# Grazer exclusion is associated with higher fast-cycling carbon pools but lower slow-cycling mineral-associated carbon across grasslands

## TL;DR

Removing livestock from grasslands increases fast-cycling carbon but decreases long-term stable carbon, potentially making soil carbon less resilient to climate change.

## Contribution

This study reveals that grazer exclusion alters both fast and slow-cycling soil carbon pools, with implications for carbon persistence in grasslands.

## Key findings

- Grazer exclusion increases fast-cycling carbon pools like plant and litter carbon.
- Exclusion of grazers leads to lower mineral-associated organic carbon, a more stable carbon pool.
- Vegetation shifts, such as increased ericoid shrubs, influence soil carbon dynamics through multiple pathways.

## Abstract

The removal of livestock grazers from historically grazed grasslands is widely considered a beneficial strategy for restoring soil organic carbon (SOC) stocks and mitigating climate change. However, this view often overlooks the functional diversity of SOC pools and the contrasting mechanisms that govern their dynamics. Drawing on a network of long-term grazer exclusion experiments on grasslands, we show that grazer exclusion is associated with lower mineral-associated organic C—a key component of long-term C persistence—despite higher levels of faster-cycling, plant-derived pools. We also identify pathways that regulate these distinct SOC pools, which are linked to grazer-induced shifts in plant community composition. Our findings underscore the need to consider both the quantity and persistence of SOC in grassland management strategies.

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.