Development of online microdialysis-mass spectrometry for continuous minimally invasive measurement of soil solution dynamics

The rate that amino acids are removed from the soil solution is poorly known but vitally important. It is possible to determine the time course of soil solution concentrations by extracting soils at different time points after adding labelled compounds, but this approach either lacks sufficient temporal resolution or generates large number of samples that require subsequent offline analysis. The aim of this study was to develop online microdialysis-mass spectrometry to enable the minimally invasive measurement of the time-course of isotope labelled amino acid added to soil. The method was subsequently tested by examining the fate of isotope labelled L- and d-alanine added to sterile and non-sterile soils. One concern with application of microdialysis to soil is if calibrations are affected by inorganic ion composition of the perfusate and the external (soil) solution. Tests showed that the presence/absence of inorganic ions in perfusate and external solution did not affect dialysate concentrations, suggesting that perfusing with an artificial soil solution matching the inorganic ion composition of the external solution does not convey any benefits. Hence water was used as perfusate for development of online microdialysis-mass spectrometry. The online system took around one minute to equilibrate to step-changes in concentration and had detection limits around 0.5 μmol L−1 for alanine. Addition of isotope labelled alanine to soils led to an almost instantaneous increase and subsequent decrease in dialysate alanine concentration. With sterile soils there was a slow abiotic decrease in dialysate concentrations, presumably due to development of a depletion shell around the microdialysis probe and adsorption of alanine to the soil. For non-sterile soils there was an additional more rapid biotic decrease in dialysate concentrations that presumably reflected microbial uptake. For l-alanine added to non-sterile soil much of the compound was taken up before it reached the probe surface and concentrations decreased to below detection limits within 5-20 min. Thus microdialysis afforded a graphic illustration of the ephemeral nature of intact l-alanine in non-sterile soil, while parallel measurements showed that added d-alanine was removed from soil solution several times more slowly.