What controls the rate of decomposition

Three major factors control decomposition: climate, quality of the litter, and the soil microbial and faunal communities, as shown in Fig. 7.3. Other factors

Factors Of110
Figure 7.3 Factors that affect how quickly litter decomposes. (Reprinted from: Prescott et al. 2000. Forest Ecology and Management 133 with permission from Elsevier.)

can be important such as soil pH and aeration but tend themselves to be influenced by the three main factors. As with most ecological processes, environmental conditions such as temperature and moisture will affect how fast the process goes. With decomposition, the quality of the food supply - the material for decomposition - must inevitably be important, together with the nature of the soil biota discussed above. In most situations, the early decomposition is usually closely related to climatic conditions and concentrations of water-soluble nutrients while later decomposition rates are more influenced by the quality of the necromass, particularly the quantity of important nutrients and compounds that tend to impede competition such as lignin and other polyphenols (Prescott et al., 2000).

7.5.1 Environmental conditions

Providing other factors such as moisture are not limiting, decomposition rates increase with temperature, usually increasing by 2-3.5-fold in freshly fallen litter with a 10 °C increase (this is called the temperature coefficient or Q10 value).

Moisture availability also regulates the rate of decomposition, especially summer precipitation when temperatures are warmest and decomposition is potentially most rapid. At the other extreme, under waterlogged conditions, oxygen availability is drastically reduced because diffusion is limited, and anaerobic conditions soon develop. In such cases, only limited decomposition is carried out by anaerobic bacteria, and with time peat can begin to form.

Indirectly, the moisture conditions of a soil can influence pH by affecting the oxygen supply, which in turn will influence the activity of decomposer organisms.

The influences of temperature and moisture interact such that under extreme moisture conditions, temperature has much less effect on decay rates. Furthermore, weather conditions influence the efficiency of decay. For example, the amount of nitrogen released per gram of carbon used varies when identical litters are incubated under different temperature and moisture conditions.

7.5.2 Quality of the necromass

Animal necromass in the form of dead bodies and waste products may be relatively small in quantity and irregularly produced but its quality, especially in terms of its nitrogen content, can have very important effects in a forest. For example, the dung and urine produced by moose have been seen to have a larger effect through increased nitrogen mineralization and other soil processes in a willow forest (diamondleaf willow, Salix planifera) in Alaska than the more obvious effects of browsing (Molvar et al., 1993). Some studies have shown that such mineralization tends to lead to nitrogen loss from soils by oxides being leached or escaping as gas. Sometimes, however, faeces can cause nitrogen immobilization at least for a time. For example, Christenson et al. (2002) found that the nitrogen in droppings (frass) of the gypsy moth Lymantria dispar in a hardwood forest of New York State was more quickly mineralized than from leaf litter (as predicted above) but was quickly immobilized by soil microbes stimulated by the readily available carbon in the frass.

As noted at the beginning of the chapter, plant litter quality varies greatly depending upon the plants it comes from; both in terms of what species are present and also how their growth is influenced by nutrient stress. Plants growing under nutrient shortage tend to produce harder, nutrient-poor, recalcitrant litter. While animals normally keep the elemental composition of their bodies within narrow bounds, plants are much more flexible so the nutrient content of their litter can vary enormously even within the same species.

A large number of different measurements of necromass quality have been implicated in limiting decomposition, including such considerations as toughness, thickness, carbon to nitrogen ratio, and the content of calcium, nitrogen, carbohydrates, lignin and other polyphenol compounds. As is often the case, different factors will be limiting under different circumstances. For example, in general, litters from tropical sites have higher nitrogen (N) values and lower lignin to N ratios (also written lignin/N) than litters from other regions. In the tropics, Mediterranean region, and temperate forests the lignin/N ratio is usually the best predictor of decomposability (i.e. litter with proportionately

Forest Litter Decomposition

Figure 7.4 The percentage oflitter remaining after 1 year plotted against the ratio of lignin to nitrogen in the litter at the start of the experiment. The experiment was conducted in a hardwood forest at the Hubbard Brook Experimental Forest, New Hampshire and in a similar forest in North Carolina, using litter of individual species placed in nylon mesh 'litter bags' which were then placed out in the forest and recollected for weighing to estimate weight loss (or biomass remaining). The equations are for the regression line fitted to each set of data: r2 values run from 0 to 1; the closer the value to 1, the closer the data fits the line. The species used are: A white or American ash Fraxinus americana; Be Beech Fagus grandifolia; CO chestnut oak Quercus prinus; FD flowering dogwood Cornus florida; PB paper birch Betula papyrifera; PC pin cherry Prunus pensylvanica; RM red maple Acer rubrum; SM sugar maple Acer saccharum; WO white oak Quercus alba; WP eastern white pine Pinus strobus. (From Melillo et al. 1982. Ecology 63.)

