Synchronous DOM and dissolved phosphorus release in riparian soil waters: linking water table fluctuations and biogeochemical processes

Abstract:

Riparian zones are often viewed as hot spots controlling N, C, P and Fe cycling and export in catchments. Groundwater and surface water flowpaths converge in these zones, and encounter the most reactive, organic-rich, uppermost soil horizons, while being at the same time zones in which soil moisture conditions temporarily fluctuate due to changes in water table depth, which can trigger biogeochemical processes. One well documented example is the process of denitrification which can remove N from riparian groundwater due to the anaerobic reduction of nitrate by soil organic matter. However, the role of riparian zones on the cycling of other nutrients such as dissolved organic matter (DOM) and dissolved P (DP) is much less well documented. In this study, we evaluated this role by using time series of DOM and DP concentrations obtained on the Kervidy-Naizin catchment, a temperate agricultural headwater catchment controlled by shallow groundwater. Over 2 years, groundwater DOM and DP were monitored fortnightly both in the riparian zones and at the bottom of hillslope domains. Two periods of synchronous DOM and DP release were evidenced, the first corresponding to the rise of the water table after the dry summer period, the second being concomitant of the installation of reducing conditions. The reductive dissolution of soil Fe oxyhydroxides initiated by the prolonged soil water saturation caused the second peak, a process which was, however, strongly temporarily and spatially variable at the catchment scale, being dependent on i) the local topographic slope and ii) the annual rainfall amount and frequency. As regard the first peak, it was due either to the flushing by the water table of DOM and DP accumulated during the summer period, or to the release of microbial DOM and DP due to microbial biomass killing by osmotic shock. This study argues for the existence of coupled and complex DOM and DP release processes in the riparian zones of shallow groundwater dominated catchments, and for the need of considering water table fluctuations as one of the key driving factor controlling the nature and magnitude of these processes. Important implications regarding DOM and DP transfer from soils to surface waters will be discussed.