University of Manchester research suggests removing livestock from grasslands could weaken soil carbon storage, challenging climate assumptions
A University of Manchester study warns that removing livestock from grasslands may compromise long-term soil carbon storage. While researchers found that ungrazed land may increase surface carbon, they also discovered it can reduce more stable forms of carbon stored deep in the soil. This raises questions about current land-management strategies aimed at tackling climate change.
The new findings are published in the Proceedings of the National Academy of Sciences (PNAS).
Rethinking livestock removal as a climate solution
Historically, removing sheep and other livestock from upland grasslands has often been promoted as a strategy to boost carbon storage and tackle climate change. However, new research presented here suggests that this may reduce the most stable forms of soil carbon.
Manchester researchers have found that removing livestock from upland grasslands can increase the fast-cycling carbon stored in plants and dead vegetation, but can also lead to losses of a more stable form of soil carbon. This long-lived carbon, known as mineral-associated organic carbon (MAOC), binds to soil minerals and can persist for decades.
This is important because grasslands store around one-third of the world’s terrestrial carbon, with the vast majority being found in soils. Removing livestock from historically grazed grasslands has often been proposed as a scalable climate solution.
“While ungrazed grasslands tend to accumulate more unprotected carbon in plants and litter, they are associated with lower levels of soil carbon protected by minerals, which is the form most resistant to warming-induced decomposition,” explained Dr Luhong Zhou, lead author of the study and visiting scholar at The University of Manchester. “Although high grazing intensity can negatively affect soil carbon, our results show that total grazer exclusion does not necessarily lead to greater long-term soil carbon storage.”
How researchers compared grazed and ungrazed grasslands
To investigate these effects, a global team of researchers analysed 12 upland grasslands across an 800km south-north gradient in the United Kingdom. At each site, they compared grasslands that had been ungrazed for over ten years with neighbouring areas that had been grazed over that time.
They found that ungrazed grasslands tend to accumulate more short-lived carbon in plants and surface litter but generally have less MAOC. Long-lived soil carbon is linked to changes in vegetation following the removal of grazing sheep. This leads to grass-dominated landscapes being replaced by dwarf-shrub-dominated landscapes, such as those of heather. The roots of the shrubs form associations with a specialised fungi called ericoid mycorrhiza. These fungi slow the decay of plant litter, increasing short-lived carbon, but also stimulate the breakdown of older, more stable soil carbon to gain nutrients to sustain plant growth.
“Viewing grazer removal as a universally beneficial strategy for carbon mitigation often overlooks the continuum of carbon durability within ecosystems, and the fact that not all carbon gains contribute equally to long-term climate mitigation,” said Dr Shangshi Liu from the Yale Center for Natural Carbon Capture who co-led this study. “ When slow-cycling carbon declines, grassland carbon stocks may become more vulnerable to future climate change. Effective climate mitigation strategies must therefore consider both how much carbon is stored and how durable it is.”
Professor Richard Bardgett, Chair of Ecology at Lancaster University, who initiated the study while at The University of Manchester, said: “Our results suggest that maintaining low-intensity grazing in upland grasslands, which cover large areas in the United Kingdom, is important for protecting the most stable forms of soil carbon.”
