A small grant from the ECT and the Ramble Worldwide Outdoor Trust has enabled a research team to look at how experimental treatments affect the carbon sequestration rate of peatlands.
Peatlands are valuable conservation landscapes with vast carbon stores. However, they are threatened by both climate change and management. In particular, prescribed heather burning is a highly contentious subject with potential negative impacts on carbon accumulation. Although there is limited detailed evidence to support such claims. Much research has concentrated on the long-term experimental burn plots on Hard Hill at Moor House which is one of the ECTs flag ship LTE sites (yet burning has been discontinued). The small experimental burns offer a known long-term chrono-sequence of:
1- Regular burns (10-yearly or 20-yearly)
2- Control plots (experimental no burn since 1954)
3- Reference areas (long-term no burn since 1926)
FIGURE 1: Peat sampling in June 2023 at Moor House Hard Hill plots (from left to right: student Anabel Cole, emeritus Prof Rob Marrs, technicians Dr Tom Holmes, Dr Luke Andrews and Dr Andreas Heinemeyer).
Previous research at Hard Hill indicated reduced carbon sequestration from burning. Initially, Garnett et al. (2000) stated a large loss of about 70 gC m-2 yr-1 on grazed and 10-yearly burnt plots versus unburnt (since 1954) plots. However, this study had some methodological limitations and did not compare all grazed treatments. Subsequently, Marrs et al. (2019), with more robust methods compared all grazed plots. They found an only marginally significant and much smaller reduction of 10 gC m-2 yr-1 in the regular burn plots (10-yearly) compared to the reference areas. However, they measured organic carbon content (%Corg) only indirectly; cores only captured the carbon accumulation rates in the top ~25 cm at 1 cm resolution. However, it is vital to measure %Corg directly and to sample at finer (0.5 cm) resolution to capture potential charcoal impacts on increased %Corg and bulk density (Heinemeyer et al., 2018). These uncertainties in previous studies may have caused an underestimation in carbon sequestration.
Supported by a small grant from the ECT and the Ramble Worldwide Outdoor Trust, a team from Liverpool University and the University of York (Figure 1) took 16 new 1 m deep peat cores in June 2023. 4 cores were taken from the regular 10-yearly and 20-yearly burns, the control plots, and the reference areas. They used 0.5 cm sections for the surface 25 cm peat section (Figure 2) for bulk density determination and direct measurements of %Corg (C/N analyses.
Figure 2: Peat core (5x5 cm) from the top 25 cm (left) which was sub-sampled for 0.5 cm sections (right) to determine organic carbon content and bulk density for carbon accumulation rates. Photo A. Heinemeyer
Figure 3: New shrinkage corrected (2023; left) and previous Marrs et al. (2019; right) carbon accumulation rates over the top ~25 cm (i.e., since 1963). Treatments are: R = no burn since 1926 reference plots; N = no burn since 1954; L = 20-yearly and S = 10-yearly burns. Different letters indicate weakly significant (P < 0.027) differences observed only for Marrs et al. (2019).
Whilst bulk density was higher under burning this was only significant for the 10-year burn. The %Corg was slightly lower, yet overall %Corg values were about 12% higher than those reported in Marrs et al. (2019).
Carbon sequestration rates were statistically not different between burnt and control plots with averages for the 1963-2023 period of about 70 gC m-2 yr-1, which was higher (by about 21 gC m-2 yr-1) than those reported in Marrs et al. (2019). However, peat shrinkage affected our new cores as water tables were lower in 2023 compared to the Marrs et al. (2019) samples.
After correcting the age-depth of peat layers for shrinkage, the sequestration rates were still about 12 gC m-2 yr-1 higher than in Marrs et al. But also not significantly different with about 61 gC m-2 yr-1 (Figure 3).
This study highlights the importance of detailed and direct peat property assessments to calculate accurate carbon accumulation rates. The team intend to fully analyse to a depth of 1 m (at 1 cm resolution) to test for any pre-experimental differences in carbon accumulation rates.
