Does Grazer Exclusion Improve Soil Carbon Storage? — a Q&A with YCNCC Postdoctoral Fellow Shangshi Liu

Earlier this year, YCNCC Postdoctoral Fellow Shangshi Liu  co-authored a peer-reviewed article in PNAS entitled “Grazer exclusion is associated with higher fast-cycling carbon pools but lower slow-cycling mineral-associated carbon across grasslands.”

YCNCC Science Communications Fellow Samantha Tracy sat down with Liu to discuss the importance of managing grazing on grasslands and the need for accurate estimates of the different stocks of soil organic carbon.

YCNCC: First off, can you explain what soil organic carbon is, and its function in the carbon cycle?

SL:  Soil organic carbon (SOC) is the carbon stored in soil organic matter. This includes decomposed plant material, roots, and microbial residues. It serves an important role in the global carbon cycle, as SOC stores more carbon than the atmosphere and forests combined. However, not all soil carbon is equal. Some SOC cycles very quickly and may return to the atmosphere within days to years, while some SOC can persist for centuries or even millennia, especially when associated with soil minerals. In considering SOC’s functionality in the carbon cycle, we have to not only consider the amount of carbon, but also the stability of SOC in soil systems. 

YCNCC: What types of impacts do grazers traditionally have on grassland ecosystems and how might they be influencing carbon storage?

SL:  Grazers have a variety of impacts on grasslands. They consume plant biomass, return nutrients through dung and urine, disturb soil through trampling, and influence which plant species dominate over time. In natural grassland ecosystems, large herbivores have long been part of the ecosystem, shaping vegetation, nutrient cycling, and soil processes. In addition to naturally occurring herbivorous species, grasslands are widely used for agricultural livestock grazing, where overgrazing is well documented to cause degradation of the ecosystem. Overgrazing has contributed to the degradation of nearly half of the world’s grasslands, creating major ecological and societal consequences, including biodiversity loss, soil degradation, reduced productivity, and large soil carbon deficits.

So grazing is not simply “good” or “bad” for soil carbon. It depends on grazing intensity, ecosystem context, management history, and the type of carbon pool. Removing grazers has often been viewed as a straightforward way to increase carbon storage, because plant biomass and litter can build up when animals are excluded. While that can increase some fast-cycling carbon pools, it is not that simple as grazing does not impact persistent SOC pools. These persistent pools carbon bound to soil minerals and resistant to decomposition can be fluctuating regardless of grazing history.

YCNCC: What are the key takeaways from the research and how might it change future rangeland management strategies?

SL:  The key message is that complete grazer exclusion does not always lead to greater long-term soil carbon storage compared to low-density grazing. In the UK upland grasslands we studied, excluding grazers for more than ten years was associated with higher fast-cycling carbon pools, such as plant and litter carbon, but lower persistent SOC.

This suggests that grassland management should avoid overly simple assumptions. Removing livestock may increase visible plant biomass or surface organic matter, but that does not always translate into more durable carbon storage. For future management, the goal may not be simply “more grazing” or “no grazing,” but finding grazing regimes that maintain plant productivity, support biodiversity, and protect the more stable forms of soil carbon.

YCNCC: What implications does this work have for future carbon accounting methods?

SL: Many carbon accounting approaches focus on total SOC stocks, but this can obscure important changes in the underlying carbon pool. Two soils might have similar total carbon, but very different proportions of fast-cycling and slow-cycling carbon.

For carbon markets, climate mitigation, and land-management decisions, we need better accounting of carbon durability. This will likely mean separating SOC into functionally meaningful pools — such as particulate organic carbon and mineral-associated organic carbon — and understanding how management affects each of them. We are also currently testing a new soil sensing approach that would allow us to predict these different forms of soil carbon at scale, which could make this kind of monitoring practically feasible across large landscapes. Without these validations, we risk overestimating the climate benefit of some land-management practices.

YCNCC: Any final thoughts?

SL: I would also emphasize that the negative impacts of overgrazing on SOC, vegetation, biodiversity, and wider ecosystem functioning are already well documented. Our study is not claiming that  grazing is always beneficial, or that grazing exclusion is always problematic, but is reporting how grazing management impacts SOC over time. 

This work further echoes the need for field-based, high-quality evidence on whether improved grazing management can increase soil carbon sequestration in grasslands. We hope our work is a useful step towards that, but there is still a great deal to learn.