FIGURE 3.6. Spatial distribution of sampling points in topsoil (a). Solid circles form systematic random lattice and open circles form a biased random cluster. An undisturbed core (b) was sampled at each point and a thin section (c) cut from the horizontal plane. Nine spatially referenced images, in which bacteria were mapped, were acquired from each thin section. Average bacterial density per thin section was calculated and the values used to study large-scale variability. Bacterial maps were divided into 100 quadrats and bacterial density in each quadrat calculated. There were 900 quadrats per thin section and these bacterial density values were used to study microscale spatial variability (from Nunan et al., 2002).

scale in the topsoil, whereas evidence for nested scales of spatial structure was found in the subsoil at both the micrometer scale and at the centimeter to meter scales. Evidence for spatial aggregation in bacteria was stronger in the topsoil and decreased with depth in the subsoil. Nunan et al. (2002) suggest that factors that regulate the distribution of bacteria in the subsoil operate at two scales, in contrast to one scale in the topsoil, and that bacterial patches are larger and more prevalent in the topsoil.

Textbooks such as those by Swift et al. (1979) and Paul and Clark (1996) cover a number of methodological approaches for estimating microbial numbers and turnover in considerable detail. In this book, we present a few principal techniques for measuring numbers and identifying members of the microbial communities. We then relate them to studies of nutrient immobilization and mineralization, covered later in Chapter 5 on decomposition processes.

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