Decomposition

7.2.1 The process of decay

When litter reaches the ground, or roots die, physical decomposition mainly involves rapid leaching of soluble substances (particularly dissolved organic matter and nitrogen - DOM and DON - see Section 8.4.2) by percolating water which can account for as much as 25% or more of weight loss from some litters (Fig. 7.1). Physical change also involves freeze-thaw and wetting-drying cycles, and the movements of animals which can contribute to the physical fragmentation of the litter. At the other extreme, fire can cause very rapid physical and chemical decomposition of litter and accumulated humus. In

Litter Decomposition

0 Arctic 5 20 100

Time (years)

Figure 7.1 Changes in plant litter over time once on the ground and decomposition begins.

0 Arctic 5 20 100

Time (years)

Figure 7.1 Changes in plant litter over time once on the ground and decomposition begins.

most soils, however, it is the 'biologically mediated' decay which accounts for most decomposition.

Decomposition of the dead material (necromass) by living organisms can be initiated by fungi living on the phylloplane of the leaf or the rhizoplane of the root (Section 5.5.1), such that decay has started before the leaf hits the ground or before the root is fully dead. Necromass is subsequently colonized by saprotrophic fungi and bacteria, some of which may be preyed upon by certain nematode worms, protozoa and rotifers, but it is this microbial conditioning which enables detritivores, such as various oribatid mites and spring-tails (Collembola), millipedes and woodlice, earthworms and potworms (Enchytraeidae) to exploit the dead remains. Molluscs, ants and termites are less dependent on such conditioning. The term detritivore, as mentioned in Section 1.5, is used here in the broadest sense to include decomposer animals that exist primarily on fungi or bacteria, as well as those which ingest necro-mass. The main role played by these animals is the breaking up (comminution) of the litter. Such fragmentation disrupts cell walls, exposing the more readily digested contents and considerably increasing the surface area of the litter, so making it more accessible to the microbiota (see below). Harding and Stuttard

(1974) estimated that the chewing up of a 60-mm-long conifer needle into 10 mm3 fragments (m, a micron, is a thousandth of a mm) by mites results in a 10 000-fold increase in surface area. Many detritivores are inefficient digesters, with as much as 80-95% of their food passing through without much alteration except being physically chewed up and coated with mucus which has a 'primer effect' encouraging the attachment of bacteria cells. Larger animals such as certain earthworms are also useful in mixing the litter into the mineral soil, helping it stay moist (and decompose quicker), and opening channels for air and water movement into soil, thus acting as ecosystem engineers (Bardgett, 2005). Paradoxically perhaps, their decay-resistant casts prevent all organic matter from being used up in the soil, which helps maintain fertility and structure. Detritivores are thus the millstones of the decomposer subsystem doing the heavy work of mixing and grinding, helping microbes that do the bulk of the actual decomposition by improving their food source and aiding spore dispersal (see Moore et al., 1988 for a more detailed review). In these ways, the soil animals help to unblock bottle-necks in energy flow and nutrient cycling and so contribute much more to the functioning of the soil than is indicated by their own energy requirements (Macfayden, 1961).

