Hydroclimatic Controls on the Seasonal and Inter-Annual Variability of Dissolved Phosphorus Concentration in a Lowland Agricultural Catchment

Abstract:

We investigated soluble reactive phosphorus (SRP) at the outlet of a lowland agricultural catchment (Kervidy-Naizin, France) to identify the hydroclimatic controls on the seasonal and inter-annual variability in concentrations. Six years of stream data have been used, including a regular 6-daily sampling and high-frequency monitoring of 52 floods. Both on an annual basis and during flood events, distinct export dynamics for SRP and particulate phosphorus (PP) revealed that SRP transport mechanism was independent from PP (Dupas et al., submitted). During most flood events, discharge-SRP hystereses were anticlockwise, which suggests that SRP was transferred to the stream via subsurface flow. Groundwater rise in wetland soils was likely the cause of this transfer, through the hydrological connectivity it created between the stream and P-rich soil horizons. SRP concentrations were highest in the beginning of the hydrological year (period A), when the stream started to flow again after the dry summer season and water table fluctuated in the wetland domain. Thus, wetland soils seemed to be a major source of SRP. Concentrations during period A were higher after a long summer period than after a short one, which suggest that a pool of labile P was constituted in soils during the dry summer period. During winter (period B), SRP concentration generally decreased compared to period A, both during floods and interflood. This could be due to depletion of a soil P pool in the wetland domain and/or dilution by deep groundwater with low P concentration from the upland domain. Concentration during period B barely decreased compared to A during wet years, probably due to increased connectivity with soils from the upland domain in wet conditions. During spring (period C), SRP concentration increased during baseflow periods. The possible mechanisms causing the release of SRP could involve reduction of Fe oxide-hydroxides in wetland soils or in-stream processes. At the same time, SRP and PP export during floods were synchronised and caused by erosion and overland flow. In-situ monitoring of soil solution and batch experiments are currently been performed to confirm hypotheses on biogeochemical mechanisms.

Dupas R et al. Distinct export dynamics for dissolved and particulate phosphorus reveal independent transport mechanisms (subm.)