Dynamics of phosphorus pools in drainage ditch sediments within an agricultural catchment

Hydrological dynamics in drainage ditch sediments may impact phosphorus (P) retention and its potential (re-)release to connected waters. We aimed at understanding the fate of P in drainage-ditch sediments under dynamic environmental conditions (i.e., drying-rewetting cycles) by examining P pool transformations through an isotopic labelling and incubation experiment. Fresh sediment samples of a drainage ditch were labeled with 18O-enriched inorganic P (Pi), followed by a twenty-four-week incubation under different environmental conditions. A five-step sequential extraction scheme was combined with δ18O analysis of Pi (δ18O Pi) to follow changes in the operationally defined P pools. Few P pools showed significant changes, but time-resolved results revealed a dynamic interplay between redox conditions, microbial activity, and P dynamics. A subsequent decline in microbial P concentrations suggested cell death and reduced microbial activity, likely caused by hydrological stress. The δ18OPi values in surface-adsorbed pools retained traces of the original Pi label after twenty-four weeks, allowing tracking of long-term P transformations and distinguishing biological and geochemical reactions.