In-situ sampling of soil pore water: evaluation of linear-type microdialysis probes and suction cups at varied moisture contents

Abstract

Environmental context. There is a need for slightly invasive techniques capable of in-situ probing of target analytes in environmental compartments. Owing to its passive sampling mode and small probe dimensions, microdialysis-based dosimetry is an appealing tool for monitoring of solute concentrations in both water bodies and pore soil waters with minimum disturbance of natural equilibrium. The development of field applications is challenging but will provide novel insights as to the speciation and bioaccessibility of environmental pollutants, e.g. trace metals, at high spatial resolution.Abstract. In-situ sampling of soil pore water is still a challenge for environmental scientists. Here, microdialysis is explored for probing metal concentrations in soil pore water at soil moisture contents ranging from 50 to 115% of the maximal water holding capacity and is compared with traditional sampling by suction cups. Metal concentrations obtained by the suction cup technique were consistently larger than those measured in the dialysate. Good agreement was obtained for Pb and Cu at soil moistures close to saturation after accounting for diffusion resistances whereas corrected Ni and Cd concentrations in the dialysates exceeded those measured by the suction cup technique. These deviations reflect inherent differences in the sampling mode and effects of soil heterogeneity at the microscale. Microdialysis offers new opportunities to probe solute concentrations at high spatial resolution and minimal disturbance of soil conditions at environmental interfaces such as the plant rhizosphere or at the transition between forest floors and the mineral soil.

Assessment of diffusive phosphate supply in soils by microdialysis

“…By this we can exclude major adsorption of phosphate in our experimental setup as well as an inappropriate molecular cut‐off of the PAES‐membrane. RR of phosphate in our experiments was between reported values for inorganic nitrogen ions ( Inselsbacher et al, ) and those expected for heavy metals ( Miró et al, ). However, this comparison has to be interpreted carefully, because experimental setups were not fully identical.…”

“…We observed a non‐linear decrease of RR with increasing flow rate. This occurs due to rate‐limited diffusive transport driven by a concentration gradient into the probe to obtain equilibrium between external medium and dialysate ( Sulyok et al, ; Miró et al, ; Inselsbacher et al, ). By this we can exclude major adsorption of phosphate in our experimental setup as well as an inappropriate molecular cut‐off of the PAES‐membrane.…”

“…The use of MD probes in soils and other semi-solid environmental media was first discussed by Miró and Frenzel (2004), emphasizing the possibility of continuous sampling with minimum soil disturbance. The use of MD in soils has targeted inorganic and organic nitrogen compounds (Inselsbacher et al, 2011;Inselsbacher and Nä sholm, 2012;Shaw et al, 2014), organic anions (Sulyok et al, 2005), and heavy metals (Miró et al, 2010). In nonmixed media, such as soils, and when the target solute is interacting with the matrix, the calibration of dialysate yields for equilibrium concentrations turned out to be critical (Peters et al, 2000;Sulyok et al, 2005;Inselsbacher et al, 2011).…”

A microdialysis perspective of soil nitrogen availability

“…and can be calculated by a linear regression of -ln(1-Ed) against 1/Qd. This has previously been used to estimate the efficiency of different membrane types on recovery (Torto et al, 1998, Buckley et al, 2017 and to predict the relative recovery of metal ions from solution (Miró et al, 2010).…”

“…Rext >> Rm >> Rd; that is, the environmental resistances to solute movement have a much greater influence than those imposed by the membrane and dialysate (Hsiao et al, 1990, Miró et al, 2010.…”