How do peat soils form? Self-weight compaction versus decomposition in the early stages of peat development

Peat soils exist because the rate of accumulation of organic matter is faster than the rate of organic matter decomposition. This balance of rates can be in favour of net organic matter accumulation if: the rate of primary productive is relative high; the decomposition rate is relative slow; or, a combination of both. Slowing the decomposition rate has been ascribed to the presence of plants composed of decay-resistant components and to water-logged conditions. Water-logged conditions limit ingress of oxygen and oxygen is rapidly consumed by the supply of organic matter leaving decomposition dependent on other, less available and less energetically favourable terminal electron acceptors such as nitrate, sulphate and iron. The water-logged conditions can occur due to position in the landscape, high precipitation inputs, and/or restricted drainage within the peatland. Classic texts on peat formation refer to the development of restricted drainage within peat profiles but are vague on how it forms – for example “At first the porous structure survives, just as a wall with a few bricks removed does. But eventually the structure collapses. The dry bulk density increases abruptly” – Clymo and Pearce (1995). However, despite this processing being referred to in most texts the process and role of compaction in the formation of peat is not detailed nor has it been studied. Therefore, in this study we measure the initial development of peat from sphagnum moss and question which is more important in the development of peat soil – is it self-weight compaction or degradation - and are these two processes independent?

Using sphagnum moss mesocosms we compared the change in peat depth with the flux of CO₂ from the peats. Over periods of more than 1 year the surface recession and CO₂ flux were monitored in 12  sphagnum mesocosms relative to water table and climatic conditions.

The results show:

• Dry bulk density did not significantly change over the course of the experiment.
• The initial surface recession was between 1.7 to 35 % of depth with a median = 9.7%
• Young’s modulus had a median = 1.9 MPa ranging between 0.4 and 13.0 MPa.
• Given the values of the Young’s modulus calculated for these mesocosms then the viscosity varied between 2.2 and 20.4 Ns/m² with a median of 6.1 Ns/m². These calculations suggest that the sudden stress is readily adsorbed.
• After 214 days a median of 23.7 % surface recession had occurred or a median of a further 33% surface occurring after the initial surface recession.
• Comparison between the extrapolated CO₂ flux and the measured surface recession across the entire experiment between -2 and 75% with a median of 29% due to self-weight compaction. There is no apparent correlation between length of the experiment and proportion of the effect due to self-weight compaction.

This study has been shown that, although self-weight compaction was a major component of the development of the peat, the initially phases of peat development were dominated by degradation of the peat. Further that degradation was able to equilibrate with initial self-weight compaction.