Iron dynamics and phosphorus availability during short periods of soil reduction

Abstract

Soils frequently have short reduction periods, in which iron forms and P availability changes. During the reduction period, the solubility of iron phosphate increase, whereas oxidation determines P adsorption on iron oxi-hidroxides, and consequently decrease P availability. The objective of this work was to evaluate the magnitude of these changes in P availability, in relation to iron oxides reactivity, after short periods of soil reduction in three uruguayan soils. We applied three hydric regimes to pots containing 1600 g of soil: (1) 20 days at 5% above Field Capacity (R₁₀₅); (2) 20 days with water content up to 50% of Field Capacity (R₅₀); and (3) 20 days at 5% above Field Capacity + 20 days with water content up to 50% of Field Capacity (R₁₀₅R₅₀). Phosphorus treatments (0 or 33,3 mg P pot^-1) were applied prior to each hydric regimes on R₁₀₅ or R₅₀, or
between periods R₁₀₅ and R₅₀ in R₁₀₅R₅₀. After each hydric regime, soil samples were analyzed for P availability (Bray 1), total Fe oxides (Fe_d), low cristalinity Fe oxides extracted with ammonium oxalate (Fe_ox,) and P retention (P₈₀: percent of P added to soil who was sorbed, from 80 mg kg^-1). Subsequently, ryegrass (Lolium multiflorum Lam.) seeds were sown in each pot to evaluate P uptake by plants after 45 and 69 days. During the reduction period, P availability and Fe_ox increased. The return of soils to oxidized conditions caused a decrease
in P availability levels because of an increase in P retention by Fe oxides recently precipitated. As a results, P uptake decreased. When phosphate fertilizer was added to soil after a reduction period, P adsorption on Fe oxides was observed also. The lowest P contents in plants were observed after 20 days of reduction conditions followed by an oxidation period of 20 days (treatment R₁₀₅R₅₀).