Fate and bioavailability of phosphorus loaded to iron oxyhydroxide nanoparticles added to weathered soils

Abstract

The low phosphorus (P) fertilizer use efficiency in weathered, P deficient soils calls for better fertilizer formulations. We previously formulated nanoparticles containing P (NP-P) that were a successful fertilizer in nutrient solution. This study was set up to test the fate and the bioavailability of nanofertilizer-P and of that of native (colloid) P naturally present in soil. The NP-P consisted of nano-ferrihydrite (~ 10 nm) loaded with phosphate (P-nFh) and stabilized with either natural organic matter (NOM) or hexametaphosphate (HMP). Natural colloid concentrations were increased with KOH addition, as deflocculating agent, to soil; all tests used samples from P deficient, highly weathered soils. Pot trials with rice seedlings did not reveal larger P uptake in the NP-P amended soils compared to equal doses of soluble PO4 or soluble HMP. Total Fe concentrations in soil solutions were unaffected by NP-P addition, whereas natural colloidal Fe and P markedly increased by KOH addition. The bioavailability of native colloidal P, mobilized by KOH addition, could not be assessed due to lack of growth, likely related to collapse of the soil structure. This study showed that P-loaded iron oxyhydroxide NPs insufficiently enhanced soluble P in soil to offer benefits over soluble fertilizers, likely because of a combined effect of lower diffusivity of NPs compared to Pi and lower bioavailability of NP-P than Pi. Smaller particles or small labile organic colloids might offer an improvement in both aspects.