Phosphorus saturation and pH differentially regulate the efficiency of organic acid anion-mediated P solubilization mechanisms in soil

Abstract

Exudation of organic acid anions by plants as well as root-induced changes in rhizosphere pH can potentially improve phosphate (P i ) availability in the rhizosphere and are frequently found to occur simultaneously. In non-calcareous soils, a major proportion of P i is strongly sorbed to metal oxi (hydr)oxides of mainly iron (Fe) and aluminium (Al) and organic anions are known to compete with P i for the same sorption sites (ligand exchange) or solubilize P i via ligand-promoted mineral dissolution. Rootinduced co-acidification may also further promote P i release from soil. The relative efficiency of these different solubilization mechanisms, however, is poorly understood. The aims of this study were to gain a better mechanistic understanding of the solubilizing mechanisms of four carboxylates (citrate, malate, oxalate, malonate) in five soils with high and low P surface site saturation. Results indicate that at a lower P saturation of solid phase sorption sites, ligand-promoted mineral dissolution was the main P i solubilization mechanism, while ligand exchange became more important at higher soil P concentrations. Co-acidification generally increased P i solubility in the presence of carboxylates; however the relative solubilizing effect of carboxylates compared to the background electrolyte (KCl) control decreased by 20-50%. In soils with high amounts of exchangeable calcium (Ca), the proton-induced Ca solubilization reduced soluble P i , presumably due to ionic-strengthdriven changes in the electric surface potential favoring a higher P i retention. Across a wider soil pH range (pH 3-8), P i solubility increased with increasing alkalinity, as a result of both, more negatively charged sorption sites, as well as DOC-driven changes in Fe and Al solubility, which were further enhanced by the presence of citrate. Overall, the relative efficiency of carboxylates in solubilizing P i was greatest in soils with medium to high amounts of anionic binding sites (mainly Fe-and Al-oxy(hydr)oxides) and a medium P sorption site coverage, with citrate being most effective in solubilizing P i .

Effects of Pig Slurry Application and Crops on Phosphorus Content in Soil and the Chemical Species in Solution

“…Phosphate in the soil solution can interact as a ligand with soluble cations such as Ca, Mg, Fe, and Al, among others, changing their availability and mobility (Girotto et al, 2010;Oburger et al, 2011). Continued applications of PS, a multielement fertilizer, are expected to induce changes in the chemical composition and distribution of P species in the soil solution, together with crop cultivation, which can alter the concentration of elements in the soil solution by absorption and accumulation in plant tissues, by exudation of organic compounds and by changes in pH through the release of OH -or H + (Gahoonia et al, 1992).…”

Bioavailability of zinc and phosphorus in calcareous soils as affected by citrate exudation

“…Although it is well documented that citrate (and protons) is able to mobilize P in soil (Hoffland 1992;Geelhoed et al 1999;Kirk et al 1999;Hinsinger 2001;Oburger et al 2011), there are contrasting reports on root exudation of citrate or other low molecular weight organic acids under Zn deficiency and/ or their potential to mobilize Zn in soil. On the one hand Rose et al (2011) proposed that enhanced exudation of malate, but not citrate, is a response of Znefficient rice genotypes to Zn deficiency.…”

Competitive sorption and desorption of phosphate and citrate in clayey and sandy loam soils

“…For both soils, P desorption in response to citrate application was higher than P desorption with water, in view of the competition of citrate with P for similar soil sorption sites, as already noted by other authors (Geelhoed et al, 1999;Johnson & Loeppert, 2006;Moradi et al, 2012). Oburger et al (2011a) observed a similar response in P desorption with citrate addition, causing a mean desorption of 13.0 % of the P applied to the soil and only 3.0 % of P was desorbed with water after an application of 24 mg kg -1 P. Phosphorus desorption by citrate can occur under natural conditions by root exudation or organic matter decomposition (Johnson & Loeppert, 2006;Pavinato & Rosolem, 2008). More than 99 % of the organic acid exudates remain within a distance of 1 mm from the root surface, increasing the chances of P desorption in this region (Jones et al 1996).…”

“…Organic acids can be retained in the soil with high energy and block P sorption sites (Haynes, 1984). The carboxylic groups, characteristic of these organic acids, can dissociate and release protons in a wide pH range and promote the dissolution of soil minerals (Andrade et al, 2003;Oburger et al, 2011a). A great effectiveness of citrate was noted in the competition for P sorption sites; this greater competitiveness is due to the higher number of carboxylic groups (tricarboxylic).…”