(Fig. 11.4). Hiltner, in 1904, used the term rhizosphere for the area of bacterial growth around legume roots. Later the rhizosphere became generally known as the soil region under the immediate influence of plant roots and in which there is a proliferation of microorganisms due to the influx of plant-derived labile organic matter into the soil. Pathways for release of plant assimilates from roots include turnover of fine roots, leakage or diffusion of molecules across cell membranes (exudation), and sloughing off of cells and tissue fragments during root growth. Root caps and tips are sites of active exudation, releasing mucilaginous material as well as root caps and cells. Analyses of the organic materials found in the rhi-zosphere reveal a wide assortment of compounds, including aliphatic, amino, and aromatic acids and amides, sugars, amino sugars, cellulose, lignin, and protein.

Complex interactions and feedbacks occur at the plant-soil interface, representing potentially important factors regulating ecosystem structure and functioning. These interactions range from mutualistic to pathogenic and have long been recognized for their role in plant nutrition and nutrient cycling. More recently, plant-microbe interactions have been found to influence the composition and diversity of both plant and soil microbial communities. Microbial abundance, activity, and community composition and diversity often reflect the plant species present in a given soil. For example, bacteria isolated from the rhizospheres of different grass species

FIGURE 11.4 Diagrammatic representation of a plant root and associated biota in an approximately 1-cm2 area. (Adapted from S. Rose and T. Elliott, personal communication.)

exhibited differential growth and C source utilization patterns (Westover et al., 1997). Plant-mediated differences in the microbial community are potentially attributable to specific variations between plant species in the quality and quantity of organic matter inputs to the soil. Plant resource quantity and quality can also be altered by disturbances such as herbivore grazing and global change (e.g., elevated atmospheric CO2, N deposition). Plant herbivory has been shown to increase C allocation to roots, root exudation, fine root turnover, soil-dissolved organic C, microbial biomass and activity, and faunal activity (Bardgett and Wardle, 2003). These changes in turn alter soil N availability, plant N acquisition, photosynthetic rates, and, ultimately, plant productivity.

Just as the plant community may be a determinant of microbial community structure, the diversity and composition of the microbial community may play a role in plant community dynamics. The diversity of arbuscular mycorrhizal fungi was observed to be a major factor contributing to plant diversity, above- and below-ground plant biomass, and soil nutrient availability in a macrocosm experiment simulating North American old-field ecosystems (Van der Heijden et al., 1998).

An increase in arbuscular mycorrhizal fungal diversity was accompanied by a significant increase in the length of mycorrhizal hyphae in the soil leading to greater soil resource acquisition. As the number of mycorrhizal fungal species increased, plant diversity, biomass, and plant tissue phosphorus content increased, while the phosphorus concentration in the soil decreased.

There is accumulating evidence that plants "culture" a soil community that then controls their long-term survival and growth, sometimes negatively. Survival and growth of several grass species were significantly reduced when grown with their own soil community rather than that of another plant species (Bever, 1994). This result was attributed to an accumulation of specific plant pathogens or a change in microbial community composition. Such a negative feedback may provide a mechanism for maintenance of plant communities in natural ecosystems, whereby an individual plant cannot dominate a community because of the accumulated detrimental effect of the soil community on plant growth. It has been hypothesized that exotic plants that become invasive have escaped control by local soil organisms at invaded sites and may even alter the microbial community to their benefit. Callaway et al. (2004) reported that spotted knapweed (Centaurea maculosa), an invasive weed in North America, cultivates a soil community in its native European soil that negatively affects its own growth, possibly controlling its spread in its home range. However, the plant cultivates a different soil community at invaded sites in the western United States, positively enhancing its own growth and contributing to its success as an invasive species.

spatial heterogeneity of soil organisms

Organism abundance and activity are not randomly distributed in soil, but vary both horizontally and vertically through the soil profile (Fig. 11.5). Different groups of organisms exhibit different spatial patterns, because they each react to soil conditions in different ways (Klironomos et al., 1999). This spatial heterogeneity, which has been observed at the scale of millimeters to hundreds of meters, has been shown in some studies to correlate with gradients in site and soil properties, including bulk density, aggregation, texture, oxygen concentration, pH, moisture, soil organic matter content, inorganic N availability, precipitation levels, and vegetation dynamics. Some of these properties are important at the microscopic scale, whereas others act over larger distances. For example, microbial biomass and collembolan abundance in an agroecosystem reflected large-scale gradients in soil C content and cultivation practices (Fromm et al., 1993). In other cases, soil characteristics have been found to explain a relatively minor amount (<30%) of the spatial variation in organism abundance (Robertson and Freckman, 1995). Spatial heterogeneity can be high even in soils that appear relatively homogeneous at the plot or field scale (Franklin and Mills, 2003). Nematode populations were strongly

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