Phosphorus Mineralization and Transport in the Vicinity of an Anion Sink: Experiment and Modeling

Abstract

Digestates from biogas plants provide valuable crop nutrients, such as P. Plant uptake of anions like PO43− causes an alkalinization of the rhizosphere due to release of HCO3−. We investigated the transport of P to an anion sink simulating the anion exchange activity of a plant root in digestate-amended soils and hypothesized that the HCO3− source induces enhanced mineralization of organic P. We expected this effect to strengthen with decreasing distance from the HCO3− source. Anion exchange resins (AERs) saturated with HCO3− were used in an incubation experiment. Available PO4–P was measured in 10 slices of the soil–digestate mixtures (1 mm each) after 2, 8, and 32 d and in the resin and total uncut samples after 2, 4, 8, 16, 32, and 64 d of incubation. Anion exchange resins increased pH by 2 to 3 units. Concentrations of available PO4–P in the uncut samples increased within 4 d, which we attribute to enhanced microbial P mineralization activity. Afterward, PO4–P concentrations decreased within 4 d, which we attribute to precipitation of calcium phosphate due to further increasing pH. Concentrations of PO4–P were lowest near the AER and showed a peak migrating away from the resin with time. A new model linking diffusive transport and mineralization intensity of organic matter as a function of distance from AER could well describe the observed spatial PO4–P distribution and accumulation in the resin. However, further studies are needed to account for mechanisms like precipitation in models for P transport.