Fall 1994 (v6n4)
H.A. Verhoef and L. Brussaard
Biogeochemistry
11:175-211. 1990
This review (105 references)
discusses the contribution of soil microfauna to nitrogen mineralization.
The article contains six major sections: 1) A functional classification
of the soil fauna based on their role in the decomposition process. 2)
Findings from studies of natural and agricultural ecosystems. 3) Interactions
between various soil fauna. 4) The effects of abiotic factors on faunal
decomposition. 5) Effects of other environmental disruptions on soil fauna.
6) Areas for further research.
Functional Classifications
Three classifications are
discussed in this section: size, habitat, and food. Size (measured in
body width) is often used since it broadly reflects the scale at which
the various fauna affect soil processes. The size distinctions and species
found in each group are:
- Microfauna (2m - l00m). Includes nematodes and protozoa.
- Mesofauna (100m - 2 mm). Includes collembola, mites, enchytraeids and others.
- Macrofauna (2mm - 20mm). Includes millipedes, woodlice, fly larvae, beetles, snails and earthworms.
Habitat classifications are
usually based on the depth in the soil profile at which the animal lives.
Earthworm species, for example, are identified as either epigeal (surface-active),
endogeic (subsurface) or anecic (active between the surface and subsurface
layers). Similar stratifications have been described for certain types
of spiders and collembola.
The third classification, food, is largely based on the trophic relationships
between different groups of soil animals. According to the authors, this
classification is more complex than either size or habitat, but it is
more useful for evaluating the effect of different soil-dwelling species
on decomposition and mineralization.
Contribution to Nitrogen Mineralization
Studies from forest and grassland ecosystems indicate that soil fauna are responsible for about 30 percent of total net nitrogen mineralization. In one study of a forest system estimates ranged from 10 to 49 percent, depending on the assimilation efficiency factor used in the calculations. The grassland study showed that amoebae and nematodes together accounted for over 83 percent of the nitrogen mineralized by all fauna. Another study conducted in an agricultural system estimated that soil fauna contributed 25 percent to net nitrogen mineralization. This is in close agreement with studies from the natural systems, especially considering the variation in faunal activity resulting from environmental and management factors.
Population Dynamics
The amount of nitrogen contributed
by the soil fauna is closely correlated with their population densities.
These numbers can fluctuate considerably even over short periods of time,
and reveal a complex set of interactions amongst the various groups of
fauna. Paradoxically, according to several studies, the rate of organic
matter decomposition appears to proceed independent of these changes in
faunal populations. How can this paradox be explained? According to the
authors, "…total organism biomass, including bacterial and fungal
biomass [flora and fauna], may be a poor indicator of biological activity.
Rather than pointing to the low importance of the fauna at the observed
level of microbial biomass, this may instead indicate that the microbial
biomass is kept at a low level by microbial grazers. In their turn, the
grazers may not build up a large biomass, but instead be consumed by predators.
Hence it appears that in a variety of ecosystems, microbial grazers and
their predators [soil fauna] significantly contribute to the mineralization
of nitrogen, despite their often low biomass."
Abiotic Factors
Soil moisture and temperature
are the key environmental variables influencing the activity of soil fauna.
Several studies have shown an important phenomenon where moisture stress
is concerned. While bacterial and fungal decomposition are slowed down
(and ultimately cease) with increasing moisture stress, the process of
nitrogen mineralization is continued to some extent where soil micro-
and meso-fauna are active.
Management Factors
It is generally accepted
that adding nutrients to a nutrient deficient system increases the rate
of organic matter decomposition. This phenomenon has been observed in
numerous studies. However, many examples exist in which addition of nitrogen
to nitrogen-deficient systems slows down decomposition, especially where
high lignin organic material is concerned. The authors' explanation for
this is that nitrogen and other fertilizers can negatively influence particular
groups of organisms, particularly microbes. Any shift in microbial composition
can have a negative effect on the soil fauna as well. As a result, decomposition
and mineralization rate may decrease.
Harvesting (particularly
in annual cropping systems) and incorporation of crop residue also have
a strong influence on mineralization, mainly because of the rapid dying-off
of roots and the addition of large amounts of carbon to the soil system.
Few studies, however, have analyzed the specific effect of harvesting
and residue incorporation on soil fauna and the associated effect on nitrogen
dynamics. It is likely that the fungi and their predators play a key role
in the mineralization process, but the specific groups involved have not
been quantitatively studied.
Further Research
The authors explore three
important lines for future research: 1) further development of models
that help us describe, predict and manage the contribution of the soil
biota to nutrient turnover in soil; 2) the ability of the soil fauna in
managed systems to make nutrients available when plants need them; and
3) more detailed study of the interactions between functional groups of
soil fauna to better understand the process of organic matter decomposition.
For more information write
to: H. Verhoef, Dept. of Ecology and Ecotoxicology, Free University, De
Boelelaan 108 7,1081 HV Amsterdam, The Netherlands.
(DEC.326)
Contributed by David
Chaney
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