Polyphosphate hydrolysis, sorption, and conversion in two different soils

Polyphosphate fertilizer is an alternative to traditional orthophosphate fertilizer, but it is unclear whether it provides greater soil phosphorus (P) mobility and availability than orthophosphate fertilizer. This study aimed to examine the chemical behaviour of polyphosphate after its application to the soil. The hydrolysis, sorption, and P transformation of two types of polyphosphate fertilizers (short-chain and long-chain) were examined for two types of soils (acidic Acrisol and calcareous Cambisol), with orthophosphate serving as the control treatment. P-31 NMR spectroscopic analyses indicated that polyphosphate in soil was rapidly converted to pyrophosphate, which was then hydrolyzed to orthophosphate at a slower rate. In general, in the calcareous Cambisol, the addition of short-chain polyphosphate fertilizer resulted in greater soil P mobility as measured by greater soil pore water orthophosphate concentration than the control. Thus, short-chain polyphosphate fertilizer tended to be more effective than orthophosphate in calcareous soils. This was in contrast to the result of an aqueous incubation experiment, in which a lower pH increased the rate of polyphosphate hydrolysis. Indeed, in both soils, the sorption of total P was greater when P was added as polyphosphate than when it was added as orthophosphate. Moreover, polyphosphate sorption appeared to be mainly controlled by soil iron and aluminium minerals, because the Acrisol showed stronger polyphosphate sorption than the Cambisol due to its greater amorphous iron content. It appears that polyphosphate was first adsorbed on soil minerals before being hydrolyzed to release orthophosphate, and calcium may have played an important role in the hydrolysis of polyphosphate as evidenced by the greater P release in calcareous Cambisol. Our results suggest that soil characteristics, such as acidity, iron, aluminium, and calcium content, should be considered when evaluating the P efficiency of polyphosphate fertilizers.Highlights P-31 NMR spectroscopy successfully tracked the fate of added Poly-P in soilPoly-P was rapidly converted to Pyro-P, and then hydrolyzed to Ortho-P slowlyP supplied as Poly-P had stronger sorption affinity than Ortho-P in all soilsShort-chain Poly-P induced greater P mobility than Ortho-P in Cambisol but not in Acrisol