Rhizosphere acidification

Especially in dicotyledonous plant species such as tomato, chickpea, and white lupin (82,111), with a high cation/anion uptake ratio, PEPC-mediated biosynthesis of carboxylates may also be linked to excessive net uptake of cations due to inhibition of uptake and assimilation of nitrate under P-deficient conditions (Fig. 5) (17,111,115). Excess uptake of cations is balanced by enhanced net re-lea,se of protons (82,111,116), provided by increased bio.synthesis of organic acids via PEPC as a constituent of the intracellular pH-stat mechanism (117). In these plants, P deficiency-mediated proton extrusion leads to rhizosphere acidification, which can contribute to the. solubilization of acid soluble Ca phosphates in calcareous soils (Fig. 5) (34,118,119). In some species (e.g., chickpea, white lupin, oil-seed rape, buckwheat), the enhanced net release of protons is associated with increased exudation of carboxylates, whereas in tomato, carboxylate exudation was negligible despite intense proton extrusion (82,120). 

The contribution of root and microbial respiration to rhizosphere acidification depends on the amount of C02 released, the amount of organic material oxidized to C02, and the initial pH of the soil (Hinsinger et al., 2003). However, since the first pK of H2CO3 is 6.36, the contribution of these processes to soil acidification will be significant only in neutral and alkaline soils. 

Beyond this effect, however, the concentration gradients measured in various solid-phase metal fractions at the soil-root interface can also be attributed to the effects of a variety of processes, such as rhizosphere acidification or alka-linization, adsorption or desorption reactions, and precipitation or dissolution phenomena, which are themselves associated with plant uptake and a range of... 

Weis, 2004). If the rhizosphere plays an important role in the sequestration of metals in wetland soils, fluxes of potentially toxic metals out of the wetland to adjacent aquatic systems will, in theory, be reduced. However, plants may also increase metal mobilization through rhizosphere acidification and the oxidation of metal-sulfide complexes (Jacob and Otte, 2003). They may also represent only a temporary sink for metals if rhizosphere Fe plaque is reduced following plant senescence. Furthermore, metals can be exported from the ecosystem if contaminated plant parts are consumed by people or wildlife. The pathways and possible health effects of metal consumption have been especially well studied in Southeast Asia, where metal contamination (notably As) of rice crops is a serious public health issue (e.g., Meharg and Rahman, 2003 Meharg, 2004). 

Gillespie, A.R., Pope, P.E., 1990. Rhizosphere acidification increases phosphorus recovery of black locust I. Induced acidification and soil response. Soil Sci. Soc. Am. J. 54, 533-537. 

Jauert, R, Schumacher, T.E., Boe, A., Reese, R.N., 2002. Rhizosphere acidification and cadmium uptake by strawberry clover. 1. Envir. Qual. 31, 627-633. 

Under Fe-deficient conditions, Strategy I plant species (i.e. all plant species but grasses) show a typical enhanced release of protons (and thus rhizosphere acidification) behind the apical root zone (Marschner et al, 1982 Romheld,... 

Putnam AR, DeFrank J, Barnes JP (1983) Exploitation of allelopathy for weed control in annual and perennial cropping systems. J Chem Ecol 9 1001-1010 Rao TP, Yano K, lijima M, Yamauchi A, Tatsumi J (2002) Regulation of rhizosphere acidification by photosynthetic activity of cowpea Vigna unguiculata L Walp) seedlings. Ann Bot 89 213-220... 

W. Petersen and M. Bottger, Contribution of organic acids to the acidification in the rhizosphere of maize seedlings. Plant Soil 132A59 (1992). 

In the first case the mechanisms are based on an increased reducing capacity of Fe(lll)-chelates, a necessary step in the uptake process, with a concurrent increase in acidification and release of organic acids into the rhizosphere in the latter case molecules having high affinity for Fe (phytosiderophores) are synthesized and released into the rhizosphere when Fe is lacking. 

In experiments with lowland rice Oiyzci saliva L.) it was found that roots quickly exhausted available sources of P and sub.sequently exploited the acid-soluble pool with small amounts deriving from the alkaline soluble pool (18). More recalcitrant forms of P were not utilized. The zone of net P depletion was 4-6 mm wide and showed accumulation in some P pools giving rather complex concentration profiles in the rhizosphere. Several mechanisms for P solubilization could be invoked in a conceptual model to describe this behavior. However using a mathematical model with independently measured parameters (19), it was shown that it could be accounted for solely by root-induced acidification. The acidification resulted from H" produced during the oxidation of Fe by Oi released from roots into the anaerobic rhizosphere as well as from cation/anion imbalances in ion uptake (18). Rice was shown to depend on root-induced acidification for more than 80% of its P uptake. 

Microbial-driven mineralization of organic matter can also contribute to acidification of the rhizosphere (Badalucco and Nannipieri, 2007). It should be noted that pH variations in the rhizosphere depend also on the soil s buffering capacity... 

Besides the fluxes of protons that occur to counterbalance charge imbalances, redox-coupled pH changes can also take place in the rhizosphere, as is the case for lowland rice. Kirk and Le Van Du (1997) showed that the precipitation of iron oxide occurring as a consequence of root-induced oxidation of the rhizosphere of rice was responsible for a significant proportion of the concurrent acidification. 

In the case of Erica arborea, which is an acidophilic species, the colonization of an alkaline soil derived from marine sediments had been so efficient that it had converted to acid conditions a mass of soil to about 60 cm in depth. As indicated by the absence of differences between the two soil compartments of this soil thickness, the process of acidification was able to transform all the bulk soil into rhizosphere soil. Indeed, Erica roots are now colonizing the horizon underneath, where dissolution of carbonates has already been accomplished, but where the bulk is still less acidic than the rhizosphere soil. At greater depths, carbonates persist and roots of Erica are rare. 

The acidification of the rhizosphere has been reported in a range of studies from pot experiments to field investigations (Marschner, 1986 Smith and Pooley, 1989 Courchesne et al, 2001 Wang et al, 2001). The acidification is considered to be mainly induced by the response of roots to ionic charge imbalances in the soil solution (Nye, 1986 Haynes, 1990). This imbalance is caused by the preferential uptake of cations or anions, as selected by plant roots. The acidification of the soil solution therefore results from a release of H+ by roots in response to an ionic charge imbalance caused by the preferential uptake of cations such as NH. Other factors, such as the exudation of organic substances by roots and COj enhancement by microorganism respiration, are also known to contribute to the acidification of the rhizosphere. 

Muranyi, A., Seeling, B., Ladewig, E., Jungk, A., 1994. Acidification in the rhizosphere of rape seedlings and in bulk soil by nitrification and ammonium uptake. Z. Pflanzenemahr. Bodenk. 157, 61-65. 

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