Understanding Soil Carbon: The Hidden Value of Grazing

Rob Beeson
44 minutes ago
3 min read

✅ KEY TAKEAWAY: While removing livestock may increase visible vegetation, this study indicates it can destabilize the deep, long-term carbon stores that are vital for climate security.

For decades, land managers and hill farmers have acted as custodians of the British uplands, balancing food production with the maintenance of unique habitats. Recently, however, there has been increasing pressure to remove livestock from hillsides under the assumption that "rewilding" or ending grazing is the best way to lock up carbon to fight climate change.

A significant study published in the Proceedings of the National Academy of Sciences (PNAS) challenges this simple narrative.

The research, led by scientists across the UK and abroad, suggests that while fencing off land creates more vegetation above ground, it may actually reduce the stability of the carbon stored below ground - where it matters most.

The Study

The researchers looked at 12 distinct grassland sites spanning an 800km gradient across the UK, from Dartmoor and Exmoor in the south to the Highlands of Scotland.

They compared plots of land that have been grazed by sheep (at typical stocking densities) with adjacent plots where livestock have been excluded for over 10 years. This allowed them to see the real-world, long-term effects of removing grazers from the landscape.

Fast Carbon vs. Slow Carbon

To understand the findings, it is important to distinguish between two types of carbon:

Fast-Cycling Carbon (Particulate Organic Carbon): This is found in plant litter and shallow roots. It breaks down relatively quickly - over years or decades.
Slow-Cycling Carbon (Mineral-Associated Organic Carbon): This carbon binds to minerals in the soil. It is incredibly stable, protected from decomposition, and can remain locked away for centuries or even millennia.

For genuine climate mitigation, Slow-Cycling Carbon is the "gold standard." It is the long-term savings account of the soil.

Key Findings

The study found that simply removing livestock is not a silver bullet for carbon storage. In fact, it leads to a trade-off that could be risky for the future.

More Biomass, Less Stability: As expected, the ungrazed plots had more vegetation and leaf litter. However, the study found that these plots had lower levels of the stable, Slow-Cycling Carbon compared to the grazed plots.
The Vegetation Shift: When sheep were removed, the land changed. There was a 27% increase in shrub cover (such as heather) but a 26% decrease in grasses.
Soil Chemistry Changes: This shift from grass to shrub dominance changed the way the soil functions. The study revealed that in ungrazed plots, the minerals that usually protect carbon were less effective.

Why Does This Happen?

The researchers discovered that the change in vegetation alters the soil biology. The roots of shrubs and the fungi associated with them (ericoid mycorrhiza) produce enzymes that can actually "prime" the soil microbes to decompose old, stable carbon.

Essentially, by allowing shrubs to dominate completely at the expense of grasses, we may be inadvertently unlocking the carbon vault in the soil.

What This Means for Moorland Management

This research validates the traditional view that the uplands are complex ecosystems that require active stewardship.

While seeing tall vegetation in fenced-off areas might look like a conservation success, this "surface greening" masks a loss of stability underground. The researchers noted that while exclusion favours fast-cycling carbon, it "coincides with lower soil organic carbon persistence."

This suggests that a balanced approach to grazing - maintaining a mix of grasses and shrubs - may be more effective for keeping carbon locked in the ground for the long term than total abandonment or exclusion.

The Bottom Line

For rural communities and conservationists, this is a reminder that we must look beyond the surface. Responsible grazing is not just about tradition or food production; it plays a complex biological role in maintaining the health and stability of our soil.