How hydrological connectivity controls sediment phosphorus release in a river–floodplain system

River–floodplain systems are multifunctional and hydrologically dynamic systems that provide key ecosystem services, including water storage and nutrient retention. Albeit reduced phosphorus (P) inputs to freshwater systems, eutrophication remains widespread. In shallow systems such as floodplains, sediment P released through microbial mineralisation can sustain high nutrient concentrations in water. Lateral hydrological connectivity further shapes sediment nutrient fluxes and microbial processes by changing biogeochemical conditions. However, the mechanistic pathways linking hydrological dynamics to sediment P release remain insufficiently understood.

Here, we synthesise findings from three complementary studies combining field campaigns along a hydrological river-floodplain gradient with experimental drought–rewetting incubations. We propose a framework in which hydrological connectivity functions as the ultimate driver, regulating microbial activity and organic matter quality, which in turn act as proximate drivers of sediment P release.

Across a hydrological gradient in a floodplain of the German Elbe River, we find that hydrological connectivity between floodplain water bodies and the main river consistently mediates sediment P release. Field measurements during floodplain connection and retraction phases revealed spatially distinct dynamics, with P release increasing progressively along the hydrological gradient during retraction. This pattern coincided with enhanced sediment phosphatase enzyme activity and organic matter concentrations. An experimental drought-rewetting incubation further showed that short-term drought modifies microbial controls on sediment P release but exerts weaker effects than long-term hydrological connectivity. Moreover, we observed P release under oxic conditions, which was linked to heterotrophic microbial carbon use and humic-like dissolved organic matter.

Our findings collectively suggest that P fluxes are shaped by hydrologically mediated shifts in microbial organic matter decomposition, with hydrological connectivity possibly defining the boundary conditions under which microbial processes operate. Ultimately, hydrological connectivity should be integrated into river–floodplain research for its simultaneous effects on phosphorus transport and turnover.