Organic acid mediated nutrient extraction efficiency in three calcareous soils.(Report)

Introduction

The secretion of organic acids by plant roots is now regarded as one of the common mechanisms involved in solubilising relatively insoluble soil nutrients (Jones et al. 1994). One of the effects linked to nutrition that organic acids have in root exudates is the acidification of the rbizosphere. Root exudation of high concentrations of organic acid anions as a result of nutrient deficiency (P deficiency) may lower rhizosphere pH, making P and micronutrients such as Mn, Fe, and Zn more available in calcareous soils (Dinkelaker et al. 1989; Hoffland et al. 1989; Jones and Darrah 1994b). However, the relationship between organic acid exudation and rhizosphere acidification is not that simple, as the extrusion of [H.sup.+] would depend on the amounts of anions absorbed by roots relative to cations (Jones and Darrah 1994a; Dakora and Phillips 2002). When organic acids are added to solution they bind cations (e.g. Ca), causing the release of [2H.sup.+] (in the case of dicarboxylates, for example) from the organic acid, which consequently lowers solution pH. In contrast, the addition of potassium salt of the organic acids tends to cause a rise in soil solution pH, presumably due to the release of C[O.sub.3.sup.-] from the CAC[O.sub.3] and the formation of HC[O.sub.3.sup.-].

Soil is acidified due to proton release from roots. As a consequence of proton release, plants accumulate organic anions, which may, if returned and decomposed in the soil, neutralise the soil acids. Despite early work suggesting that organic acids can acidify the rhizosphere (Marschner 1995), it now appears that [H.sup.+] release and organic acid release are probably 2 biochemically separate but spatially coordinated transport events (Kirk 2002). Excretion of organic anions may solubilise P by changing the soil pH and by displacing P from adsorption sites. Changes in pH may be important where the anion is excreted in large quantities in association with protons, but in general the plant's inorganic cation-anion balance has a much larger effect on rhizosphere pH. Plants mobilise less readily available soil phosphates (Marschner 1995). Lowering of the soil pH when organic anion is excreted in association with [H.sup.+] ions may cause displacement of P from soil adsorption sites. However, there will only be a substantial pH change where very large quantities of organic anion are excreted (Kirk 2002).

The exudation of low molecular weight carboxylates into the rhizosphere has been hypothesised to play an important role in controlling metal solubility in soil (Jones 1998; Jones et al. 2003; van Hees et al. 2005). The organic acids could potentially lower the pH of the rhizosphere and therefore increase the dissolution of metals and improve their availability for plant uptake (Jones et al. 2003; van Hees et al. 2005). This is particularly apparent for metals that become more soluble with decreasing pH (e.g. Fe, Zn, Mn, etc.). The organic acid anions (e.g. [citrate.sup.3-], [malate.sup.2-], [oxalate.sup.2-) are also capable of forming organo-metallic complexes with metal ions, consequently modifying the mobility of the metals in the rhizosphere (i.e. by reducing their sorption potential, increasing their rate of diffusion, or dissolving the solid phase such as rock and mineral fragments, secondary minerals, and organic materials).

Plant nutrient availability in soil is usually assessed by performing batch extracts whereby small amounts of soil (e.g. 1-5 g) are shaken with a relatively large volume of solution (e.g. 10-50mL) for short periods of rime (e.g. 0.5-3h). The transfer of solutes to the aqueous phase provides a measure of plant availability (Wang et al. 2003). Here we hypothesise that low molecular weight organic acids (LMWOAs) used in laboratory batch extracts in the absence of roots will provide a model system for investigating the behaviour of LMWOAs in the rhizosphere. We used this approach because of the inherent difficulties in obtaining very small representative volumes of soil solution from the rhizosphere and their subsequent analysis. According to Westerman (1999), it is desirable to know the composition of solutes existing in the soil water at field water content. However, because present methods of obtaining soil water samples at field water contents are not practical for routine purposes, soil solution extracts are generally used. Because the absolute and relative concentrations of various solutes in the extract are influenced by the soil/water ratio, the ratio must be specified and it is desirable that it be standardised to obtain results that can be applied and interpreted uniformly. The soil/water ratio in the aqueous ratio extracts of saturated soil pastes is the lowest practical ratio for which enough extract for analysis can be readily removed from the soil with low pressure or vacuum (Rhoades and Miyamoto 1990). Of the organic acid anions released from the root, citric acid (H-citrate) and oxalic acid (H-oxalate) are often released in the highest amounts and evidence suggests that they may be effective at solubilising metals in calcareous soils (Strom 1997). Based upon studies in acid soils, we hypothesise that the reaction of organic acids with metals in calcareous soils may also be time- and concentration-dependent (Jones and Darrah 1994a). As pH is known to be a major factor regulating the metal complexation reactions of organic acids in solution, the ionic state of the organic acids during release from the root may also be extremely important. Normally, it is assumed that organic acids are released in the acid form ([H.sub.3]citrate or [H.sub.2]oxalate); however, there is also evidence to suggest that they are released in the form of potassium salts ([K.sub.3]citrate or [K.sub.2]oxalate; Ryan et al. 2001). Depending upon the buffering capacity of the soil, the release of organic acids could therefore cause an acidification or alkalisation of the soil. This may have a profound effect on their metal mobilisation capacity. This reaction can also be expected to be highly soil type-dependent. The objective of this study was therefore to investigate the efficacy of different organic acid anions for solubilising nutrients from 3 calcareous soils.

Materials and methods

Soil sampling