The soil in forests contains a far greater diversity of organisms than those we notice above-ground. The role of soil animals in forest soils has long been recognized as being of fundamental importance (Wallwork, 1970). A single square metre of soil in temperate woodland may contain more than 1000 species of animals, from protozoa to earthworms. Some of these will be carnivores and herbivores feeding on roots but the majority are detritivores. Generally when large decomposers such as earthworms, woodlice and millipedes are absent, this is compensated for by higher numbers of smaller animals. Northern conifer forests, with their acidic mor humus, are renowned for the high numbers of mesobiotic organisms (see Fig. 1.11): a square metre may contain up to 6.3 million nematode worms and 400 000 springtails and mites (Sohlenius, 1980; Hole, 1981). Enchytraeid worms (macrobiota) are considered to be keystone detritivore species in such soils (Bardgett, 2005). Despite these large numbers of small animals, the total biomass of soil animals increases by a factor of six from northern conifer forests to temperate and subtropical areas, declining again towards tropical forests. Anderson and Swift (1983) point out that tropical rain forests generally have a lower weight of soil fauna than temperate deciduous forests, but the overall activity of soil organisms is higher. The effect of earthworms is being dramatically shown in many northern forests in North America which had no native earthworms after the last ice age and which are now being invaded by worms, mainly of European and Asian origin with the help of humans (such as worms carried on vehicle wheels, throwing away of unused fishing bait and disposal of horticultural waste) - see Bohlen et al. (2004). Their effect is still not completely known but they seem to shift the soil system from a slower cycling fungal-dominated system to a faster cycling, bacterial-dominated system with complex changes to nutrient and carbon cycling, invertebrate and vertebrate populations. The direction of these changes varies depending upon the system and the reasons are not always easy to unravel. Most strikingly, a number of studies have shown that the abundance and diversity of understorey vegetation and tree seedlings declines dramatically with the arrival of worms. A study by Hale (quoted in Bohlen et al. 2004) in hardwood forests of northern Minnesota showed that over 40 years a lush, diverse forest was reduced to only one species of native herb and virtually no tree seedlings by earthworm invasion.

Mites, of which the free-living oribatids are particularly interesting, play important roles in all soils from the poles to the equator. Soil mites are not only hugely abundant and diverse in soils everywhere, they are also most important in organic decomposition and nutrient recycling. Their feeding habits vary from the grazing of bacteria, and the consumption of all kinds of decomposing materials, to brutal carnivory.

Above-ground factors can have a large influence on the soil fauna. Wardle et al. (2001) looked at the effect of introduced browsing animals, including deer and goats in New Zealand (there are no natural large forest-dwelling herbivores there) on above- and below-ground flora and fauna using a series of exclosures. These browsing animals not surprisingly reduced the overall vegetation density and the number of palatable broadleaved species, and promoted other less palatable types and hence lower quality litter. Below-ground, the microbes and nematodes were largely unaffected but the larger animals, such as collembola, mites and snails were reduced in browsing areas with consequent (though variable) negative effects on decomposition. Dead wood lying on the soil has also been seen to influence the larger soil fauna (Spears et al., 2003); Jabin et al. (2004) found that dead wood had a positive influence on the numbers of beetles, spiders, millipedes and centipedes in a 120-year-old oak-beech woodland in Germany. They found there were usually twice as many individuals of these groups in soil less than 10 cm from dead wood than in soil more than 5 m from such wood. This is because the wood supplies food, and also keeps the surface soil horizons moister (particularly in spring) and warmer.

The microbiota, and particularly the fungi and bacteria, are the real chemical decomposers, and account for about 80% of the energy flow within the decomposer subsystem. Soil bacteria are a very diverse group consisting of probably over 500 000 species, 95% of which do not show up in traditional culture methods and have only recently been found using modern molecular techniques (Fitter, 2005). Not only are they difficult to find, they are also remarkably numerous. There may be more than 1000 species and more than 200 million bacterial cells in one gram of soil! The cells tend to be anchored to soil particles particularly in the rhizosphere where they benefit from organic acids (such as malate, citrate and oxalate) secreted by the roots. The plants also benefit since the soluble nutrients mineralized by the bacteria in the rhizosphere are readily absorbed by the plant. Moreover, the rhizosphere and its microbiota have been implicated in many soil processes including the mobilization and uptake of less soluble nutrients such as phosphorus, and the detoxification of metals such as aluminium (see Section 2.3.4).

Ecological interactions between the multitudinous species occurring above the ground have been investigated for centuries. In contrast, many questions regarding the ecological significance of the biological diversity of the soil, and how it affects ecosystem function, have not been investigated until recently due to methodological difficulties. New technical developments, including isotopic and molecular methods, together with fresh experimental and modelling approaches, have thrown light on the ways in which the biodiversity of the soil influences the vital processes taking place within it (Bardgett, 2005; Bardgett et al., 2005). The soil is no longer a 'black box' since increasingly we are learning which does what to, or for, whom (see Box 7.1 for an example).

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