Effect of time and temperature on the Phosphorus availability in soils fertilized with sewage sludge and derived materials

Recycling phosphorus (P) from sewage sludge and its derivatives such as ash and biochar is key for closing nutrient cycles. Although these materials initially provide less available P than fully water-soluble fertilizers, their long-term effectiveness as plant P sources is poorly understood. We hypothesized that soils fertilized with sewage sludge or derivatives would show gradual increases in water-extractable P concentrations (WEP) during an incubation experiment. Given that higher incubation temperatures can accelerate P release and binding processes, we expected variations in WEP to be more pronounced at higher temperatures. We employed three complementary approaches to assess soil P availability over time following fertilization with sewage sludge from different wastewater treatment processes (P removal with FeCl3 and biologically (SS), Fe2(SO4)3 and biologically (DSS), both with anaerobic digestion, and Fe2(SO4)3 with aerobic mineralization in reed beds (MSS)), a sludge-derived biochar or ash, or triple superphosphate (TSP). The approaches were: (1) a soil incubation experiment conducted at 7°C and 21°C over 600 days, with periodic measurements of WEP; (2) sorption-based kinetic modeling to predict WEP; and (3) a pot experiment growing perennial ryegrass in the previously incubated soils. At 21°C, WEP declined more rapidly in TSP-fertilized soils than at 7°C, consistent with increased P sorption, which could be replicated by sorption models. In contrast, WEP increased over time in the ash treatment, indicating slow but continuous P release. Sewage sludge treatments showed variable patterns; SS had higher WEP at 21°C, but models failed to replicate measured trends, suggesting different release mechanisms compared to TSP. In the third study, plant growth and P uptake in the biochar-treated soil was similar to that in the unfertilized control, reflecting biochar’s low P solubility. In contrast, all sludge treatments resulted in higher P uptake than the control, and plant growth was comparable to that achieved with TSP, highlighting the potential of sewage sludge as an effective P fertilizer. However, the mechanisms governing P availability from these materials differ from those of soluble P sources.