Agriculture Reference
In-Depth Information
Decomposition rates
The rates of disappearance of decomposing resources have been widely measured by
exposing known amounts of leaf and other litter in 'litter bags' made from a mesh of
an inert fibre (see Anderson and Swift‚ 1983; Anderson and Ingram‚ 1993; Bockheim
et al.‚
1991; Schowalter
et al.‚
1991; Montagnini
et al.‚
1993; Thomas and Asakawa‚
1993; Basaguren and Pozo‚ 1994 in terrestrial ecosystems‚ Irons
et al.‚
1994 in fresh
water ecosystems). Weighing the material remaining at several time intervals permits
the calculation of decomposition rates. To avoid the problems resulting from litter
confinement (lack of faunal access‚ altered water status) some authors prefer to study
the decomposition of individual leaves tethered by nylon line (Swift
et al.‚
1979;
Vitousek
et al.‚
1994). Another common and simpler method compares annual inputs
with the accumulated decomposing leaf litter. In a system at steady state‚ the annual
input of dead organic matter (I) is assumed equal to the amount decomposed annually
(Olson‚ 1963). Under such conditions‚ and assuming a simple exponential pattern of
decomposition‚ the decomposition constant (
k
) may be calculated:
where
k
is the % of resource decomposed annually‚ I is the annual input and Xss
the mean annual weight of accumulated‚ decomposing material. Time for decomposition
of a given proportion of the residue is calculated from the equation:
where X/X0 is the proportion of the resource decomposed in a given period of time
(t) with a decomposition rate
k
(Bernhard-Reversat‚ 1982). However‚ this equation is
unsuitable for litters with complex decomposition patterns. The X/X0 value varies
greatly with a maximum of approximately four (half the input mass decomposed in
3 months) in tropical rainforests and a minimum of 0.01 (half the mass decomposed
in 100 years) in tundra ecosystems (Figure I.43). The data compiled by Heal
et al.
(1981)
show a clear climatic effect‚ with
k
values decreasing from tropical to tundra ecosystems.
Other more complicated models have been used to characterise litter decomposition
rates. The more realistic of these estimate different decomposition constants (
k
values)
for each of the major litter components (Jones‚ 1990) although few models cope satis-
factorily with the important interactions that occur among them in many plant materials.
Transfers
Decomposition is often associated with the movement of part or all of the decomposing
resource although roots and most under-ground resources‚ are little affected by this process.
In contrast‚ above-ground litter is progressively translocated deeper into the soil as
it decomposes. Organic matter leaching through the profile is usually metabolised
(sugars‚ amino-acids) or immobilised in the soil (phenolic compounds) by iron cations
(Toutain‚ 1974). The latter are precipitated as organic and organo-mineral concentrations
or become associated with clays and other mineral particles. The older‚ partly-decomposed
litter is buried under successive layers of more recently-deposited material (Figure I.42)
