Agriculture Reference
In-Depth Information
decomposition of litter by white-rot fungi in the absence of anecic earthworms and
mulls may therefore develop on acid soils (Toutain‚ 1981). In humid tropical areas‚
decomposition is generally rapid due to the warm temperatures and favourable moisture
regimes and mulls occur frequently‚ despite the large litter biomasses input to the
soil surface.
In both areas‚ any limitations that exist among the climatic‚ edaphic or resource-
quality determinants of biological activity may impede the formation of mull litter
systems. In the Amazonian rainforest‚ decomposition rates may be depressed where
drainage is impeded‚ where much of the incident rainfall is rapidly lost through runoff
(Lucas and Chauvel‚ 1992)‚ where soils are acid and nutrient-poor (Herrera
et al.‚
1978)
and in places where low-quality litter is deposited at the soil surface (Leroy
et al.‚
1992).
2.2.3
MODER
In moder systems‚ decomposition is slow due to climatic‚ edaphic or trophic (resource
quality) conditions that limit or preclude the activities of anecic decomposers (in neutral
mull) and active white-rot fungi (in acid mulls) (Toutain‚ 1987a). Epigeic saprophages
may be abundant and exploit the decomposition products of fungal activity. A large num-
ber of predators also exploit the wide range of microhabitats and food resources present
within the substantial litter layers. The litter system is thick since decomposing material
from previous years accumulates at the soil surface forming discrete layers of decom-
posing material. These may comprise an Ol layer formed of poorly decomposed leaves
and an Of that consists of plant fragments of recognisable structure and the large faecal
pellets of millipedes and epigeic earthworms. The underlying Oh layer consists of the
accumulated faecal pellets of smaller invertebrates (micro-arthropods and
Enchytraeidae) that feed on larger pellets and other debris accumulated within the Oh
layer (Figure IV.15).
In this system‚ phenol-protein complexes are largely decomposed by white-rot
basidiomycote fungi and the litter mass is progressively reduced through the activities of
arthropods and enchytraeid worms. Since these invertebrates are largely confined to the
litter layers and rely mainly on the external-rumen type of digestion‚ they progressively
transform the decomposing material into an Of and subsequently an Oh horizon.
This occurs through continuing processes of ingestion‚ egestion and re-ingestion of litter
materials until the nutritive value of the decomposing materials for invertebrates has
been largely exhausted‚ deep within the Oh layer. In a two-year experiment studying leaf
decomposition in litter bags‚ Garay
et al.
(1986a) observed that leaves were incorporated
into the Of1 layer after 6-9 months still retaining 78 % of their initial mass; one year later
(18-21 months) they were found in the Of2 layer (
ca.
40 % of initial mass)‚ and after
2-3 years‚ they were incorporated into the Oh layer and colonised by fine roots (20 %
of initial mass) (Figure I.42).
