Ionic interactions and their effects on the adsorption-desorption processes of phosphorus and nitrate in typical oxisol clay-sized minerals

• Grant number: 12/25178-4
• Support Opportunities: Regular Research Grants
• Duration: April 01, 2013 - March 31, 2015
• Field of knowledge: Physical Sciences and Mathematics - Geosciences
• Principal Investigator: Marcelo Eduardo Alves
• Grantee: Marcelo Eduardo Alves
• Host Institution: Escola Superior de Agricultura Luiz de Queiroz (ESALQ). Universidade de São Paulo (USP). Piracicaba , SP, Brazil

Abstract

Phosphorus adsorption is stronger in tropical soils than in those of temperate zone. This difference can be ascribed mainly to the clay mineralogical composition of the first ones which comprises greater amounts of kaolinite, gibbsite, hematite and goethite. This mineralogical composition is usually found in the clay fraction of Oxisols, a class of deeply weathered soils that cover great part of Brazil. Although adsorption contributes to the phosphorus storage in the soil region explored by the plant roots, the aging of this retention and the precipitation of calcium, iron and aluminum phosphates contribute for the so-called fixation of P. Therefore, the decrease in adsorption strength could contribute to increase P availability for cropped plants. According to the literature some anions derived from the ionization of organic acids such as acetic, citric, formic, malic and oxalic contribute to reduce the amount of P adsorbed by minerals and so, at least briefly, to retard the more intense P retention that take place in tropical soils. In this context, the present research aims to evaluate if, besides the well-known decreasing effect on P adsorption, the coadsorption of organic anions also weaken the phosphorus retention by kaolinite, gibbsite, hematite, and goethite. For this, studies will be carried out on the individual effects of anions acetate, citrate, formate, malate and oxalate on the kinetics desorption of P adsorbed on each above-mentioned mineral considering that to a faster desorption can be associated a greater weakening organic anion effect on the P retention. Like phosphorus, nitrogen is also essential to plants that uptake it from the soil mainly as ammonium (NH4+) and nitrate (NO3-). Due to be easily leachable in the soil, nitrate not only can be less available to the crops but also can pollute subterranean aquifers. Considering the known increase in the positive surface charges of iron oxides and gibbsite caused by the adsorption of divalent cations, the present proposal will also investigate the individual effects of some plant nutrients (K+, Ca2+, Mg2+, Cu2+, Mn2+ e Zn2+) on the amounts of nitrate adsorbed by kaolinite, gibbsite, hematite and goethite. Furthermore, the effects of the possible co-adsorption of those cations and nitrate on the release of this anion will be studied in each of the above-mentioned minerals by using a kinetic approach. (AU)