essential for the mineralization of carbon and nu-
trients that are bound to dead organic matter and
the provision of resources for primary production.
To quantify the functional significance of soil or-
ganisms, for instance for nutrient mineralization,
species have been assembled in guilds or function-
al groups (Fig. 6.1). The rationale behind collecting
species in functional groups, based on similar food
types, predators, metabolic efficiencies and loca-
tion in the soil profile, is that they may affect
ecosystem processes in a similar way (Moore
et al. 1988). Interestingly, the assumption that
these shared characteristics of species reflect simi-
lar functions is seldom tested. The flow of carbon
and nutrients through soil largely depends on tro-
phic interactions between these functional groups,
and these trophic relationships can be visualized
in so-called connectedness food webs. A connect-
edness food web shows the architecture of trophic
relationships within soil communities; for exam-
ple, in Fig. 6.1 a connectedness food web is shown
for a pine forest soil in the temperate region (Berg
et al. 2001). This type of food web indicates only the
most dominant feeding relationships and does not
necessarily depict all physiologically possible tro-
phic interactions between functional groups.
Often, only those trophic interactions are included
for which we have proof that they matter for car-
bon and nutrient flows. A food web, therefore, is
notmorethanareflectionofarealcommunity.It
does not necessarily depict the real organization of
the soil community, because other types than feed-
ing interactions are excluded, such as facilitation
or mutualism. More examples of connectedness
food webs are given by Huntet al.(1987;short-
grass prairie), de Ruiteret al. (1993; agricultural
field) and Schro ̈teret al. (2003; coniferous forests).
Given the importance of detritus, both quantita-
tively and qualitatively, for the structure of soil
communities, we need a closer look at the compo-
sition of soil organic matter, the process of organic
matter decay and its spatio-temporal distribution
in soil.
6.3 Soil organic matter
Detritus, or soil organic matter, is a collection of
various organic compounds, such as plant remains,
dead animals and metabolic by-products of micro-
bial degradation. Input of detritus onto the forest
floor is dominated by leaves, flower heads, seeds,
twigs and bark and is often characterized by a
seasonal pattern. Although plant species vary in
their timing of leaf litter abscission over a year,
most temperate tree species shed their litter in au-
tumn. In coniferous forests needle fall is more or
less continuous over a year, but also peaks in au-
tumn. On some occasions the input of plant re-
mains other than leaves can be high. After a
severe storm large amounts of branches are blown
from the trees and unusually heavy rains cause
mast seeding and pulses of primary production
(Ostfeld and Keesing 2000). Annual fluctuations in
the supply of detritus to soil organisms may be a
reason why many species of soil biota show strong
seasonal patterns in activity or abundance (Wardle
2002).
In mixed forests where tree species that pro-
duce litters of different qualities coexist, a patchy
horizontal spatial distribution in detritus quality
may occur. Patchy spatial distributions of soil
biota communities may be correlated with spa-
tial variability in the amount of litter produced.
For example, in the Amazon forest Dicorynia
quianensisproduces litter with a high content of
polyphenolic complexes, whereas Qualea spp.
produce litter with low levels of polyphenols,
but high aluminium levels (Wardle and Lavelle
1997). Earthworms are absent from the litter of
D. quianensis, whereas in litter which accumu-
lates under Qualea endogenic earthworms are
found, with a high abundance near the tree
trunk where litter input is highest. More evi-
dence that the spatial positioning of plants is a
key determinant of the spatial patterning in soil
biota can be found in Wardle (2002).
The organic compounds that make up detritus
differ greatly in physical attributes and chemical
complexity and, subsequently, in degradability.
The process of organic matter decay is subdivided
into two main phases. In the initial phase the readi-
ly decomposable organic compounds, mainly poly-
saccharides and non-lignified carbohydrates, are
degraded. In a subsequent phase the more recalci-
trant compounds, such as lignified carbohydrates,
lignin and organic intermediates, are broken downSPATIO-TEMPORAL STRUCTURE 71