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 were labeled with 18 O-enriched inorganic P (P i ), followed by a 24-week incubation under different environmental conditions. The sediment material was incubated at three different hydrological regimes, short-pulsed logged (weekly cycles drying-rewetting), long-pulsed logged (biweekly cycles drying-rewetting), and permanently water-logged (consistently water saturated), and two different temperatures (20°C and 5°C). Samples were taken at specific times (24 h after labelling, early phase, intermediate phase, late phase). A five-step sequential extraction scheme was combined with δ 18 O analysis of P i (δ 18 OP i ) to follow changes in the operationally defined P pools. First, 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 along with decreasing alkalinity suggested cell death and reduced microbial activity, likely caused by hydrological stress. However, extracellular acid phosphatase activity remained stable. This suggests that while microbial populations were affected, their enzymatic functions persisted, revealing microbial dynamics and stress responses under drying-rewetting conditions. The δ 18 O Pi values in surface-adsorbed pools retained traces of the original P i label after 24 weeks, allowing tracking of long-term P transformations and distinguishing biological and geochemical reactions. Our results further reflected dynamic transformations of P i into organic P and vice-versa, which was largely influenced by the hydrological regime and raised the re-mobilization potential of P i in the long term. This relationship should be integrated into management strategies aimed at mitigating P losses from agricultural catchments, particularly under changing climatic conditions. Future studies should consider pronounced dynamics within P pools that can only be observed through close temporal monitoring, which may not be evident in simple before-and-after comparisons. • Dynamics in P pools of drainage-ditch sediments caused by drying-rewetting cycles • Unexpected role of organic P pools in the retention and permanent storage of P • Intense enzymatic turnover of P over weeks to months • Reduced retention of external P by repeated drying-rewetting over months • Increase in moderately labile P raising the P re-mobilization potential