳）以及一定程度上的POC相关，但与更低的稳定慢速循环MAOC库相关。放牧排除还与植被组成的显著转变相关，表现为杜鹃花类菌根（ericoid mycorrhizal, ErM）灌木的盖度高于丛枝菌根（arbuscular mycorrhizal, AM）草本植物。这种植被转变可能在调控SOC中发挥双重作用：一方面通过输入难降解凋落物以及提升土壤湿度，促进POC含量提升；另一方面则通过激发效应以及降低SOC的矿物保护作用，导致MAOC含量下降。本研究结果表明，尽管放牧排除往往有利于快速循环碳库，但同时会导致SOC持久性降低，这可能会提升草原SOC库对未来气候变化的脆弱性。