Iron-related phosphorus in eroded sediments from agricultural soils of Mediterranean areas

Abstract

A fraction of P in eroded sediments is attached to Fe oxides (Fe-related P). Reduction of Fe oxides in the bottom of water reservoirs is considered to mobilize P bound by poorly crystalline Fe oxides, which is supposed to be the main Fe(III) form available for microbial reduction in anoxic environments. The main objective of this work was to compare Fe and P extracted from suspended sediments of representative agricultural soils of the Guadalquivir Valley (SW Spain) with those in their source soils in order to predict the proportion of Fe-related P in eroded sediments that can be released under anoxic environments. A sequential extraction involving NaOH (P OH), citrate bicarbonate (P cb and Fe cb), citrate (P c and Fe c), citrate ascorbate (P ca and Fe ca), citrate bicarbonate dithionite (P cbd, and Fe cbd), acetate (P OAc), and HCl (P HCl) was used for this purpose. The P content in suspended sediments was four times higher than in the source soils. This is the result, in part, of the increased Fe concentration in sediment compared to total soil. The sum of P OH and P cb accounted for 38% of the combined P fractions in sediments, while 24% was released by reductants (P ca+P cbd). Overall, the P ca/P cbd ratio in sediments was higher than in soils (0.75±0.38 vs. 0.25±0.1). This can be ascribed to the higher ratio of Fe related to poorly crystalline Fe oxides to that related to crystalline oxides in sediments (Fe ca/Fe cbd=0.19±0.07) than in soils (Fe ca/Fe cbd=0.05±0.02). The ratio of Fe ca to Fe cbd was higher at low sediment charge in runoff. The mean P/Fe mole ratio in citrate ascorbate extracts (essentially dissolves poorly crystalline Fe oxides) was higher in soils (0.068±0.056) than in sediments (0.022±0.009); in citrate bicarbonate dithionite extracts (dissolves oxides not reduced by citrate ascorbate), it was the same for soils and sediments (0.01). This indicates that poorly crystalline oxides with low P contents are preferentially eroded. The concentration of P fractions associated to Fe oxides in sediments decreased at increased sediment concentration in runoff, due to a decrease in Fe (ca and cbd extractable) with increasing sediment concentration in runoff.