Rates of decomposition

7.6.1 k values and mean residence time

With so many factors potentially affecting decomposition it is no surprise that decomposition rates vary tremendously around the world. These rates can be expressed in absolute terms as the weight of litter that disappears in an area

Rate Decomposition Ammonia

Biomass of forest floor (t ha 1)

Figure 7.6 Estimates of the decomposition rate constant (k) for different forests, defined as the ratio of amount of annual litterfall to the amount of organic matter in the forest floor. Higher values of k indicate more rapid long-term decomposition rates. The inverse of k is called the mean residence time and gives an estimate of how quickly litter decomposes. (Redrawn from data from Olson, 1963. Ecology 44 and Rodin and Bazilevich, 1967. Production and Mineral Cycling in Terrestrial Vegetation. Oliver and Boyd.)

Biomass of forest floor (t ha 1)

Figure 7.6 Estimates of the decomposition rate constant (k) for different forests, defined as the ratio of amount of annual litterfall to the amount of organic matter in the forest floor. Higher values of k indicate more rapid long-term decomposition rates. The inverse of k is called the mean residence time and gives an estimate of how quickly litter decomposes. (Redrawn from data from Olson, 1963. Ecology 44 and Rodin and Bazilevich, 1967. Production and Mineral Cycling in Terrestrial Vegetation. Oliver and Boyd.)

over a set time. Such figures can be useful in helping physiologists understand how much of a nutrient is available over time. For the forest ecologist an equally useful measure is to get a feel for the overall long-term decomposition rate by looking at the ratio of how much litter (or better still, total necromass) falls in a year relative to the amount of organic matter stored in the forest floor. This figure is referred to as k or the decomposition rate constant (implying that this figure stays constant in any particular forest). If a great mass of litter falls in a year and very little of the organic matter remains in the soil (so k is high) then the litter must be disappearing soon after it hits the ground and decomposition rates are high. Conversely, if the same mass of litter arrives but there is a good accumulation of organic matter in the soil (giving a low value of k) then decomposition rates, evened out over the years, must be low. Globally, the value of k varies from over 4 in some tropical forests to less than 0.01 k in subalpine forests with a cold, short growing season (Fig. 7.6).

These values of k reflect two general trends. The yearly production of necromass tends to decrease with increasing distance from the equator, and the amount of organic matter building up in the forest floor is, on the whole, the reverse: it increases with latitude. Thus organic matter contents should be low in tropical forest soils and high in boreal forests. As a gross generalization this is true but there is a very wide variation, including many organic-rich tropical soils, such as in cloud forests, deep tropical peat bogs and freshwater swamp forests merging to mangroves, and very sandy, impoverished northern soils resulting from severe fires. Typical approximate figures for the accumulation of organic matter in forest floors are 15-1001 ha-1 in northern boreal forests, 7.5-12.5 tha-1 in temperate broadleaved forest and 1-2.5 (and up to 10-12) tha-1 in tropical forests (Vogt et al., 1986).

Another way of looking at these types of data is to consider the mean residence time, defined as 1/k or how long it takes for the equivalent mass of litter to disappear completely. In other words, if 1 tonne of litter falls into an area in one year, how long does it take for 1 tonne of weight to disappear from the forest floor (although not necessarily the same tonne that was added)? The material whose destruction is responsible for this weight loss will be made up predominantly of the softer and easily digested parts of the new litter but will be supplemented by some of the tougher components from older organic matter slowly decomposing lower in the soil. Since this is the inverse of k, in tropical forests with a k value of 4, it will take % = 0.25 years for that amount of organic matter to decay and disappear. Estimates of the mean residence time vary from 0.25-2.5 years in tropical forests (longest in those with the most infertile soils) through 3-4 years in warm temperate deciduous forests to 60-350 years in boreal conifer forests. On a global scale, average residence time for litter is around 5 years and for coarse woody debris 13 years (Matthews 1997); see Section 7.7.

