Assessing the impact of land-use on soil carbon stored in deep peat

Peatlands accumulate organic material in waterlogged soil and provide stable, long-term storage for carbon. However, peatland carbon stocks are sensitive to disturbance from land-use change and the increase in vascular plant cover. Peat below 0.5 meters is rarely investigated in peatland research but is generally assumed to be recalcitrant and resistant to decomposition, while representing the largest source of potential carbon loss. In this study I measured peat depth and sampled peat from 0.5, 1.5, and 3 m across varying levels of land-use change within Vestreimsmyrane peatland complex in Kaupanger, Sogndal municipality, western Norway. In total I collected 36 peat cores and 68 depths with vegetation surveys, from 6 sites ranging from intact to drained and cultivated. I calculated soil organic matter (SOM) and bulk density in the lab to estimate carbon density and stocks. I identified patterns of soil carbon loss at depth by using mixed-effect linear modelling to examine the relationships of land-use and vegetation on SOM and bulk density. The study area had a mean depth of 2.3 m, and the deepest spot was over 9 m. SOM (36.1–97.8%) was 34.6% lower in drained and cultivated sites than intact, while bulk density (0.04–0.32 g/cm3) was higher, together indicating carbon loss down to 3 m. Vegetation composition was associated with bulk density at 0.5 and 1.5 m, but with SOM only at 1.5 m. Carbon density ranged from a mean of 143 kg/m2 up to 360 kg/m2 and closely reflected the depth of the peat. Of the plant types surveyed, only the presence of Sphagnum species was an indicator of relative depth, but did not preclude large depths in its absence. My results show that the impacts of land-use on soil carbon in peatlands extends down to at least 3 m, and that the depth of a peatland is critical to carbon stocks or losses. As Sphagnum cannot reliably indicate depth, carbon stock estimates should measure depth when making calculations. Peatland carbon and emissions are likely underestimated in Norway, and a thorough assessment of peatland characteristics is needed to support nature management and planning.