Introduction to Soil Biota

Plants

Plants are the original source of energy in most soil systems, and this is certainly true for all forests of the southern US.  Leaf fall during autumn provides belowground systems with large inputs of energy and habitat for a vast array of soil organisms.  Furthermore, the roots of plants are an integral part of the biotic makeup of soils.  Whether live or dead, roots are important food resources in soils, and represent a basic source of energy for soil organisms from microbes all the way up the food chain to top predators in soil systems.

Microbes

Microbial organisms are an extremely important component of the biologically active portion of soil ecosystems.  The word "microbe" encompasses organisms of several different types that are not possible to observe without the help of a microscope.  The primary microbes in soil are bacteria and fungi.  Soil bacteria fall into several categories depending on their functions, but the primary role of bacteria in soil is recycling nutrients from dead plant material into forms that are available for use by live plants.  Soil fungi fall into two general categories: saprobic and mutualist.  The saprobic fungi are microbes that derive energy primarily from decaying wood, leaves and other organic material.  Mutualist fungi (sometimes called mycorrhizae) form associations with the roots of live plants that are mutually beneficial to the plant and to the fungus.  In these mutualisms, the plant provides the fungus with sugars (energy), and the fungus assists the plant in acquiring nutrients.  Although most bacteria and fungi could be considered beneficial to the functioning of soil systems in southern forests, it is important to mention that not all microbes are desirable members of the soil community and some can seriously impact the health of plants in the forest.  The activities of these undesirable, or pathogenic, microbes can lead to the mortality of valuable trees, and can cause significant economic loss in some forests.

Other microscopic organisms that can be grouped with the microbes are the protists including algae, amoebae, and ciliates.  Together, the actions and interactions of microbes and protists are largely responsible for the breakdown and recycling of dead plant material, and can strongly influence the fertility of a given site.

Soil Invertebrates

Soil invertebrates make up an extremely diverse assemblage of organisms ranging widely in size and function.  The most abundant soil organisms are also some of the smallest.  For example, soil nematode worms are sometimes found in densities greater than one million individuals per square meter of soil area, but they are tiny with body lengths generally between 0.1 and 1.2 mm.  Other types of very small but very abundant organisms include mites and springtails.  Both mites and springtails are somewhat larger than nematodes and can occur in densities in the tens of thousands of individuals per square meter. Because of their small size, these organisms are sometimes called microinvertebrates.  On the other end of the size spectrum are the largest soil invertebrates, sometimes called macroinvertebrates, a group that includes earthworms, millipedes, centipedes, and many insects.  For a list and some photos of some important soil organisms in southern forests, see Table:Soil Invertebrates in the Southeast.

The functions that soil invertebrates perform are mostly related to the breakdown and recycling of organic material in the litter layer and in the soil.  Many of the organisms mentioned above have been shown to influence the rate at which leaf litter or dead roots are processed into soil organic matter, and subsequently the availability of nutrients to plants.  However, many of the organisms living in soil, especially nematodes and beetle larvae, feed directly on the roots of living plants.  When these organisms reach high population densities, they can significantly damage the vegetation at a site.  This is usually only a problem in systems that are intensively managed for production of plants (e.g. in agricultural fields, nurseries, or tree plantations), and is generally negligible in diverse forest stands.

Habitats for Soil Organisms

Although soil may seem to be a fairly uniform and simple environment at first inspection, there are a large number of specific habitat types in a typical forest soil that are available for soil invertebrates to inhabit.  From the soil surface where mites, springtails and other insects inhabit freshly fallen leaf litter, down through layers of decomposing leaves and further down through mineral horizons of soil where nematodes feed on the deepest roots of trees, the entire soil volume is occupied by living creatures.  Not only is the soil a varied habitat vertically, but even more diverse habitat types exist horizontally across a forest landscape.  For example, the soil environment (moisture and temperature) on a ridge top is very different from that found in a valley bottom.  At finer scales, there can be significant differences in the chemical, physical and biological characteristics of soil underlying individual trees of different species.  So, from soil surface to deep depths, and across the whole forest landscape, the soil is a very complex mosaic of habitat types supporting a highly diverse community of organisms.