more lignin to nitrogen decomposes more slowly), as shown in Fig. 7.4. Certainly the rate of N mineralization depends strongly on the lignin/N ratio in these areas (Rees et al., 2001). However, N does not appear to be a reliable chemical predictor of decomposition in northern areas. For example, Prescott (1995) conducted a range of experiments looking at the effect of nitrogen on decomposition of litter of jack pine Pinus banksiana in the Canadian boreal forest. In some of these, N was applied externally as fertilizer (an 'exogenous' supply) and in others, litter was taken from trees grown with and without fertilizer which gave different amounts of internal N ('endogenous') -1.56% N in litter from fertilized trees, 0.33% N without fertilizer. In all cases Prescott found that the speed of decomposition was the same or poorer with added N and she concluded that internal or external N does not control rates of litter decomposition in these northern forests. Attiwill and Adams (1993), reviewing published studies, go further and state that in warm-temperate areas and the tropics, and most likely also in northern boreal forests, available phosphorus (P) is much more likely to be the factor limiting decomposition.

The lignin/N ratio has been found to differ between species of trees by a factor of two or more. This should, and does, give quite a spatial variation in soil fertility within a forest depending upon species composition and arrangement of trees. Surprisingly, however, this pattern is not necessarily reflected in how well vegetation grows. For example, Finzi and Canham (2000) found that variations in N availability within a New England forest made little difference to sapling growth; light was far more important, explaining up to 79% of the variation in growth. Only the growth of sugar maple Acer saccharum and red maple A. rubrum was related to N and even here it explained less than 7% of the variation in growth. This is unexpected given the clear role that N plays in forest growth on a regional level but may be explained by the wide-spreading roots of most trees and nutrient movement across root grafts (see Section 1.3.2).

The nitrogen and phosphorus content of litter can be manipulated by the plant reabsorbing nutrients from the leaves before they fall. On a common-sense basis we might expect that (a) evergreen plants would reabsorb more nutrients than deciduous plants, and (b) that plants on more nutrient-poor soils should reabsorb a greater proportion of nutrients to reuse the next growing season. In both cases the consequence would be to make the C/N and C/P ratios of the litter poorer and so more resistant to decomposition. Aerts (1996) conducted a wide-ranging review of this using data from 44 studies primarily from the USA and Europe. Overall he found that an average of 50% of N and 52% of P was reabsorbed from leaves. Phosphorus reabsorption was similar across evergreen and deciduous woody plants but less N was reabsorbed by evergreen woody plants (47%) than by their deciduous counterparts (54%). But evergreen leaves started out with some 40% lower concentrations of N and P than did deciduous leaves (Table 7.2) so the litter of evergreen species is poorer in these nutrients. This agrees with a study by Killingbeck (1996) who collated published data on woody plants from around the world. He similarly found that the litter of evergreens was lower in P than deciduous species, and heading that way in N although the data were not significantly different (Fig. 7.5). It follows from this that the evergreen litter is likely to be more difficult (and so slower) to decompose than that from deciduous species. Importantly to the second intuitive guess above, the proportion of N and P recovered did not vary in any consistent way with the nutrient status of the plant. So it seems that species on poor sites are not reabsorbing higher amounts of nutrients from their leaves. In that case, how

Table 7.2. Mean nitrogen (N) and phosphorus (P) concentrations in mature leaves from evergreen and deciduous woody plants (trees and shrubs) growing primarily in the USA and Europe, and litter from these plants (note, these last figures are calculated from the mature leafconcentrations knowing the reabsorption rates. The data can be converted to percentages by dividing the given figures by 10).

Table 7.2. Mean nitrogen (N) and phosphorus (P) concentrations in mature leaves from evergreen and deciduous woody plants (trees and shrubs) growing primarily in the USA and Europe, and litter from these plants (note, these last figures are calculated from the mature leafconcentrations knowing the reabsorption rates. The data can be converted to percentages by dividing the given figures by 10).

Mature leaves (mg g 1)

Litter (mg g^1)

N

P

N

P

Evergreen

13.7

1.02

7.30

0.50

Deciduous

22.2

1.60

10.21

0.79

Source: Data from Aerts, 1996. Journal of Ecology 84.

Source: Data from Aerts, 1996. Journal of Ecology 84.

1.00

1.00

Imagines Ecology Center

0.10

0.05

Deciduous Evergreen Deciduous Evergreen

Figure 7.5 Mean nitrogen (N) and phosphorus (P) concentrations measured in the shed leaves of 45 species of deciduous woody plants and 32 species (N) or 31 species (P) of evergreen woody plants from around the world. These data can be compared with Table 7.2 by multiplying the figures by 10 to get mg of N or P per gram of leaf (mg g-1). P values in the boxes indicate that the two nitrogen concentrations are not significantly different but the two phosphorus figures are. (Redrawn from Killingbeck, 1996. Ecology 77.)

