The ecology of peat-forming processes: a review

Peat is a predominantly organic material derived from plants that accumulates in certain types of ecosystems. Its formation is dependent on an excess of local plant productivity over the respiratory processes of organisms. Such an imbalance may be more closely related to the retardation of microbial activity than to high productivity, hence peat accumulates where physical conditions serve to reduce the rate at which detritovores and decomposers can consume the available organic resource. Many factors reduce the respiratory activity of aerobic microbes, such as low oxygen concentration, poor access to the food resource, limiting mineral elements, low pH, low temperature, and of these, the lack of oxygen is perhaps the most common and dominant cause of peat formation in nature. Waterlogging is frequently associated with low oxygen availability, and for this reason peat formation is closely linked with hydrologic factors. Mires (a general and useful term extensively applied in European literature to peat-forming ecosystems) can be classified into two types according to their hydrology. Rheotrophic mires are fed directly by both rain- and ground-water flow, and ombrotrophic mires receive water input only in the form of rainfall. These two types of mires differ not only in the quantity of water they receive, but also in their dissolved and suspended inorganic sediment load. These factors lead to differences in the vegetation, because some plants demand richer supplies of inorganic nutrient materials than others, as well as in the organic: inorganic ratio (that is, the amount of organic matter relative to the ash content) in the peat. Microbial activity in peat profiles is strongest in the upper layers (acrotelm) where the oxygen supply is relatively richer, but this activity continues at deeper levels (catotelm) as a result of the respiratory activity of anaerobic microbes. The preservation of organic matter depends upon various factors including its palatability to microbes, where it is located in the peat profile, how long it remains in the acrotelm, and the degree to which it is submitted to compression and, therefore, rendered less accessible to the microbial population of the catotelm. The proportion of inorganic material (as measured by ash content) in a peat varies by virtue of a variety of processes too. The inorganic content of its constituent plants, the allochthonous input sedimented or otherwise trapped from the water supply, aerial dust input, and the degree to which the original organic component has been decomposed are some of the factors. Because many of these variables alter during the course of hydroseral succession, inorganic fractions of peats also change as plant succession proceeds. The variety of mire types, their successional relationships and the nature of the peat they produce are discussed and considered in relation to the fossil record in coal.