Agriculture Reference
In-Depth Information
In the litter colonizing microflora pectinolytic and cellulolytic fungi were aboundant
from the start of decomposition, followed by the chitinolytic ones that increased after six
months of decomposition. Ligninolytic fungi, even if isolated in the first month of
decomposition increased when lignin decay started. Considering their taxonomic
diversity species of typical genera of the Mediterranean area were mixed to those of
ubiquitous genera found also in other regions.
I
NTRODUCTION
The cycle of organic matter and related mineral elements play a key role in the
relationships between the soil, the vegetation and the surrounding environment.
The primary net productivity of forest vegetation is dependent on exogenous
environmental factors such as soil and climate, and to endogenous factors such type of tree
cover and age (Santa Regina et al., 1997). Trees retain a part of their production in perennial
structures, whose nutritive elements form the mineralomass of the phytocenosis. In the forest
ecosystems, the largest fraction of net primary production is delivered every year to the upper layers
of soil and, consequently, provides the main above-ground contribution of carbon and nutrients
to the forest floor. Litter deposit depends on the productivity of plant communities, which, in
turn, is affected by plant species composition, soil fertility, water retention and climate
(Meentemeyer et al. 1982; Kouki and Hokkanen, 1992; Pausas et al., 1997). An inverse linear
relationship between total litter production and latitude has been found (Vogt et al., 1986).
Leaf tissues account for about 70% of aboveground litterfall (Meentemeyer et al. 1982), even if the
deposition of woody litter tends to increase with forest age (Harmon et al., 1987).
Most of detritus is delivered to the upper layers soil where it is subject to degradation. Litter
disappearance is dependent on physical and chemical degradation, heterotrofic consumption and
decomposition. The last process, carried out by bacteria and fungi with the help of pedofauna,
involves the mineralization and humification of organic matter with the progressive release
and/or immobilization of nutrients (Aber and Melillo, 1991; Coûteaux et al., 1995). In this
view it is an important factor controlling nutrient cycling and soil humus formation, in particular, in
forest ecosystems.
The decomposition process is affected by litter quality, climatic factors and soil organisms
(Virzo De Santo et al., 1993; Berg et al. 1993, 1995 a; Rutigliano et al. 1996; Moore et al., 1999;
Osono e Takeda, 2001; Fioretto et al., 2001 a and b). The quality depends on chemical and
biochemical features of the litter, in terms of relative abundance of water soluble substances, polymer
carbohydrates including pectin, hemicellulose and holocellulose, lignin and other aromatic
compounds, lipids and waxes as well as nutrient availability. C/N ratio, lignin/N,
holocellulose/lignin, cellulose/lignin/N are indices that reflects the fraction of labile compounds
(carbohydrates, proteins) and recalcitrant compounds (mainly lignin, but also suberins, resins, fats
and waxes) (Mac Claugherty and Berg, 1987; Taylor et al., 1989; Vallejo 1993; Rovira and Vallejo,
2002). As a consequence, the litter quality affects the amount and chemical features of humus
as well as the microclimate of the soil surface.
Water soluble substances and other labile compounds are degraded in the early stage of
decomposition by fast growing microorganisms that require high concentration of nitrogen
(Swift et al., 1979). Cellulose and lignin, the most abundant components of litter, constituting
70-80% of fallen litter, are slowly decomposed.