The time taken for an individual piece of litter to decompose will be longer than the mean residence time (as noted above, apparent decomposition of fresh litter is helped by other organic matter decomposing). The technical reason for this is that for a single piece of litter k is not really a constant (Fig. 7.6); decomposition slows down as the most easily digested compounds are used up and the more recalcitrant portions are left. Olson (1963) modelled this process in 1963 and suggested that since the rate of decomposition is negatively exponential it would take a period of 3/k to reach 95% total decomposition and 5/k for 99% decomposition. Thus in a forest where k = 0.5, the mean residence time of a new input of litter would be 2 years (1/k), but an individual leaf might take 6 years (3/k) to 95% decompose and 10 years (5/k) for 99% of it to decay.

Different types of litter will, of course, have their own k values which contribute to the overall forest k value. Vogt et al. (1983) record that the 99% disappearance time in a fir stand in Washington state was 2.2 years for species of Allectoria lichen and leaves of blueberries Vaccinium species, 33 years for Pacific silver fir Abies amabalis cone scales and 38 years for mountain hemlock Tsuga mertensiana cones. Similarly, different soil horizons have their cumulative k value. In the Vogt et al. (1983) study the mean residence time for the litter was 11-12 years and for the organic matter below the litter it was 32 years in the 23-year-old stand (3.2% annual turnover) and 69 years (1.5%) in the 180-year-old stand.

An interesting corollary of Olson's work is that if decay of a leaf becomes progressively slower with time (i.e. negatively exponential) such that it takes 5/k years for 95% to be lost, then the amount of organic matter in the forest floor will be slowly increasing through that time. So for a northern or subalpine forest where k values are low (0.01) the amount of organic matter in the soil may be slowly increasing for centuries after a disturbance (5/0.01 = 500 years for 95% decomposition) before it reaches a nominal steady state, and in the case of the subalpine forest Olson suggests that it may actually take 3000 years to reach that steady state. This is long after the time the vegetation above has reached maturity when most ecologists would have considered any change long finished; a salutary lesson not to ignore what happens in the soil!

The existence of different rates of decomposition for different fractions of the soil organic matter has led soil scientists, particularly those interested in climate change and carbon storage, to separate soil organic matter into three distinct pools of organic matter (but bear in mind that the 'pools' are conceptual artifacts to help understand a continuous trend). The active carbon pool, consisting of root exudates and the rapidly decomposing components of fresh litter, has a mean residence time of a matter of years and so is very susceptible to rapid change. The slow carbon pool is made up of the material coming from the active pool which is more resistant to decomposition and typically contains a higher proportion of components such as lignin. This pool has a turnover time that is measured in decades. Finally, the passive carbon pool contains the humus which, despite continual microbial attack, is the most stable organic matter with a turnover time of millennia.

7.6.2 The final product - humus

The amorphous and colloidal humus in the slow carbon pool is the most persistent part of the soil organic matter. This is partly because the humus is made up of the more inert residues of decaying organic matter (including a good deal of lignin) and complex microbial products synthesized during decomposition. Its persistence is also due to the humus tending to combine with clay to form stable clay-humus complexes which are more resistant to decay. It is perhaps not surprising then that humus may account for 60-90% of the soil organic matter. As well as being the major store of carbon in the soil, the humus is also important because it has a high cation exchange capacity (see Section 2.2.1) and so has a strong influence on nutrient cycling in a soil.

Humus can be rich in nitrogen but this is mostly locked up in complex molecules and so largely unavailable to plants.

Humus can be separated by chemical extraction into three distinct components: humins, humic acids and fulvic acids. Humins are black, insoluble and have the highest carbon content and the lowest oxygen content of all three. Humic acids are dark brown to black and are not soluble in acidic water; they are the most easily extracted components of humus. Fulvic acids tend to be yellow or yellow brown, are soluble in water at any pH, have the lowest carbon content but the most oxygen. The ratio of humic acids (HA) to fulvic acids (FA) changes predictably among soils. Forest soils often have an HA/FA ratio of less than 1 (i.e. comparatively low amounts of humic acids) compared with a ratio of more than 2 in grasslands.

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