Resource Quality Climate And Litter Breakdown

Litter breakdown rates vary between and among ecosystems on localized and broad geographic scales, as functions of soil biota, substrate quality, microclimate, and ecosystem condition. In general, we view breakdown and decomposition as the result of biota acting on substrates of varying quality within the constraints of climate and soil properties.

Resource quality is defined principally by the chemical composition of organic residues deposited on or in the soil. Sugars and starches (i.e., labile substrates) are easily digested by microbes and other soil biota, whereas tannins, lignins, and other compounds rich in polyphenols (i.e., recalcitrant substrates) can be utilized directly only by certain specialized organisms (e.g., white-rot fungi). Cellulose and hemicelluloses are intermediate in their degradability. Hence the relative proportions of these classes of compounds in organic materials greatly influence the overall rate of decomposition of those materials (Fig. 5.2) (Berg, 1986). Organic litter in most terrestrial ecosystems is a mixture of relatively labile and relatively recalcitrant substrates—thin, calcium-rich dogwood (Cornus florida) leaves versus thick, highly lignified, oak leaves (Quercus spp.) or conifer needles (Pinus spp.), for example. Even in agricultural systems, differences between leaves and stalks of corn (Zea mays), for example, represent different substrate qualities with different breakdown rates. Woody litter, high in tannins and lignins, may have breakdown rates measured in decades or even centuries for large logs in cool climates (Harmon and Chen, 1991). Fine root turnover may be measured in days, but coarse roots, with highly suberized tissues, turn over in years.

On a broad geographic basis, the change in breakdown rates as a function of latitude is generally predictable (Fig. 5.3) (Meentemeyer, 1978). However, the effect of latitude is not strictly a direct effect of climate; the abundance of the various soil biotas also changes with latitude (Fig. 5.4) (Swift et al., 1979). For example, adaptations of the soil biota to desert conditions allow breakdown rates to proceed more rapidly than predicted by temperature-moisture considerations. Members of the desert soil biota are active nocturnally, when temperatures moderate and light dew may accumulate. Litter breakdown in tropical

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FIGURE 5.2. Model for decomposition of some organic components in Scots pine (Pinus silvestris) needle litter. In the early phase of decomposition, high concentrations of nutrients such as nitrogen (N), phosphorus (P), and sulfer (S) exert a rate-enhancing influence on mass-loss of the nonlignified parts of the litter. Also, high concentrations of easily degraded solubles and celluloses influence a high mass-loss rate. In the late stage where mainly lignified material remains, lignin mass-loss is governing, which in its turn is negatively affected by high nitrogen concentrations and positively by high concentrations of celluloses in the lignified material. The negative effect of lignin on cellulose degradation is indicated by black arrows. A (+) indicates a rate-enhancing influence and (-) a negative one. HLQ designates the quotient between holocellulose and lignin plus holocellulose (from Berg, 1986).

Time

FIGURE 5.2. Model for decomposition of some organic components in Scots pine (Pinus silvestris) needle litter. In the early phase of decomposition, high concentrations of nutrients such as nitrogen (N), phosphorus (P), and sulfer (S) exert a rate-enhancing influence on mass-loss of the nonlignified parts of the litter. Also, high concentrations of easily degraded solubles and celluloses influence a high mass-loss rate. In the late stage where mainly lignified material remains, lignin mass-loss is governing, which in its turn is negatively affected by high nitrogen concentrations and positively by high concentrations of celluloses in the lignified material. The negative effect of lignin on cellulose degradation is indicated by black arrows. A (+) indicates a rate-enhancing influence and (-) a negative one. HLQ designates the quotient between holocellulose and lignin plus holocellulose (from Berg, 1986).

systems may be strongly influenced by seasonality of litterfall as well as faunal abundance. González and Seastedt (2001) found that all three groups of factors (climate, substrate quality, and soil fauna) independently influenced the decomposition rate of leaf litter in tropical dry and subalpine forests. Soil fauna had a disproportionately larger effect on litter decomposition in a tropical wet forest than in tropical dry or subalpine forests.

Decomposition rates may vary along elevational gradients as well, but not as predictably. In a study conducted in Arizona in the United States, plant litter decomposition was measured along a gradient from desert to pinyon-juniper woodland and up into a ponderosa pine forest (Murphy et al, 1998). Decomposition was more rapid at the upper, cooler elevation that was also moister. In these systems, moisture—not temperature—

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