Phosphorus in Digestate-Based Compost: Chemical Speciation and Plant-Availability

A compost (CHW) from the residue of the dry-batch digestion of household waste and one (CSD) from the solid fraction of anaerobically digested maize were tested for their phosphorus (P) release by means of sequential extraction. The products were then compared in a soil incubation (30 mg P kg-1) for their capacity to supply potentially available P (Olsen-P), and in a pot trial to test plant-available P on Italian ryegrass over 112 days. A municipal solid waste compost (MSWC) and an inorganic P source (P-chem) were added as reference in addition to a not-fertilized control (Control). Sequential extraction showed that CSD had the greatest labile P (NaHCO3 30%> H2O 29%> HCl 23%> NaOH 10%). On the contrary, CHW showed the greatest recalcitrant P (HCl 44%> NaHCO3 10%> H2O 11%> NaOH 8%), being similar to MSWC (HCl 65%> NaOH 13% NaHCO3 12%> H2O 5%). This was in agreement with the different amorphous or crystalline degree of the sample as determined by XRD. Also in soil, CSD showed the highest level of Olsen-P at the beginning of incubation (28.5 mg kg-1), while CHW and MSWC had the lowest level of Olsen-P throughout incubation (4.6 and 6.0 mg kg-1 on average), very close to the Control (3.0 mg kg-1). Plant test showed that CSD also had the greatest available P (ARF: 12.5%), followed by P-chem (5.6%)> MSWC (4.2%)> CHW (2.5%). From sequential extraction and physical analysis, it therefore appears that the presence of significant amounts of Ca, such as in CHW and MSWC, shifted P toward low soluble Ca-P compounds, thus reducing the P available for plant nutrition.