horizons (Pearson correlation coefficient for C,r¼
0.999; for N,r¼0.996). Moreover, total food web
biomass did not explain the observed decrease in
decay rates of detritus with depth. This indicates
the importance of indirect contributions of soil
fauna, via enhancing the mineralization rates of
microbes, sometimes exceeding the direct contribu-
tion of fauna to C and N mineralization rates. The
results imply that the regulation of ecosystem pro-
cesses is inextricably linked to the structure of the
soil community, and that accounting for vertical
variation in food web structure seems essential for
understanding energy and nutrient flows in soils.
Empirical evidence that micro-stratification of soil
organisms can have specific effects on ecosystem
processes is given by Briones and Ineson (2002).
Using radiocarbon techniques, they found that
enchytraeids in a blanket bog assimilate carbon
components of predominantly 5–10 years old.
Vertical movement of enchytraeids due to abiotic
factors does not affect this as they show similar
(^14) C enrichment values at various depths. In re-
sponse to warming, however, worms are forced
deeper into the soil, where they feed on older
carbon components. Similarly, the vertical strati-
fication of fungivores determines their impact on
ecosystem processes (Faber 1991). Surface-living,
epigeic species affect the colonization of fresh
leaf litter, and potentially enhance the immobili-
zation of nutrients. Hemiedaphic species en-
hance the net mineralization and nutrient
mobilization in fragmented litter. Euedaphic spe-
cies living in the humus horizon have the poten-
tial to affect plant growth by interference with
mycorrhizal establishment or nutrient uptake by
the roots. Therefore, aggregating organic hori-
zons, with their own species compositions and
specific impact on processes, is an oversimplifi-
cation that obscures our understanding of the
mechanisms behind many soil processes.
6.7 Spatio-temporal scales of community studies
There exists a disparity between the short tempo-
ral scale, days to weeks, and small spatial scale,
centimetres to metres, at which most soil organ-
isms operate and the long temporal scales, years to
decade, and large spatial scales, plots to land-
scape, adopted in many studies. In community
and food web studies we have to use the scales
at which organisms operate and include their
basal resources to fully understand their role in
communities and ecosystems. Time and space are,
however,thesamesideofthecoin.Theyactsi-
multaneously on species in the soil community
over time by biochemical and physical changes
in resources and across space by burrowing
organic matter owing to input of fresh litter. Spa-
tio-temporal modifications in detrital characteris-
tics as degradation progresses are perceived
differently by bacteria and fungi (Bosatta and
A ̊gren 1991). Bacteria have shorter generation
times than fungi (Rooneyet al. 2006), and react
differently to disturbances (Orwinet al. 2006).
Hence, modification in detritus affects the fungi-
or bacteria-based energy or nutrient channels of
the food web in a different way, and this may
contribute to the observed high within-year varia-
bility in community composition within organic
horizons. Moreover, bacteria gain importance
from litter to humus, because they benefit from
theincreaseinresourcesurfaceowingtoadecline
in particle size (Fig. 6.6). Fungi penetrate whole
leaves, a trait that bacteria lack, and do not profit
fromthedecreaseinparticlesize.Adeclinein
particle size and alteration in chemical composi-
tion with soil depth and time may explain the
often observed succession of soil organisms
when degradation proceeds, and result in specific
local communities in subsequent organic horizons
(Fig. 6.6).
Resource-based modification in food web struc-
ture in turn feeds back to organic matter degrada-
tion, because food web composition strongly affects
the rate of C and N mineralization. Trophic interac-
tions within local communities can result in either
immobilization of nutrients, in litter, or mobiliza-
tion of nutrients, in fragmented litter and humus.
This shift in the role soil organisms play in the
decomposition processes warrants the subdivision
of the organic horizon in its successive layers, each
representing a different phase in the process of
decomposition, each with a specific soil community
composition (Fig. 6.6). However, spatio-temporal
changes in resources are not described adequatelySPATIO-TEMPORAL STRUCTURE 79