Sediment phosphorus mobility in a semi‑arid wetland with highly fuctuating hydrology

Abstract

Background: Phosphorus (P) mobility and internal loading from sediments are critical processes driving eutrophica‑ tion in wetlands, particularly in those experiencing strong water level fuctuations. In such systems, redox oscillations, temperature variations, and microbial activity interact to infuence the bioavailability and release of sediment-bound P. However, the relative roles of abiotic and biotic mechanisms in P mobilization remain poorly understood, particu‑ larly in semi-arid wetlands afected by prolonged drought and intermittent fooding. Results: This study experimentally assessed the release of soluble reactive phosphorus (SRP) from sediments col‑ lected in Las Tablas de Daimiel National Park (central Spain), a semi-arid wetland with contrasting hydrological regimes. Sediments were incubated under both aerobic and anaerobic conditions across three air temperatures (4, 20, and 35 °C), with treatments isolating the efects of abiotic and biotic processes. We hypothesize that microbial processes, particularly those linked to Fe–P cycling, are key drivers of P mobilization in wetlands. Regression analy‑ ses confrmed a strong negative correlation between Fe:PO₄ ratios and SRP concentrations (R=−0.598, p<0.001), indicating a key role for iron redox dynamics in controlling internal P loading. Biotic+abiotic treatments exhibited consistently lower SRP release than abiotic-only controls, suggesting that microbial processes may suppress rather than enhance P mobilization. SRP release was highest in sediments exposed to recurrent anoxia and high Fe content, and lowest in intermittently dry sites where prolonged desiccation likely stabilized P in mineral phases. A structural equation model confrmed that Fe2 ⁺ was the dominant driver of SRP release, while Fe3 ⁺ and microbial activity had weaker, but signifcant, efects. Site-specifc interactions between dissolved oxygen availability and redox conditions were also critical in determining P fuxes. Conclusions: The fndings emphasize the dominant role of abiotic Fe–P coupling in wetland sediment dynamics, challenging the common assumption that microbial activity is the primary driver of phosphorus release. The study also highlights the infuence of temperature, redox conditions, and site-specifc hydrological regimes on SRP mobiliza‑ tion. While microcosm experiments provided mechanistic insights, their limitations underscore the need for comple‑ mentary feld-based studies. These results have direct implications for wetland restoration and eutrophication control. They also inform catchment-scale land use planning by highlighting the vulnerability of internal phosphorus release in semi-arid systems. In such environments, climate-driven hydrological variability may intensify internal nutrient loading.