Agriculture Reference
In-Depth Information
High lignin contents
Lignin is one of the plant components most resistant to decomposition and its relative
concentration‚ usually expressed as lignin to nitrogen ratio‚ is often inversely and closely
linked to decomposition rates (Meentemeyer‚ 1978; Melillo
et al.‚
1982; Laishram and
Yadava‚ 1988). Lignin degradation of is also highly dependent on the presence of a
specific microflora‚ mainly the 'white-rot' fungi (discussed in sections 2.2 and 2.5.4)‚
and may accumulate in ecosystems where these fungi are lacking (Bernhard-Reversat
and Schwartz‚ 1997). The lignin to nitrogen ratio (L/N) of decomposing material has
been used to describe decomposition rates in the Century model of soil organic matter
dynamics (Parton
et al.‚
1983). Lignin and phenolic compounds may have similar
inhibiting effects on decomposition rates‚ although different mechanisms are involved.
Ratios of carbon to nutrient elements
At any stage of decomposition‚ nutrient-element deficiencies may limit microbial
activity and thereby block the release of nutrient elements available to plants and other
micro-organisms. This occurs when the C:nutrient ratio of the decomposing resource is
high compared with that of the living micro-organisms and nutrients remain immobilised
within the microbial biomass. In the case of the C:N ratio‚ this is 5-7:1 for bacteria and
7-25:1 for fungi (Swift
et al.‚
1979).
In these situations‚ decomposition is slowed or halted if sufficient quantities of
the particular nutrient are not available in the immediate environment (Figure IV.10).
This often occurs in nutrient-limited soils and the C:nutrient ratio is adjusted through the
progressive elimination of carbon as respired by micro-organisms leading to
a mobilisation of the nutrients contained within their biomass. In the case of nitrogen‚ a
C:N ratio of
ca.
20-25 represents this limit and other nutrient elements may be immobilised
or mineralised following the same rule. Therefore‚ depending on the initial values of
the C:nutrient ratios‚ some nutrients may be released while others are immobilised.
A relationship between decomposition rates and soil nutrient contents has often been
postulated for the humid tropics. However‚ apparent exceptions exist‚ possibly due to
the dominant effect of a determinant from a higher level in the hierarchy (see‚ for
example‚ Spain and Lefeuvre (1987) and reviews by Anderson and Swift (1983).
The unexpected significant positive correlation found between the C:N ratio and the rate
of N mineralisation from heavy organic fractions is another example of such effects
(Sollins
et al.‚
1984). In this case‚ the organic matter fractions with the lowest C:N ratios
were the most prone to physical protection through adsorption onto the surfaces of clay
minerals and their mineralisation rates were subsequently decreased. They may also
have had a greater chemical complexity than SOM from the light fraction since this has
a higher C:N ratio.
Resistant humic compounds
At all stages of decomposition‚ humification results in the synthesis or relative accumu-
lation of highly-condensed humic molecules. They may be derived from microbial
syntheses or the insolubilisation of water-soluble organic matter when associated with
clay and sesquioxides (permanent or variable charge clay minerals). They may also
contain plant components that have resisted decomposition (see Chapter II). Some humic
