According to a new study conducted by University of Manchester researchers, removing livestock from grasslands, a practice commonly considered beneficial for climate change mitigation, may actually damage the long-term carbon storage in soils.
The findings, published in the Proceedings of the National Academy of Sciences (PNAS) journal, indicate that while removing livestock may increase surface carbon, it can also reduce a more stable form of carbon stored in the soil.
When livestock is removed from upland grasslands, it can increase the fast-cycling, plant-stored CO2, but it can also cause losses to a type of long-lived soil CO2 known as mineral-associated organic carbon (MAOC), which binds to soil minerals and can remain stored away from the atmosphere for decades.
Lead study author and visiting Manchester University scholar Dr. Luhong Zhou explains,
“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.”
“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,” he says.
To reach these findings, a global team of researchers assessed 12 upland grasslands across an 800km south-north gradient in the UK, comparing ungrazed and grazed sites that have each been in that state for over 10 years.
This work showed that the ungrazed grasslands generally have less MAOC, and they tend to accumulate more short-lived carbon in plants and surface litter.
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The paper details that long-lived soil CO2 has a link to vegetation changes unlocked through the removal of grazing sheep, as this change leads to sites previously dominated by grass to be replaced with dwarf-shrub-dominated landscapes.
The roots of the shrubs then form associations with ericoid mycorrhizal fungi, known to slow the decay of plant litter, boosting short-lived CO2, but also catalyzing the breakdown of older, more stable soil carbon to gain access to the nutrients necessary for plant growth.
Drawing on these findings, study co-lead Dr. Shangshi Liu from the Yale Center for Natural Carbon Capture says, “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.”
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