Phosphorus solubility changes following additions of bioenergy wastes to an agricultural soil: Implications for crop availability and environmental mobility

Abstract

Pathways for replacing chemical fertilisers (reliant on rock P resources) with alternative P-bearing materials require assessment of soil processes, crop nutrient acquisition and potential pollution consequences. We examined bioenergy waste materials, individually and as combined ash and anaerobic digestate in terms of plant P availability and mobility. We compared initial effects on mixing of amendments with a test soil and effects after 6-weeks pot trials, with and without wheat growth, against soil and chemical fertiliser controls. Chemical extractions, 31P NMR spectroscopic determination of P forms and phytase-labile P assays examined processes of P release. 31P NMR analysis revealed that ash comprised dominantly inorganic orthophosphate P with inherent low P solubility. Initial ash mixing with soils increased solution pH, soluble P in water (pure ash alone) and in citrate (ash alone and in blends). Digestate comprised a diverse array of orthophosphate and organically-complexed P forms, similar to the test soil P compositions, with limited P solubility on initial mixing. Following no plant incubations high water-soluble P with ash additions remained but all effects on citrate-soluble P were normalised. Incubations with plants increased water-soluble P in digestate only and blended amendment treatments relative to initial mixing. When comparing to chemical fertiliser the digestate plus ash blends led to smaller water-soluble P, but equal P in above-ground biomass after incubation. The ash-digestate C:N:P ratios and P form diversity appeared to promote microbial regulation of plant P availability versus potential leaching. The results suggest that the initial days-months are important periods for amendment interactions with soils during plant establishment and the lag before strong growth when system P mobility may induce polluted runoff. Biogeochemical P solubility controls require further study across differing soils and timescales to inform management of bioenergy wastes as fertilisers, particularly in terms of trade-offs such as crop nutrition versus system P losses.