Struvite-compost blends outperform triple superphosphate for sustained phosphorus release in acidic forest soils

Over-consumption and environmental impacts of traditional chemical fertilisers are creating an urgency to source P-alternatives from recovered wastes. Recovering phosphorus (P) from municipal waters as struvite (magnesium ammonium phosphate) is gaining traction globally, especially in Europe, but challenges remain in optimising the nutrient content and release from this recycled fertiliser. Here, we describe an organo-mineral fertiliser called COM-VITE (compost + struvite), which combines different municipal solid waste composts (MSWC) with struvite. It was hypothesised that these blends would provide longer-term/sustained P-release in acidic Irish forest soils, increase/balance soil pH, offering both performance and environmental advantages over standard practice with triple-super-phosphate (TSP). In total, 27 large, reassembled soil cores (> 30 kg) from three forest plantations were collected and fertilised with two types of COM-VITE (green and brown COM-VITE made with garden and food waste, respectively) or with TSP at the equivalent to 250 kg ha -1 on the basis of granulated rock phosphate (11% P) (equivalent to 27.5 kg P ha -1 ). Temporal porewater-P measurements were collected over 227 days, with the diffusive gradients in thin-films technique (DGT) spatially resolving DGT-labile profiles. Complementary fertiliser bags were deployed also at the sites to environmentally calibrate the rate of P loss from each fertilizer over 112 days. Life Cycle Analysis (LCA), parameterised from the experimental data, were also conducted. Results confirmed the lower solubility of COM-VITE (< 20 mg P day -1 ) than TSP under field conditions (80.2 mg P day -1 ). Porewater analysis highlighted large and early pulses of P loss/depletion from TSP, by contrast, COM-VITE provided a more sustained P release over time. Losses of P were also found to be greater and more unpredictable in TSP, with P-maxima at lower soil depths, varying more with soil type. COM-VITE emerges as a promising alternative P source in acidic soils, but broader impact categories especially for struvite production also need consideration.