Agriculture Reference
In-Depth Information
The Soil Biomass
A variety of small to microscopic organisms belonging to the plant kingdom—
the “decomposers”—interact with many larger organisms, predominantly ani-
mals that live in and on the soil, called the “reducers.” Collectively, this host of
organisms is called the soil biomass. The biomass is concentrated in the top-
soil (A horizon) because that is where the food supply is most abundant, being
made up of plant and animal residues falling on the surface and C compounds
released by roots.
The most important decomposers are the Archaea, bacteria, fungi,
Actinobacteria (actinomycetes), and algae. There is also a group of very small ani-
mals in soil—the protozoa and nematodes. The latter do not fit neatly into either
the decomposer or reducer groups because they feed predominantly on the true
decomposers. Collectively, these small organisms, many of which cannot be seen
with the naked eye, are referred to as the microbial biomass or, simply, as microor-
ganisms. Measured in terms of C content, the microbial biomass ranges from 200
to 1500 kg/ha in the top 15 cm, with the larger number applying to soils under
permanent grassland.
In vineyards, the most important reducers are earthworms, wood lice, mites,
springtails, millipedes, centipedes, and the adult and larval stages of insects. The
relationships both within and between the reducer and decomposer groups are
complex, but in the context of describing how organisms influence SOM turn-
over we may generalize as follows.
First, although the reducers partially digest organic residues, their main ben-
efit is in “reducing” or breaking litter and dead roots into small fragments that
the decomposers can colonize and feed on. Earthworms, termites, and ants are
examples of such reducers, but of these only earthworms are important in most
vineyard soils. Second, life in the soil is very competitive, and, during the course
of evolution, organisms have evolved that feed on other organisms. Conceptually,
we recognize a hierarchical arrangement of trophic or feeding levels with plant lit-
ter, dead roots, exudates, and detritus from organisms at the bottom. Successively,
going up the “food web” from this level, there are
• Archaea,bacteria,fungi,andactinomycetesthatfeedondeadorganic
matter (although small unicellular algae can be included at this level, they
do not feed on dead organic matter)
• Protozoathatfeedonbacteria,fungi,andalgae;thosemitesthatfeedon
fungi
• Omnivorousnematodesandnematodesthatfeedspeciicallyonbacteria,
fungi, or other nematodes
• Toppredators—mitesandspringtails—thatfeedonallotherorganisms
