Scale Of Soil Habitat

The soil habitat is characterized by heterogeneities across all measured scales, from nanometers to kilometers, which differ in chemical, physical, and biological

FIGURE 2.4 Hierarchical model of soil aggregation and binding agents functioning for aggregation. Larger aggregates are often composed of an agglomeration of smaller aggregates. Different factors are important for aggregation at each of the levels in the hierarchy of soil aggregates. (A) A macroaggregate composed of many microaggregates bound together mainly by a sticky network formed from fungal hyphae, bacterial cells, and fine roots. (B) A microaggregate consisting mainly of fine sand grains and smaller clumps of silt grains, clay, and organic debris bound together by root hairs, fungal hyphae, and microbial biofilms. Submicroaggregates consist of fine silt particles encrusted with organic debris and bits of plant and microbial debris (called particulate organic matter) encrusted with even smaller packets of clay, humus, and Fe or Al oxides. White bar indicates 10 p,m. (Photos courtesy of D. A. Angers, C. Chenu, and S. Recous.)

FIGURE 2.4 Hierarchical model of soil aggregation and binding agents functioning for aggregation. Larger aggregates are often composed of an agglomeration of smaller aggregates. Different factors are important for aggregation at each of the levels in the hierarchy of soil aggregates. (A) A macroaggregate composed of many microaggregates bound together mainly by a sticky network formed from fungal hyphae, bacterial cells, and fine roots. (B) A microaggregate consisting mainly of fine sand grains and smaller clumps of silt grains, clay, and organic debris bound together by root hairs, fungal hyphae, and microbial biofilms. Submicroaggregates consist of fine silt particles encrusted with organic debris and bits of plant and microbial debris (called particulate organic matter) encrusted with even smaller packets of clay, humus, and Fe or Al oxides. White bar indicates 10 p,m. (Photos courtesy of D. A. Angers, C. Chenu, and S. Recous.)

characteristics in both space and time (Table 2.1). At various levels within this continuum of scales, different soil properties used to characterize the soil habitat can assume greater or lesser importance depending on the function or attribute that is under consideration. For broad general issues, such as climate change at a global scale, the general distribution of broad hydrologic features may be appropriate. Evaluations of more specific processes impacting on individual species' functioning may be possible only at the scale of habitat niches that occur at microsites in soil.

The habitat includes the physical location where a particular organism resides as well as the characteristics of the habitat that influence the growth, activities, interactions, and survival of other organisms found in it. Together these characteristics define the range of habitats available for organisms and their enzymes to occupy. They vary vertically, through the soil vadose zone, and horizontally across the landscape.

For higher organisms, such as animals that range over wide territories, the habitat may be on the scale of a landscape or watershed and even beyond. At the other extreme, microorganisms, the habitat occurs on a microscale and therefore has been referred to as microhabitat. The spatial characteristics of the general habitat, as well as the fine features of the microhabitats, must be considered in describing the activity of soil microorganisms. Microregions within soil aggregates control the nature and availability of nutrient resources, directly impacting population dynamics. Metabolically similar groups of microorganisms, referred to as guilds, occupy a unique ecological niche that encompasses their life strategy. Sets of guilds, which carry out interdependent physiological processes, form microbial communities. While ecological theory has proposed that no two species ever occupy exactly the same ecological niche for long, in soils it is the rule that different species share aspects of their niche with others. Soil habitat spatial heterogeneity is an important contributor to the coexistence of species in soil micro-bial communities, enhancing overall soil biodiversity by promoting the persistence of individual populations.

Studies have confirmed that soil organisms are usually not randomly distributed but exhibit predictable spatial patterns over wide spatial scales (Fig. 2.5).

Spatial patterns of soil biota also affect the spatial patterns of microbial activity and processes they carry out. Accumulations of inorganic N may not be observed where sites of plant residue decay are adjacent and closely linked to those where ammonification of SOM is occurring. Inorganic N can accumulate and nitrification can occur where these microbial processes are physically separated in soil space. Recent studies of "trigger molecules" have identified substrates that appear to promote metabolic linkages over spatially diverse soil microbial communities (De Nobili et al., 2001).

Although the main factors influencing the gross behavior of soil organisms are known, their relative importance and influence on spatial distribution have not been studied in detail. For soil microorganisms, there are few methods currently available that enable the study of their detailed activity in situ at the level of the soil microhabitat. In fact, when collecting samples in the field from a soil profile, it is common practice for soil scientists to homogenize the samples before analysis, after removal of the plant debris and macrofauna, by passing the soil through a 2-mm sieve.

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