The method controls the story - Sampling method impacts on the detection of pore-water nitrogen concentrations in streambeds

Este artículo contiene 19 páginas, 5 tablas, 5 figuras.
Biogeochemical gradients in streambeds are steep and can vary over short distances oftenmaking adequate characterisation of sediment biogeochemical processes challenging. This paper provides an overviewand comparison of streambed pore-water samplingmethods, highlighting their capacity to address gaps in our understanding of streambed biogeochemical processes. Thiswork reviews and critiques available pore-water sampling techniques to characterise streambed biogeochemical conditions, including their characteristic spatial and temporal resolutions, and associated advantages and limitations. A field study comparing three commonly-used pore-water sampling techniques (multilevel mini-piezometers,miniature drivepoint samplers and diffusive equilibriumin thinfilm gels) was conducted to assess differences in observed nitrate and ammonium concentration profiles. Porewater nitrate concentrations did not differ significantly between sampling methods (p-value = 0.54) with mean concentrations of 2.53, 4.08 and 4.02 mg l−1 observed with the multilevel mini-piezometers, miniature drivepoint samplers and diffusive equilibrium in thin-film gel samplers, respectively. Pore-water ammonium concentrations, however, were significantly higher in pore-water extracted by multilevel mini-piezometers (3.83 mg l−1) and significantly lower where sampled with miniature drivepoint samplers (1.05 mg l−1, pvalues b0.01). Differences in observed pore-water ammonium concentration profiles between active (suction: multilevel mini-piezometers) and passive (equilibrium; diffusive equilibrium in thin-film gels) samplers were further explored under laboratory conditions. Measured pore-water ammonium concentrations were significantly greater when sampled by diffusive equilibrium in thin-film gels than with multilevel mini-piezometers (all p-values ≤0.02). The findings of this study have critical implications for the interpretation of field-based research on hyporheic zone biogeochemical cycling and highlight the need for more systematic testing of sampling protocols. For the first time, the impact of different active and passive pore-water sampling methods is addressed systematically here, highlighting to what degree the choice of pore-water sampling methods affects research outcomes, with relevance for the interpretation of previously published work as well as future studies.
This research was funded by The Leverhulme Trust project “Where rivers, groundwater and disciplines meet: A hyporheic research network” and from the authors' institutions. Additional funding was also provided from NERC through a Central England NERC Training Alliance Studentship, NERC standard grant NE/L004437/1, and the European Union's H2020-MSCA-RISE-2016 project 734317. Jay Zarnetske and Joseph Lee-Cullin were partially supported by the US NSF Award Number 1446328.