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patterned in an agroecosystem despite many years of soil tillage and monoculture cropping, suggesting that spatial heterogeneity may be even higher in less disturbed systems (Robertson and Freckman, 1995).

There is little detailed information available on the spatial distribution of soil biota, and spatial variability has historically been regarded as random noise in the system. Geostatistics provide a method for quantifying spatial heterogeneity and providing information on the underlying causes of observed spatial patterns. Information on spatial patterns can be used to design more statistically powerful experiments, to improve our understanding of how soil communities develop, and to determine what factors are important for regulating and maintaining soil function. It has been hypothesized that high levels of soil biodiversity are attributable to spatial heterogeneity in resource availability as influenced by land-use patterns and plant community dynamics (Ettema and Wardle, 2002). In particular, spatial isolation may be an important determinant of microbial community structure by facilitating species coexistence. In a simulated soil environment, one bacterial species dominated the community under saturated conditions where the pore network was highly connected (Treves et al., 2003). However, under low moisture conditions (i.e., discontinuous water films) spatial isolation of microbial populations allowed a less competitive species to become established in the community. These results concur with the observation that saturated subsurface soils have lower microbial diversity compared to unsaturated surface soils (Zhou et al., 2004).

vertical distribution within the soil profile

The abundance and biomass of most soil organisms are highest in the top 0-10 cm of soil and decline with depth in parallel with organic matter contents and prey availability. Approximately 65% of total microbial biomass is found in the top 25 cm of the soil profile. Below that depth, microbial densities typically decline by 1-3 orders of magnitude (Fig. 11.6). Hyphal density of and root colonization by mycorrhizal fungi decrease substantially below 20 cm. Mycorrhizal fungal spores are typically not found below the plant rooting zone. Numbers of microbial grazers (e.g., protozoa, collembola) also decrease with depth, often more rapidly than either their bacterial or their fungal prey. For example, collembolan numbers peak at 1-5 cm below the soil surface and drop to almost none below 10 cm. While generally low, the numbers and activities of soil organisms at depth vary spatially depending on gradients in texture, pH, temperature, water availability, and organic matter content. Interfaces between layers often generate localized regions of greater saturation, where microorganisms may exhibit increased numbers or activity due to improved access to nutrients.

In addition to abundance and activity, microbial community composition and diversity also change across the soil profile. Abundances of gram-negative bacteria, fungi, and protozoa are highest at the soil surface, while gram-positive bacteria and actinomycetes tend to increase in proportional abundance with increasing depth (Fierer et al., 2003). Microbes in deeper soil horizons tend to be more C limited than surface microorganisms. Mycorrhizal species change along a vertical gradient, differing in their preference for the organic or mineral soil layers. Only 4 of 22

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