Distinct export dynamics for dissolved and particulate phosphorus reveal independent transport mechanisms in an arable headwater catchment

Abstract

International audienceThis paper investigates particulate phosphorus (PP) and soluble reactive phosphorus (SRP) concentrations at the outlet of a small(5 km²) intensively farmed catchment to identify seasonal variability of sources and transport pathways for these two phosphorusforms. The shape and direction of discharge-concentration hystereses during floods were related to the hydrological conditions inthe catchment during four hydrological periods. Both during flood events and on an annual basis, contrasting export dynamicshighlighted a strong decoupling between SRP and PP export. During most flood events, discharge-concentration hystereses forPP were clockwise, indicating mobilization of a source located within or near the stream channel. Seasonal variability of PPexport was linked to the availability of stream sediments and the export capacity of the stream. In contrast, hysteresis shapes forSRP were anticlockwise, which suggests that SRP was transferred to the stream via subsurface flow. Groundwater rise in wetlandsoils was likely the cause of this transfer, through the hydrological connectivity it created between the stream and P-rich soilhorizons. SRP concentrations were the highest when the relative contribution of deep groundwater from the upland domain waslow compared with wetland groundwater. Hence, soils from non-fertilized riparian wetlands seemed to be the main source ofSRP in the catchment. This conceptual model of P transfer with distinct hydrological controls for PP and SRP was validthroughout the year, except during spring storm events, during which PP and SRP exports were synchronized as a consequenceof overland flow and erosion on hillslopes

Groundwater control of biogeochemical processes causing phosphorus release from riparian wetlands

“…This connection between MRP concentrations in RWs and streams completely explains stream observations made earlier by Dupas et al (2015) that periods of high MRP concentrations coincided with the first water table rise in RWs and periods of anoxic conditions in RW soils. This study also confirms that RWs can be major sources of MRP in agricultural watersheds through their enrichment in particulate P delivered from upland fields via erosion followed by the solubilization-mobilization of this allochthonous P in water saturated conditions (Roberts et al, 2012;Stutter et al, 2009).…”

“…The schist contains mainly quartz, muscovite and chlorite and, to a lesser extent, K-feldspar and plagioclase (Pauwels et al, 1998). The weathered schist is not likely to be a source of MRP in the watershed as mean MRP concentration measured below the soil depth (1 m) was 7 mg l À1 (unpublished results), which is lower than the long term mean baseflow concentration in the stream (18 mg l À1 , Dupas et al, 2015). Soils are silty loams, classified as Luvisols.…”

“…Because we were interested in subsurface transfer, we focused analyses on baseflow concentrations by discarding grab samples collected during storms (surface runoff may contribute to storm flow). We considered storms as events with >10% discharge rise and >20 l s À1 discharge (Dupas et al, 2015). All samples were filtered (<0.45 mm cellulose acetate filter) within 6 h after collection and kept refrigerated until analysis within 3 days.…”

Evolution of scientific questions over 50 years in the Kervidy-Naizin catchment: from catchment hydrology to integrated studies of biogeochemical cycles and agroecosystems in a rural landscape

“…For this purpose, around 200 soil samples were collected. Batch experiments were performed to analyze the effects of environmental drivers such as water content, temperature and redox potential on carbon and phosphorus release (Dupas et al, 2015a(Dupas et al, , 2015bGu et al, 2017;Humbert et al, 2015). New surveys were performed to examine antibiotics in manure, soil and water, considered as specific organic molecules.…”

New molecular evidence for surface and sub-surface soil erosion controls on the composition of stream DOM during storm events

“…It was previously air-dried and 2 mm sieved to remove the organic debris. The use of a 1 : 1000 soil : water ratio was based on the turbidity recorded in-stream at the outlet between 2007 and 2010, which rarely exceeded 1 g L −1 in storm flows (Dupas et al, 2015).…”