Contributions of the rhizosphere interactions to soil biological fertility

Availability of nutrients in the rhizosphere, which is defined as the soil around the root that is influenced by the root, is controlled by the combined effects of soil properties, plant characteristics, and the interactions of plant roots with microorganisms and the surrounding soil (Bowen and Rovira 1992). The rhizosphere extends up to a few millimetres from the root surface into the surrounding soil and is characterised by a high concentration of easily degradable substrates in root exudates (Lynch and Whipps 1990), which leads to a proliferation of microorganisms (Foster 1986, Curl and Truelove 1986, Rouatt and Katznelson 1961) (Table 1). Root exudation is greatest at the root tip (Marschner 1995) where the microbial density is low (Schönwitz and Ziegler 1989). With increasing distance from the root tip, exudation generally decreases while microbial density increases. Thus, the region of greatest release of root exudate and the region of highest microbial population density are spatially separated. Compared to the bulk soil, nutrient concentrations in the rhizosphere may be increased or decreased (Hendriks et al. 1981) (Table 1). The rhizosphere pH is increased by plant nitrate uptake while ammonium uptake leads to a pH decrease. A pH decrease is also observed in the rhizosphere of N2 fixing legumes (Römheld 1986) (Table 1). (Table presented) Besides serving as a carbon source for microorganisms, root exudates also play an important role in nutrient release by chelation and desorption of poorly soluble nutrients such as phosphorus (P) and iron (Fe) (Dinkelaker and Marschner 1992, Gerke 1994, Jones and Darrah 1994, Römheld 1991, Uren and Reisenauer 1988). For example, increasing amounts of citrate or oxalate adsorbed to the soil matrix result in increasing P mobilisation by ligand exchange and dissolution of P-sorbing Fe and aluminium (Al) sites (Gerke et al. 2000). Some exudates, such as amino acids, are even taken up again by the roots, thus minimising nitrogen (N) loss (Jones and Darrah 1994). Despite the importance of rhizosphere processes in influencing nutrient availability, until recently these processes had not received major consideration in modern agriculture, where the practice has been to provide N, P, and potassium (K) in luxurious quantities as synthetic fertilisers (Schaffert 1994). This has generated concern that the selection and breeding of new plant genotypes for agriculture has resulted in the development of cultivars that are highly responsive to fertilisers, but that do not have traits that are necessary for growth under nutrient-limiting or adverse soil conditions (Duncan and Baligar 1990). With the current emphasis on developing better cultivars for sustainable, low-input agriculture, a better knowledge of the rhizosphere processes that contribute to nutrient uptake efficiency has become essential for nutrient-limiting soils (Crowley and Rengel 1999). In this chapter, the importance of the interactions in the rhizosphere for soil biological fertility will be discussed with respect to root exudates, P, Fe, manganese (Mn) and plant growth-promoting rhizosphere organisms. © 2007 Springer.Petra Marschner and Zdenko Renge