Changes to soil phosphorus cycling parameters after long-term treated wastewater irrigation in agroforestry lands of a humid region

In water-rich regions, treated wastewater (TWW) irrigation is promoted to align circular economy and climate adaptation goals, by recycling water and nutrients, but its sustainability depends on how soils and potentially downstream water bodies respond to long-term loading. In sensitive watersheds such as the Chesapeake Bay, TWW reuse can reduce direct effluent discharges to surface waters; however, it can also increase nutrient inputs to soils, where accumulation over time can elevate the risk of subsequent phosphorus (P) losses. This study evaluated the effects of >50 years of TWW irrigation on soil P dynamics, physicochemical properties, and microbial activity in forest and agricultural plots at the “Living Filter” site in the Susquehanna River watershed (Pennsylvania, USA). Compared with nonirrigated controls, TWW-irrigated plots showed higher pH, electrical conductivity, water-extractable P (WEP), microbial biomass P, and phosphatase activities, alongside shifts in soil organic C. Sequential fractionation and sorption–desorption experiments showed higher labile P (WEP, S/L-P, Ex-P), reduced sorption capacity, and greater P desorption, indicating a mobile legacy P pool, particularly in forest soils where WEP exceeded ranges associated with rapidly rising dissolved P losses. Although isotherm-derived degree of P saturation (DPS) for forest soils remained below 15–20 % thresholds and agricultural soils fell within this range, elevated WEP and labile P under humid conditions suggest that P losses could occur. Thus, DPS should be interpreted cautiously and within site-specific risk assessments. Future work on TWW irrigation in humid regions should resolve seasonal dynamics and transport of P along landscape positions, providing information needed to cautiously manage TWW irrigation in humid agroforestry systems.