Serita D Frey

Introduction

Geographical Differences in Soil Biota Association of Soil Organisms with Plants Spatial Heterogeneity of Soil Organisms Vertical Distribution within the Soil Profile Microscale Heterogeneity in Microbial Populations References and Suggested Reading introduction

Soil biota, which represent a large proportion of Earth's biodiversity, have a worldwide distribution. Once thought to be restricted to the top few meters of terrestrial ecosystems, soil organisms are now known to survive and grow in some seemingly unlikely and often inhospitable places, including in the canopies of tropical forest trees, in deep subsurface environments, in recently deposited volcanic materials, under deep snow in alpine systems, in Antarctic Dry Valley soils, and in cryoconite holes, pockets of meltwater containing windblown soil on the surface of glaciers (Fig. 11.1). While there are still large gaps in our understanding of how soil organisms are distributed, there has been a dramatic increase in information obtained in this area over the past decade. This chapter summarizes what is known about the distribution of soil biota, from geographic differences at the regional and global scale down to variability in microbial populations at the microscale.

Sediment Ice

FIGURE 11.1 Cryoconite hole formed on the surface of an Antarctic glacier. (Top) The ice of the cryoconite hole reflects light differently compared to the surrounding ice, making it easy to spot on the glacial surface. (Middle) Ice core from the cryoconite hole showing the accumulation of windblown sediment at the top of the hole. (Bottom) Organisms associated with sediments in the cryoconite hole. The green rods are cyanobacteria. (Photos courtesy of Dorota Porazinska, University of Florida, and Thomas Nylen, Portland State University. Used by permission.)

FIGURE 11.1 Cryoconite hole formed on the surface of an Antarctic glacier. (Top) The ice of the cryoconite hole reflects light differently compared to the surrounding ice, making it easy to spot on the glacial surface. (Middle) Ice core from the cryoconite hole showing the accumulation of windblown sediment at the top of the hole. (Bottom) Organisms associated with sediments in the cryoconite hole. The green rods are cyanobacteria. (Photos courtesy of Dorota Porazinska, University of Florida, and Thomas Nylen, Portland State University. Used by permission.)

geographical differences in soil biota

Most macroscopic plant and animal species have a restricted geographic distribution because of natural barriers to migration (e.g., mountain ranges) and climate sensitivity. This isolation has, over geological time, led to the evolution of new species and the development of geographically distinct plant and animal communities. Global distributions of most of the world's flora and fauna are generally known. There has been much less emphasis on understanding and mapping the biogeography of microscopic organisms. Although molecular analysis has revealed that microbial diversity in soils far exceeds that of macroscopic organisms, the geographical patterns of this diversity and the factors controlling these patterns are only beginning to be examined. Most studies that have been done have focused on human and animal pathogens (e.g., Escherichia coli, Haemophilus influenzae). A common perception is that microorganisms are cosmopolitan in their distribution, being capable of growth in many different places worldwide. This idea goes back more than a century to Martinus Beijerinck, a Dutch soil microbiologist, who suggested that "everything is everywhere, the environment selects," meaning that microbial species can be found wherever their environmental requirements are met. However, the concept of a common occurrence of soil microorganisms may be more apparent than real due to a lack of information on microbial distributions. As more details of microbial populations are delineated with biochemical and molecular techniques, it may be found that at least some soil organisms are restricted to specific geographical areas (i.e., they are endemic).

Certainly microorganisms have had the opportunity over time to become distributed worldwide. Due to their small size and large numbers, they are continuously being moved around, often across continental-scale distances. Dispersal mechanisms include water transport via rivers, groundwater, and ocean currents; airborne transport in association with dust particles and aerosols, especially during extreme weather events such as hurricanes and dust storms; transport on or in the intestinal tract of migratory birds, insects, and aquatic organisms; and human transport through air travel and shipping. Even isolated environments, like those found in the Antarctic, show a wide range of microbial species that appear to have been introduced from other places. Of 22 fungal genera identified in Antarctic soil samples, 12 taxa were found exclusively in the area surrounding the Australian Caseyu Research Station (Azmi and Seppelt, 1998). These fungi, dominated by Penicillium species, were presumably introduced into this environment by human visitors to the station. Thus almost any microbial species has the means to achieve widespread dispersal.

Further support for the idea that microorganisms have global distributions comes from a study of protozoa living in the sediments of a crater lake in Australia (Finlay et al., 1999). This system is geographically isolated from northern Europe, where most known protozoan taxa have been isolated and identified. Of 85 species collected from the Australian system, all had been previously described and are known from northern Europe. They apparently reached the isolated crater by dispersal from other freshwater, soil, and marine environments. Another argument for the cosmopolitan distribution of protozoa is the relatively low number of species found globally. For example, there are about 3000 known species of free-living soil ciliates. This is in comparison to 5 million insect species, many of which have geographically restricted ranges. The implication here is that lower endemicity in protozoa results in low global species richness since geographic isolation leading to speciation will be rare (Finlay, 2002).

While it is generally agreed that many protozoan species are cosmopolitan in their distribution, this may not be the case for other microbial groups. Heterotrophic soil bacteria, for example, have been shown to exhibit strict site endemism. Fluorescent Pseudomonas strains isolated from soil samples collected at 10 sites on four continents showed no overlap between sites. The same genotype was found only in other soil samples from the same site and not at other sites in a region or on other continents (Cho and Tiedje, 2000). Intraspecific differences in the optimum growth temperature, pH, and substrate (NH 4) concentration for the nitrifiers are also known to exist. Penicillium is abundant in temperate and cold climates. Aspergillus predominates in warm areas. Fusarium wilt of bananas is inhibited in areas where the clay mineral smectite predominates. Cyanobacteria are commonly found in neutral to alkaline soils, but rarely under acidic conditions. The fungal component of the microbial community appears to be particularly susceptible to changes in soil conditions. For example, fungal biomass and fungal-derived organic matter have been shown to be positively related to soil moisture as influenced by gradients in mean annual temperature and precipitation (Figs. 11.2 and 11.3). Thus regional differences do reflect the ability of soil organisms to respond to specific environmental controls.

In addition to species distributions, it is useful to consider regional and global patterns in soil microbial biomass, which represents between 2 and 5% of total terrestrial soil C. Climate, vegetation, soil characteristics, and land-use patterns all interact to influence microbial abundance and biomass at a given location. Microbial biomass is generally positively related to soil organic matter contents in most ecosystems, peat and organic soils being an exception. Levels of microbial biomass are also typically correlated with soil clay content. Clay minerals promote microbial growth by maintaining the pH in an optimal range, buffering the nutrient supply, adsorbing metabolites that are inhibitory to microbial growth, and providing protection from desiccation and grazing through increased aggregation. Total amounts of microbial biomass are also impacted by land use, with lower levels typically observed in arable compared to undisturbed forest and grassland soils due to cultivation-induced losses of organic matter. Microbial biomass is also correlated with latitude, with microbial biomass tending to be lower but more highly variable at high latitudes. This increased variability in microbial biomass with increasing latitude is attributed to higher interseasonal variation in temperature (Wardle, 1998).

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