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Abstract:
We examined the molecular composition of forest soil water during three different seasons
at three different sites, using electrospray ionization Fourier transform ion cyclotron resonance
mass spectrometry (ESI-FT-ICR-MS). We examined oxic soils and tested the hypothesis
that pH and season correlate with the molecular composition of dissolved organic
matter (DOM).We used molecular formulae and their relative intensity from ESI-FT-ICRMS
for statistical analysis. Applying unconstrained and constrained ordination methods, we
observed that pH, dissolved organic carbon (DOC) concentration and season were the
main factors correlating with DOM molecular composition. This result is consistent with a
previous study where pH was a main driver of the molecular differences between DOM from
oxic rivers and anoxic bog systems in the Yenisei River catchment. At a higher pH, the molecular
formulae had a lower degree of unsaturation and oxygenation, lower molecular size
and a higher abundance of nitrogen-containing compounds. These characteristics suggest
a higher abundance of tannin connected to lower pH that possibly inhibited biological decomposition.
Higher biological activity at a higher pH might also be related to the higher
abundance of nitrogen-containing compounds. Comparing the seasons, we observed a decrease
in unsaturation, molecular diversity and the number of nitrogen-containing compounds
in the course of the year from March to November. Temperature possibly inhibited
biological degradation during winter, which could cause the accumulation of a more diverse
compound spectrum until the temperature increased again. Our findings suggest that the
molecular composition of DOM in soil pore waters is dynamic and a function of ecosystem activity, pH and temperature.
