Rhizosphere nutrient cycling

In addition to the interactions between plants and microorganisms, a third factor, the soil, also plays a role in determining root exudation and the activity and diversity of rhizosphere microbial populations. In this section, physical and structural aspects of the soil are discussed in relation to their effects on root exudation and microbial populations. Consideration is also given to the role of agricultural management practices on rhizosphere processes. In addition, the role of other biotic factors, such as microfaunal predation, is discussed in relation to nutrient cycling in the rhizosphere. 

Enhanced nutrient cycling in both the rhizosphere and bulk soil may depend on the bacterial grazing by protozoa or nematodes with release of inorganic N. Nematodes appear to be the primary consumers of bacteria in the rhizosphere, whereas protozoa are equally prevalent in rhizosphere and bulk soil (41,97). Estimated C-to-N ratios of bacterial-feeding nematodes range from 5 1 to 10 1 (98,99) and are generally higher than those of their bacterial food source thus the excess N is excreted as ammonia (100,101) by nematodes. The estimated... 

This activation of nutrient cycling is based upon a stimulation of microbial activity in the rhizosphere by labile C released by roots (De Nobili et al., 2001). 

Zoysa, A.K.N., Eoganathan, P. and Medley, M.J. (1999) Phosphorus utilisation efficiency and depletion of phosphate fractions in the rhizosphere of three tea (Camellia sinensis L.) clones. Nutrient Cycling in Agroecosystems 53, 189-201. 

Fresh organic matter plays a fundamental role in plant nutrition by supplying nutrients released through degradation processes however, humified organic substances also become a source of nutrients when subjected to mineralization processes. The main aspects of the cycle of organic matter at the rhizosphere soil are reported in Chap. 6. 

Mucilage has protective functions for the root meristem and improves root-soil contact by inclusion and aggregation of soil particles. It may also contribute to P desorption and to the exclusion of toxic elements (Al, Cd, Pb) by complexation with galacturonates, mainly in exchange with Ca2+ (Neumann and Romheld, 2002). Secreted enzymes contribute to the extracellular enzyme pool it has been shown that the activity of extracellular enzymes, such as phophatases, proteases, and aryl-sulfatases, exhibit more activity in the rhizosphere relative to the bulk soil and may have a dramatic effect on the cycling of nutrients such as P, N, and S (Badalucco and Nannipieri, 2007). 

The extremely complex interrelationships that exist in the rhizosphere between detrital processing, mineralisation of organically-bound nutrients and nutrient uptake, makes field investigation of these processes difficult. Consequently, an alternative approach has been to use sterilised soil and added consumer and decomposer organisms (microcosms) in various combinations in the laboratory to investigate the role of the microfauna in the soil P cycle. 

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