Abstract
We have used an integrated approach to study the mobility of inorganic phosphorus (P) from soil solid phase as well as the microbial biomass P and respiration at increasing doses of citric and oxalic acid in two different soils with contrasting agronomic P status. Citric or oxalic acids significantly increased soil solution P concentrations for doses over 2 mmol kg-1. However, low organic acid doses (<2 mmol kg-1) were associated with a steep increase in microbial biomass P, which was not seen for higher doses. In both soils, treatment with the tribasic citric acid led to a greater increase in soil solution P than the dibasic oxalic acid, likely due to the rapid degrading of oxalic acids in soils. After equilibration of soils with citric or oxalic acids, the adsorbed-to-solution distribution coefficient (Kd) and desorption rate constants (k-1) decreased whereas an increase in the response time of solution P equilibration (Tc) was observed. The extent of this effect was shown to be both soil and organic acid specific. Our results illustrate the critical thresholds of organic acid concentration necessary to mobilize sorbed and precipitated P, bringing new insight on how the exudation of organic acids regulate chemical-microbial soil phosphorus transformations.
Highlights
Modern agriculture is dependent on phosphorus (P) fertilizer applications to maintain crop productivity
The specific objectives of this study were (a) to determine the critical concentration of citric and oxalic acids sufficient to significantly increase P availability in soils with contrasting P status; (b) to evaluate the amount of inorganic and organic P that can be mobilized by increasing doses of citric or oxalic acid; (c) to evaluate the impact of citric acid on adsorbed-to-solution distribution coefficients, desorption rate constants and other parameters reflecting P resupply from soil solid phase; and (d) to evaluate to what extent the effect of lowmolecular-weight organic acids (LMWOA) on microbial biomass stimulation promotes the sequestration of released P, as well as the degradation of LMWOAs decreasing their net effect on P mobilization
PH and Phosphorus Concentrations in Soil Extracts. Both citric acid and oxalic acid resulted in the acidification of soils, but the extent of this effect was both soil and LMWOA type specific
Summary
Modern agriculture is dependent on phosphorus (P) fertilizer applications to maintain crop productivity. As a major plant macronutrient, P is perhaps the most limited with respect to bioavailability due to its rapid precipitation and adsorption in soils.[1] Plants have evolved several mechanisms to increase P bioavailability in the rhizosphere.[2] The exudation of lowmolecular-weight organic acids (LMWOA) is proposed as a key mechanism for increasing plant P uptake in soils.[2−4] There is an increasing scientific interest in the selection and genetic engineering of plants that exudate LMWOAs into the rhizosphere (root-soil-interface) as a means to enhance P uptake and plant yields.[5,6] some studies have shown difficulties in obtaining success in increasing plant P uptake with the expression of this trait in planta.[7] Among the LMWOAs, the tribasic citric acid and the dibasic oxalic acid are reported to be among the most commonly produced root and microbial exudates affecting rhizosphere P availability.[8,9] Citric and oxalic acids have been shown to induce higher P mobilization than other organic acids.[10,11] The mechanism involved is generally assumed to be related to the ability of LMWOAs to complex metal cations and compete with P for adsorption sites on soil colloids, thereby releasing precipitated and adsorbed P.3,12. Little is known about how these LMWOAs affect the desorption kinetics of soil P once solid-to-solution phase equilibria are reached
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