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

Digestates from biogas plants provide valuable crop nutrients, such as P. Plant uptake of anions like PO 4 3− causes an alkalinization of the rhizosphere due to release of HCO 3 − . 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 HCO 3 − source induces enhanced mineralization of organic P. We expected this effect to strengthen with decreasing distance from the HCO 3 − source. Anion exchange resins (AERs) saturated with HCO 3 − were used in an incubation experiment. Available PO 4 –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 PO 4 –P in the uncut samples increased within 4 d, which we attribute to enhanced microbial P mineralization activity. Afterward, PO 4 –P concentrations decreased within 4 d, which we attribute to precipitation of calcium phosphate due to further increasing pH. Concentrations of PO 4 –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 PO 4 –P distribution and accumulation in the resin. However, further studies are needed to account for mechanisms like precipitation in models for P transport.