Root-Induced Changes in the Soil

The following root-induced changes in the soil occur as an inevitable consequence of the nature of submerged soils and plant adaptations to them. 

Mobile inorganic reductants in the soil are oxidized, particularly Fe which is precipitated as Fe(OH)3 on or near the root. As a result the concentration of Fe + near the root falls and more Fe diffuses in from the bulk soil. This is then oxidized resulting in a zone of Fe(OH)3 accumulation near the root. The oxidation of inorganic reductants generates H+  

The net effects of these processes will depend on their rates versus the rates at which the resulting changes are buffered by processes in the soil. In the following sections I give available information for different soil conditions. 

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Plant roots and root-induced chemical changes in the rhizosphere strongly affect the bioavailability of trace elements (Hinsinger, 1999). First, root-induced changes in the ionic equilibria influence the bioavailability of trace elements. The differential rates of plant uptake of water and ions in the soil solution result in a depletion or an accumulation of the ions in the... 

Youssef, R.A., Chino, M., 1988b. Root-induced changes in the rhizosphere of plants. I. pH changes in relation to the bulk soil. Soil Sci. Plant Nutr. 35, 461 68. 

Effects of the root-induced changes on the general microbiology of submerged soils are discussed in Chapter 5 and effects on methane production and consumption are discussed in Chapter 8. 1 here discuss specific effects on plant nutrients. 

The primary objective of this study was to investigate the fractionation of copper, cadmium and lead in the rhizosphere soil in an attempt to obtain a better understanding of their availability and subsequent uptake by plants. In this regard, the present chapter focuses on (1) root-induced changes in various fractions of copper, cadmium and lead in the maize rhizosphere using rhizobox and sequential extraction techniques and (2) influences of pH, root exudates and microbial activity on metal fractionation in the rhizosphere of maize, wheat, pea and soybean, based on the results of specially designed experiments. 

The root-induced pH changes, shown on the right side of Fig. 10, were very different from those caused by addition of root exudates alone (Fig. 10, left). In addition to acidification effect of root exudates, root-induced changes in pH occur as a consequence of root respiration and the excretion or reabsorption of H or HCOj (Nye, 1981 Marschner and Romheld, 1996). As shown in Fig. 10 (right), the pH values in all sterilized and some unsterilized rhizosphere soils increased less than 0.3 units compared with the bulk soil. The two exceptions were unsterilized wheat and pea rhizosphere soils, which were a little more acidic than the bulk soil. 

H. Marschner and V. Romheld, In vivo measurement of root-induced pH changes at the. soil-root interface—effect of plant species and nitrogen. source, Zeitschrift fur Pfianzenphysiolgie 111 24 (1983). 

Nye PH. 1981. Changes in the pH across the rhizosphere induced by roots. Plant and Soil 61 7-26. 

Hinsinger, E, Flassard, C.,Tang, C., and Jaillard, B. (2003). Origins of root-induced pH changes in the rhizosphere and their responses to environmental constraints A review. Plant Soil 248,43-59. 

The increase of atmospheric CO2 may have decreased the pH of precipitation very slightly, but PcOj in soils is far more important for the acid-base status of surface waters. Variation in forest soil Pco is related to the temperature and moisture content of soils as well as the release of excess soil CO2 to the atmosphere. Warmer conditions increase the rate of microbial and root respiration in the soil, thereby increasing soil Pco above the long-term average value and producing short-term increases in runoff ANC, and vice versa. Norton et al. (2001) found that intra-seasonal variations in Pco caused by variable snowpack thickness could induce variation in ANC in runoff of 10-15 peqL Such variability is comparable to variability in ANC caused by a 15-20 p.eqL change in SO4 in runoff. Decline in soil Pco, despite increased temperature and possibly increased soil respiration could result from a lower soil moisture content and a greater efflux of soil CO2. 

Application of S, either as gypsum or from other S sources, generally decreases the uptake of Mo by crops. Studies of soybeans have shown that decreases in plant Mo are not limited only to Mo-S interactions in the soil, because foliar-applied Mo has also been shown to decrease Mo concentrations in soybean seeds and leaves in the presence of soil-added S. On low-Mo soils this can lead to induced Mo deficiency, which can decrease yields and crop quality. On soils with high amounts of Mo, application of S can prevent the accumulation of high Mo concentrations and decrease the potential for Mo toxicity to livestock. Competition between sulfate and molybdate anions, competition between bicarbonate and molybdate, and root-zone pH changes have been suggested as explanations for the action of S to reduce Mo uptake. 

Li, H., Yang, X., Kirk, G.J.D., Dobbermann, A., 2002. Root-induced changes of potassium in the rhizosphere of lowland rice (Oriza saliva L.). In 17th World Congress of Soil Science, 14-21 August 2002, Bangkok, Thailand, pp. 1282-1-1282-11. 

Certain factors controlling metal fractionation in the soil rhizosphere are time-dependent. This is one of the reasons explaining some different results of root-induced rhizospheric changes in metal fractionation. For instance, using a rhizobox system, Chino et al. (1999) found that soluble copper increased near the root, while its total content showed little or no change. Cherrey et al (1999), however, reported a depletion of copper extracted with EDTA, DTPA or CaCl2 in the rhizosphere soil. 

At least three processes triggered by root exudates can induce certain changes in metal fractionation in soil. The introduction of root exudates to a soil can cause pH change, provide ligands for metal complexation and enhance microbial activity. Each of these processes may mobilize or immobilize metals in the soil. In this study, these processes were investigated separately. To examine... 

Dynamic changes in fractionation of copper cadmium, and lead in maize rhizos-phere soil were observed during a 100-day cultivation period. The initial increases in exchangeable copper and cadmium indicate the effect of root-induced metal mobilization. Decreases in the metal-exchangeable fractions occurred after plant root absorption dominated the change. In the studied soil, pH did not have a significant influence on the metal fractionation. Complexation of root exudates and microbial activity in the rhizosphere were the major causes of the corresponding alterations of metal fractions in the rhizosphere soils. 

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