The formation of peat—Decreasing density with depth in UK peats

Abstract

Increasing bulk density with depth in a peat profile has been seen as key to the formation of peat. Increasing bulk density with depth causing the changes in porosity and permeability of peat soils has been proposed as a mechanism to explain how waterlogged, stagnant conditions persist in peat bog soils. However, a previous study (Clay & Worrall, 2015; Soil Use & Management, 31, 77) observed, in passing, that this was not always the case, but this previous study could not test the nature of the peat bulk density profile. Thus, this present study examined 22 peat cores from 13 locations across climatic gradients of the UK, including valley fens, blanket and raised bogs, and both intact peatlands and former peat extraction sites. At none of the 13 locations was there a significant increase in dry bulk density with depth in the peat profile. The oxidation state of the organic carbon (Cox) in the peat profile showed no common pattern of change with depth. The only measured property that showed a consistent change down the peat profile among all 13 locations was an increase in the degree of unsaturation of C bonds. The change in degree of unsaturation shows a trend away from vegetation biomass composition and toward lignin‐like compositions with an average rate of change of 0.2 π‐bonds/ka. The measured pattern of bulk density between locations and types of peat shows that peat density reflects the contemporary peat environment, rather than the nature of peat formation. The bulk density profile likely reflects average water table position, as determined by topographic and land‐management factors, with peat compaction and accelerated decomposition within the aerobic zone. The presence of gas bubbles may also contribute to low bulk density at depth. The common bulk density profile found across the UK for peat ecosystems shows that peat formation is not controlled by porosity change but more specifically by water flow and hydraulic conductivity.