Combining microdialysis with metabolomics to characterize the in situ composition of dissolved organic compounds in boreal forest soil
Randewig D, Marshall JD, Nasholm T, Jamtgard S

Low-molecular weight organic compounds (LMWOC) play a key role in soil respiration. Thus, a detailed understanding of their dynamics is important for any attempt to describe carbon cycling of ecosystems. Measurements of LMWOC have been compromised by the perturbation associated with traditional soil extraction methods. Due to the fast turnover of LMWOC, there are potential losses of compounds caused by ex situ methods. These losses should be reduced by microdialysis, a novel soil sampling technique that samples the delivery of compounds through induced diffusive flux rates (DFR), representing what an active organism is exposed to in situ, in the undisturbed soil. The present study used microdialysis to measure the DFR of LMWOC in the O-horizon of a boreal forest soil in northern Sweden at four occasions during the middle to end of the growing season. LMWOC was analyzed by untargeted metabolomic profiling, which identified 50 compounds, and targeted mass spectrometry, which quantified seasonal shifts in the abundance of 36 amino acids, organic acids and sugars. The amino acid proportion of the DFR decreased by 85% during the growing season. However, because the decrease in amino acids was compensated for by increases in organic acid and sugar fluxes, the total DFR remained approximately constant during the season. The organic acid pool was dominated by lactate (80%) in the middle of the season and consisted mainly of oxalate (40%) during the later season. A lower amount and more diverse composition of sugars were found compared to previous studies. Six sugars and polyols were detected, with mannitol, arabitol and glucose being the most abundant. Microdialysis has previously been explored to monitor amino acids and inorganic compounds, but the present research demonstrates the potential of the in situ microdialysis sampling when combined with metabolomics. This broadens the application of microdialysis to describing quantitative and qualitative changes in the dynamic composition of the DFRs in undisturbed soil and a similar approach may be used in future studies to study the actual soil processes governing soil LMWOC.

e-link to publication