Agriculture Reference
In-Depth Information
Nevertheless, because lignin physically protects most of cellulose and hemicellulose from
enzymatic hydrolysis, neither group of compounds decompose independently (Cooke and
Whipps, 1993). Each fraction has characteristic exponential kinetics of decomposition, so that
the total mass loss of litter is the sum of a number of exponential functions (Minderman,
1968).
Decomposition process consists of 2 main phases: I) the former phase, that lasts 1-2
years, in which the decomposition rate is regulated by the C and N availability, as well as
litter morphological characteristics (Melillo et al., 1989); II) the latter phase, lasting more
than 3 years, in which the rate is low and regulated by lignin and Mn concentration, being
such element essential for some lignin-degrading enzymes, such as Mn peroxidases (Berg et
al., 1995 b). On the other hand high lignin content has a rate reducing influence (Berg et al.,
1996; Rutigliano et al., 1996; Coûteaux et al., 1998; Tian et al., 2000; Fioretto et al., 2005 a)
in particular, when associated with high N content, because new and stable complexes are
formed (Berg and Ekbohm, 1991; Coûteaux et al., 1995).
S
TUDIES ON
L
ITTER
D
ECOMPOSITION
:
A
P
ERSPECTIVE
The studies on litter decomposition, considered by Odum (1983) an emergent property of
ecosystems being sensitive to changes in ecosystem function, started more than 30 years ago.
Numerous researchers studied the decomposition dynamics in relation to climatic
condition and substrate quality (Virzo De Santo et al., 1993; Coûteaux et al., 1995; Cortez et
al. 1996; Coûteaux et al., 1998; Berg et al., 1998), to nutrient release (Laskowski et al., 1995;
Berg and Laskowski 1997; Fioretto et al., 2001a), to nature and abundance of decomposer
organisms and their interactions with the fauna (Cortez 1998; Cortez and Bouché 1998;
Andrén et al. 2001).
Many data have driven to conclude that the climate is more important than substrate
quality as a predictor of decomposition rates over large scales (Meentemeyer and Berg,
1986). In particular climate is a dominant factor in areas subjected to unfavourable weather
conditions. Litter quality, instead, largely prevails under favourable climatic conditions
(Couteaux et al., 1995).
Soil microrganisms play a key role in the decomposition of organic matter. In boreal
forests, microbial communities mainly fungi and bacteria transform more than 90% of the
plant litter carbon, leaving only 5% of it to soil animals (Berg and Laskowski 2006). In
temperate forest soil the microorganisms are also the dominating primary decomposers.
Fungi play the main role among the decomposers for the degradation of plant organic
material. Saprophytic microfungi are ubiquitous decomposers and respond rapidly to the
addition of new substrate, because of their hyphal growth pattern, production of vegetative
spores, specific survival strategies and capacity to produce a great variety of enzymes
important in the decomposition process (Kjøller and Struwe, 2002). They contribute up to
90% of the total respiration of soil organisms (Kjøller and Struwe, 1982) and a lot of them
(Basidiomycetes) can attack the lignocellulose matrix that other organisms are unable to
metabolize. Due to their different versatility the composition of the fungal community
changes during decomposition establishing a microbial succession (Frankland, 1998).
