---

  Peat

(with some remarks on mires and peatlands)

Hans Joosten & John Couwenberg

1.1.1. A mire is a landscape with a positive (or in any case not-negative, cf. Clymo-ceiling) carbon balance (production > decay), in which the carbon surpluss is accumulated as peat.

1.1.2. An indicator for active peat accumulation is the absence of a chronostratigraphical hiatus between the actual vegetation and the underlaying peat.
 

1.2. A peatland is a landscape, the surface layer of which consists of peat, that has originated on the spot.
 

1.3. Peat

1.3.1. Peat is (a layer of) sedentarily accumulated dead organic (non-biomass) material, that has remained longer than normal.

1.3.2. A layer implies, that peat is covering an area.

1.3.3. "Sedentary" means that the organic material has not been transported after its production and death. Peat differs in this respect from organic sediments like gyttjas, forest litter and graveyards.

1.3.4. "Accumulated" implies that over time (see 1.3.8) the cumulative production of dead organic material is larger than the cumulative decomposition.

1.3.5. With "organic" is meant that the material results from carbon chemical biosynthesis. Organic materials belong to the "organogenic" materials, that include all substances that have originated from organisms. Corals, for example, are organogenic, but not organic sedentates.

1.3.6. Peat is no biomass. The dead organic material of biomass is always physically and also functionally connected to a concrete living organism (e.g. the wood of a tree).

1.3.7. Peat may contain some organisms and some living or dead biomass (e.g. micro-organisms), but the concept of peat is defined on a coarser scale than on which these organisms can be observed.

1.3.8. "Longer than normal" means longer than the most common value on a coarser scale in space and time.

2.1.1. The accumulation of peat implies an imbalance of production and decay of dead organic material.

2.1.2. Eventually (= on a large timescale) all dead organic material is destroyed (if not, we would see a thick layer of dead organic material covering the whole Earth).

2.1.3. Directly after death (= on a short time scale), there is always a dominance of produced over decayed dead organic material.

2.1.4. For these reasons, the imbalance associated with peat accumulation can only be described meaningfully on a specific temporal scale.

2.1.5. This time scale has to be determined separately for various climate zones (or landform vegetation units sensu Ritchie?), as the turn-over time of dead organic material varies geographically (cf. tropics versus arctics).
 

2.2.1. An indicator for peat accumulation is a minimum thickness of the layer of peat (cf. the pedo- and geological definitions of peatlands).

2.2.2. Such depth criterium must, however, be made more objective, by defining a minimal dry weight content of organical material in a column of specific dimensions, e.g. 150 gr dry organic material in a column of 1 dm² wide and 3 dm deep. (remember: the difference between peat and water may only be some percents of organic material. A peat thickness of 3 dm says nothing if it is not stated how many water [or sand or clay, all with different specific weights!] is contained in that layer).
 

2.3. The imbalance: production

2.3.1. An imbalance may be caused by both sides of the balance, i.e. by the production side and by the decay side.

2.3.2. Peat accumulation may occur, when the rate of production of organic material is larger than normal.

2.3.3. Increased production may be brought about by additonal fertilization with plant nutrients, leading to increased primary production and (eventually) to an increased production of dead organic material. Factors stimulating production include

• a) CO₂: higher atmospheric CO₂ concentrations (and consequent lower oxidative O₂ concentrations) may have played an important role in Carboniferous and Tertiary peat accumulation.
• b) NPK: higher plant productivity by NPK-fertilization is mostly associated with increased decay rates, because simultaneously more favourable conditions are created for decomposing micro-organisms. In areas, however, where other factors (e.g. low temperatures) limit decay, NPK fertilization may result in peat accumulation (e.g. guanogenic bird tops in the Arctics, Van der Knaap 1988, Lag 1990).

2.4. Rate of decay: endogenic factors

2.4.1. The rate of decay may be smaller than normal. This may have both endogenic and exogenic reasons.

2.4.2. The endogenic factor for peat accumulation is the chemical and structural composition of the organic material, determining "decayability". This decayability varies with species (e.g. Phragmites versus Typha), plant parts (e.g. rhizoms versus flowers) and substances (e.g. sporopollenine versus sugars).

2.4.3. Under identical environmental conditions, eventually endogenic aspects are responsible for (selective) conservation (N.B. time scale!) of the produced dead plant material. Examples of (aerobic!) peat accumulation, primarily attributable to endogenic factors, are the moss cushion mires in the Antarctic (Fenton 1980).

2.4.4. The distribution of mires and peatlands over the Earth indicates that endogenic factors nowhere are the limiting factor for peat accumulation: some plant material may turn into peat everywhere.

2.4.5. Rates of decay are a function of both endogenic decayability and exogenic factors.
 

2.5. Rate of decay: exogenic factors

2.5.1. Exogenic factors limiting decay include:

the absence, limited presence or inhibition of decomposing and decomposition facilitating organisms,

the absence (limited presence) of oxidators,

low temperatures, that diminish the rate of all physical (diffusion), chemical (oxidation) and (most) biological processes.

2.5.2. Decomposing and decomposition facilitating organisms are inhibited by

(a) low temperatures,

(b) high temperatures,

(c) the absence (= limited avilability) of oxidators (primarily oxygen),

(d) the absence of water,

(e) the absence of necessary nutrients,

(f) the presence of inhibiting substances ("qualitatively" e.g. poison; "quantitatively" e.g. disturbed osmoregulation by high concentrations of acids, salts and sugars).

2.5.3. The absence (or limited presence) of oxidators can be caused by:

(g) little supply of oxidators to the system, because of a large penetration resistance, cf. the limited diffusion rate of oxygen in fluids and solids,

(h) large losses of oxidators from the system, because of the abundant presence of reductors (oxidator consumers), e.g. rapidly oxidizing plant materials (important in all mires).

2.5.4. Of these factors "high temperatures", "absence of water" and "presence of inhibiting substances" usually also inhibit the production side of the required imbalance, and therefore normally only play a minor role in peat accumulation.
 

2.6. Exogenic factors may (rarely) change the properties of dead organic material toward less decayability, e.g. humification, or fire changing decayable wood into virtually "undecayable" charcoal.

3.1. The presence of water leads to lower temperatures (because of the large heat coefficient of water) and (more importantly) to lower availability of oxygen (because of the limited diffusion rate of oxygen in water). Both conditions inhibit decomposing and decomposition facilitating organisms. All these factors lead to a decreased rate of decay of dead organic material. Other exogenic factors may contribute to a decrease, but normally do not play a major role.
 
 

3.2. The presence of water may also lead to an inhibition of (plant) production. Peat accumulation therefore only takes place in that traject of water "availability" (both in space, cf. water levels, and time, cf. seasons), in which decay of organic material is inhibited more than its production.

Fenton, J.H.C., 1980. The rate of peat accumulation in Antrctic moss banks. J. Ecol. 68: 211 - 228.

Lag, J., 1990. Peat accumulation in steep hills at Alkhornet, Spitsbergen (Norway). Acta Agriculturae Scandinavica 40: 217 - 220.

Van der Knaap, W.O., 1988. Palynology of two 5500 year old skua-mounds of the Arctic Skua (Stercorarius parasiticus (L.)) in Svalbard. Polar Research 6: 43 - 57.