Agriculture Reference
In-Depth Information
2.6
Organic matter fluxes in the litter system
Nutrient and carbon fluxes are major consequences of litter acquisition and breakdown.
Major issues influencing ecosystem function and the global environment are the storage
and conservation of C and nutrients and the release and recycling of litter components.
The total and relative amounts of nutrients that are recycled annually through the litter
system are highly variable.
Nutrient release occurs at characteristic times and places in different litter systems.
A major challenge in the management of degraded ecosystems, is to restore or enhance
litter systems that provide nutrients to plants and energy to litter transformers and ecosystem
engineers, while conserving nutrients and storing carbon (Woomer and Swift, 1994).
General patterns of carbon and nutrient fluxes in litter systems are clearly influenced
by the hierarchical suite of factors that govern decomposition processes (Figure IV.8)
and the specific properties of individual nutrient elements. Consequently, three general
trends occur in litter-system dynamics: (i) a marked dependence of accumulation and
decomposition rates on climatic conditions, soil properties and resource-quality
attributes; (ii) the accumulation of nutrients in plant biomass and litter rather than
the soil in nutrient-deficient systems; and (iii) markedly different turnover times for
individual elements.
2.6.1
CARBON MINERALISATION IN THE LITTER SYSTEM
The amounts of carbon stored in litter systems differ greatly between humus types.
In mull humus environments with shallow litter systems, little carbon accumulation
occurs and this contrasts with the increasingly larger amounts found in moder and mor
systems, respectively (Heal
et al.,
1981; Figure IV.3).
release from the litter system may be slow and even, or undergo marked
fluctuations over different scales of time. The seasonal changes of litter mass recorded
in temperate and tropical systems suggest that associated changes in temperature and
moisture availability are responsible for differences in their rates of C release (Figure
IV.28) (see Section IV.2.3.2). At the broader temporal scale of ecosystem development,
changes in litter mass may also occur. In an age series of floristically-similar Australian
Eucalyptus
forests, Polglase and Attiwill (1992) reported a progressive increase of litter
mass at the soil surface, mainly due to an increase in the amount of woody litter present
(Figure IV.29). Significant changes were also noted in the successional process described
by Bernier and Ponge (1994) in an alpine Spruce
(Picea abies)
forest (Figures IV.20
and 21). Organic matter accumulated in the litter layers during the mature phase of
the forest; when the old trees died and new trees started to grow, the altered litter
quality allowed earthworms to invade the unit. Through this, a significant proportion
of the accumulated organic matter was decomposed and incorporated into the soil as
mineral-bonded organic matter while the large flush of nutrients boosted the growth of
the young trees.
