PH Changes in the Rhizosphere and Bioavailability of Trace Elements

Without doubt, most plants growing under natural conditions cause pH to rise in the rhizosphere. As pointed out by Cunningham [16], 62 plant species contain an average of 250 meq of absorbed cations per 100 g of oven-dry shoots and 360 meq anions per 100 g of oven-dry shoots. In general, roots release HCO3 or OH to maintain electrical neutrality in the rhizosphere: an excess of 1100 meq of HCO3 per 100 g dry matter, rather than H+ ion, and the soil near roots become more alkaline instead of more acid.

Some other workers have showed that, when plants are supplied with NH4+ rather than NO3, the pH in the root zone falls [17-19]. Nitrate normally constitutes more than half the total absorbed anions (180 meq per 100 g dry matter). If this amount of nitrogen is taken up as NH4+ instead of NO3-, cations are 430 meq per 100 g dry matter and anions are 180 me per 100 g dry matter; this yields a net release of 250 meq/100 g dry matter. For legumes, where nitrogen is fixed symbiotically and little is taken up as NO3- or NH4+, the net effect is 70 meq H+ released per gram of dry matter. The changes in the rhizosphere are at a maximum when the pH in soil is about 5.3 because, at this value of pH, diffusion of acidity is low [20]. The thickness of the zone influenced by the root decreases to about 1 mm. It is from this thin zone that most of the micronutrients and phosphate uptake probably occurs and in which a large microbial population exists [21].

Nye [21] proposed that at soil pH 8 changes in the pH are small because HCO3- concentration is high in the soil solution. The gradients in the rhizosphere will have a more significant effect on pH in alkaline soils. The release of H+ or HCO3- and the production of CO2 and the variation of soil pH depend upon the plant root. At pH values above 7 for normal inputs of H+ or HCO3- by the root, the soil may have a relatively high acidity diffusion coefficient and thus pH gradients are predicted to be very small. However, the production of CO2 by the root may have a more significant effect. The solubility of CO2 is greater than O2 in soil water; this means that small changes in partial pressure of CO2 lead to relatively large concentration gradients and therefore rapid diffusion [21].

Increased buffer capacity increases the time necessary to establish a given profile, and increased water content reduces the pH changes in the rhizosphere because the ions can diffuse more rapidly. Mitsios and Powell [1] measured changes around single onion roots giving average pH values in small volumes of soil. The preceding processes result in severe pH changes in the rhizosphere, which are directly involved in the dissolution of minerals [2] such as silicates [22], carbonates [23], and phosphates in the rhizosphere [24]. In the rhizosphere, pH can also affect phosphate uptake.

If plants can induce the release of Ca from Ca-carbonates and phosphates due to release of protons by roots, this process is also likely to induce a release of trace elements from them. Hinsinger and Gilkes [24] showed an increase in Ca and P concentration in the rhizosphere of ryegrass and subclover due to release of protons by their roots. It is well known that phosphorus fertilizers are a source of input of Cd in agricultural soils due to substitution of Cd that occurs in phosphate rocks used for manufacturing phosphorus fertilizers [25].

The released proton by plant roots can cause an increased dissolution of goethite. It is known that most plant species (all but grasses) have been described as strategy I plants that respond to Fe deficiency. Marschner and Roemheld [26] concluded that the response to Fe deficiency is due to increased acidifying and reducing capacity of their roots.

Fenn and Assadian [27] showed that, in the rhizosphere of Cynodon dactylon, pH changes and dissolution of carbonates could mobilize Pb, Cu, and Mn in the rhizosphere and accumulate them in the leaves. Youssef and Chino [28,29] found that the mobility of Zn and Cu increased in the soil surrounding the roots because of the acidification of the rhizosphere. Neng-Chang and Huai-Man [30] studied the chemical behavior of Cd in wheat rhizosphere and concluded that the mobility of

Cd increased in the soil-root interface due to acidification of the rhizosphere. The form of nitrogen taken up by the plants was the main factor responsible for acidification process.

Some works has been done to investigate pH changes and the mobility of trace elements in the rhizosphere of hyperaccumulator species. Bernal and McGrath [31] studied the effects of pH and heavy metal concentrations in solution culture on the proton release growth and elemental composition of Alyssum murale and Raphanus sativus.

Bernal et al. [32] compared redox potential and pH changes in the rhizosphere of the Ni hyperaccumulator Alyssum murale and the nonhyperaccumulator Raphanus sativus. These workers concluded that the form of N taken up by the plants was the main factor responsible for pH changes and that the plants were able to reduce system more effectively than the hyperaccumulator. These results indicate that the hyperaccumulator mechanisms may be due to other rhizosphere processes, such as the release of chelating agents, or to differences in the number and affinity of metal root transporters. McGrath et al. [33] studied the heavy metals uptake and chemical changes in the rhizosphere of Thlaspi caerulescens and Thlaspi ochroleucum grown in contaminated soils. Knight et al. [34] investigated the Zn and Cd uptake by the hyperaccumulator Thlaspi caerulescens in contaminated soils and its effects on the concentration and chemical speciation of metals in soil solution. They found that the decrease in the mobile Zn fraction could explain only less than 10% of the total Zn uptake by the plants. The mobile fraction of Zn was depleted by Thlaspi caerulescens more than the closely related, but nonaccumulating Thlaspi ochroleucum.

In the rhizosphere of Thlaspi caerulescens, no significant differences in pH were observed. To explain these results, Knight et al. [34] suggested two possible mechanisms: T. caerulencens is able to mobilize Zn from the soil, or the soil studied had a large capacity to buffer the concentration of Zn in soil solution. Recently, Hamon and McLaughlin [35] showed that there is no difference in specific activity of Cd or Zn taken up by T. caerulescens or wheat. This indicates that the hyperac-cumulator plant was able to access the same pools of metals available to the wheat plants. However, the Zn added in biosolids was highly labile, and the T. caerulescens in this experiment acts more as a Cd-tolerant species than as hyperaccumulator for Cd. These results show that hyperaccumulator plants seem to take up from the same phytoavailable metal pools, from which the other plants can take up metals when this pool is large enough. Mechanisms of hyperaccumulators, such as root exudation, may exist in the rhizosphere that can support metal uptake from less accessible pools.

Moving to the field conditions, where metals are returned to the soil from several sources, the soil is an easily available metal pool.

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