Peatland carbon stocks and burn history: Blanket bog peat core evidence highlights charcoal impacts on peat physical properties and long-term carbon storage

Peatlands are globally important carbon stores, yet both natural and human impacts can influence peatland carbon accumulation. While changes in climate can alter peatland water tables leading to changes in peat decomposition, managed burning of vegetation has also been claimed to reduce peat accumulation. Particularly in the UK, blanket bog peatlands are rotationally burned to encourage heather re-growth on grouse shooting estates. However, the evidence of burning impacts on peat carbon stocks is very limited and contradictory. We assessed peat carbon accumulation over the last few hundred years in peat cores from three UK blanket bog sites under rotational grouse moor burn management. High resolution (0.5cm) peat core analysis included dating based on spheroidal carbonaceous particles, determining fire frequency based on macro-charcoal counts and assessing peat properties such as carbon content and bulk density. All sites showed considerable net carbon accumulation during active grouse moor management periods. Averaged over the three sites, burns were more frequent, and carbon accumulation rates were also higher, over the period since 1950 than in the period 1700-1950. Carbon accumulation rates during the periods 1950-2015 and 1700-1850 were greater on the most frequently burnt site, which was linked to bulk density and carbon accumulation rates showing a positive relationship with charcoal abundance. Charcoal input from burning was identified as a potentially crucial component in explaining reported differences in burning impacts on peat carbon accumulation, as assessed by carbon fluxes or stocks. Both direct and indirect charcoal impacts on decomposition processes are discussed to be important factors, namely charcoal production converting otherwise decomposable carbon into an inert carbon pool, increasing peat bulk density, altering peat moisture and possibly negative impacts on soil microbial activity. This study highlights the value of peat core records in understanding management impacts on peat accumulation and carbon storage in peatlands.