0.10

0.05

Deciduous Evergreen Deciduous Evergreen

Figure 7.5 Mean nitrogen (N) and phosphorus (P) concentrations measured in the shed leaves of 45 species of deciduous woody plants and 32 species (N) or 31 species (P) of evergreen woody plants from around the world. These data can be compared with Table 7.2 by multiplying the figures by 10 to get mg of N or P per gram of leaf (mg g-1). P values in the boxes indicate that the two nitrogen concentrations are not significantly different but the two phosphorus figures are. (Redrawn from Killingbeck, 1996. Ecology 77.)

are nutrients conserved on poor soils? It appears that leaves live longer (this is as much to conserve carbon as nutrients) and they start with a lower initial nutrient concentration.

As well as the absence of nutrients, the presence of certain compounds has also been strongly implicated in slowing decomposition. In most cases these

are polyphenols, a range of compounds found in many plants but here referring mainly to those of higher molecular weight such as lignin and tannins that are found almost exclusively in woody plants. Lignin and other phenolics are often referred to as anti-herbivore compounds but they may have just as large an after-effect on decomposition.

Lignin is a highly complex and variable compound (a macromolecule) that forms between a quarter and a third of the weight of wood (and so is the second most abundant organic material after cellulose). It gives strength and rigidity to the cellulose wall, is what makes a woody plant woody, and what makes brown paper brown and tough; it also protects structural polysaccharides such as cellulose from microbial attack, whether the plant is alive or dead. The lignin molecule is twisted into a random, highly heterogeneous and unordered structure. Since enzymes work by fitting themselves to the shape of their target compound, this makes lignin very difficult to digest and requires a variety of enzymes to do the job. For most organisms it is too expensive to produce all the required enzymes, so lignin is fairly immune to decomposition (see Fig. 7.1) except by specialists such as the white rots. Lignin concentration is thus often negatively correlated with decomposition rates in litter - the more lignin, the slower decomposition. It is usually the case that plants on nutritionally poorer sites produce more lignin, primarily as an anti-herbivore defence to protect the scarce nutrients. But, perplexingly, this does not always appear to be the case. Kitayama et al. (2004) looked at the amounts of lignin in leaves in tropical rain forest leaves in Borneo and found that lignin concentrations changed little with productivity of forests (as judged by the amount of litter-fall). Because lignin is resistant to decay, its concentration increases during the decomposition process and eventually forms a significant proportion of humus (Section 7.6.2).

Tannins are also polyphenols and are common enough to be the fourth most abundant plant compound after cellulose, lignin and hemicellulose. Leaves and bark may contain up to 40% tannin by dry weight, and in leaves and needles tannin concentrations can exceed lignin levels (see Kraus et al., 2003). Tannins are useful in plant defence because they bind to proteins making these less digestible to herbivores, and may also inhibit fungal and bacterial activity. The ability oftannins to bind to protein and so inhibit decomposition has long been exploited in preserving leather. Their effect on decomposition in the soil can be as important as the nitrogen content of litter especially in their binding to proteins to form polyphenol-protein complexes (PPC). These are formed either in the dying plant cell or in the soil. The formation of PPC causes the brown colouration of dying leaves and can make up 20% of the dry weight of the leaf (see Hattenschwiler and Vitousek, 2000). Polyphenols and PPC are resistant to decay except by fungi with the appropriate enzymes (polyphenol oxidase) and to a lesser extent certain earthworms and millipedes. Polyphenols may have other roles in the soil such as helping to make P more available in acidic soils by preventing its adsorption by soil particles, but more study is needed to see how widespread such a mechanism is.

A good deal of research has been carried out on the effect of mixing litter from different species. The results have not always been predictable but in 67% of the cases looked at by Gartner and Cardon (2004), rates of decomposition were faster when the litter of several species was mixed together than when kept separate; the litter disappeared quicker, releasing more nutrients with a greater abundance and activity of decomposers. The reason behind this is that different litters complement the deficiencies or problems of each other. For example, nutrients released from easily decayed litter can stimulate decay in adjacent more recalcitrant litter. There may also be a number of indirect reasons as well. For example, a study by Hansen and Coleman (1998) and Hansen (1999) found that mixtures of yellow birch Betula alleganiensis and sugar maple Acer saccharum decomposed quicker if leaves of red oak Quercus rubra were added even though red oak litter is much harder to decompose. The underlying reason is that the oak leaves support a more diverse and abundant community of mites, and the activity of these mites in turn increased the moisture-holding capacity of the litter which itself helped further decomposition. In some cases decay can be slowed down when litters are mixed because of some antagonism caused by the release of inhibitory compounds such as phenolics or tannins. For example, compounds released in leachates from black oak Quercus nigra are known to slow the decay of leaves of sweetgum Liquidambar styraciflua. But, as with many aspects of soil biology, the bigger picture of what these beneficial mixtures contribute to the growth of whole stands is obviously complex and as yet less fully understood. A review of the nutritional status of mixed stands around the world by Rothe and Binkley (2001) found that the pool of soil nutrients available to plants generally did not differ between stands containing a mixture of trees and monocultures, despite the differences in decomposition rates observed above.

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