The Molecular Composition of Dissolved Organic Matter in Forest Soils as a Function of pH and Temperature

<div><p>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-ICR-MS 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.</p></div

H/C and O/C data for the different groups of molecular formulae exhibited clear trends with pH and DOC concentration.

<p>The data for each group of formulae are summarized as centroid data in the van Krevelen diagram. The number of formulae per group is represented by the scaled size of the symbols and given in brackets in the legend.</p

Ordination plots from PCA, based on all detected molecular formulae and their normalized FT-ICR-MS signal intensities.

<p>DOC concentration and pH were not used for PCA, but plotted as supplementary variables. Variability explained: PC1, 32.9%; PC2, 15.7%; PC3, 12.5%. (a) Plot of first and second axes, (b) plot of first and third axes.</p

Overview of the different groups of molecular formulae that correlated significantly in different combinations with DOC concentration and pH.

<p>Numbers of molecular formulae in each group are in brackets.</p

H/C data for unique molecular formulae for March, May and November subdivided into nitrogen-free and nitrogen-containing formulae presented as boxplots.

<p>H/C data for unique molecular formulae for March, May and November subdivided into nitrogen-free and nitrogen-containing formulae presented as boxplots.</p

Species diversity diagram based on RDA with molecular formulae as responsive variables and pH, DOC concentration and length of growing season as explanatory variables.

<p>Variability explained: Axis 1, 30%; Axis 2, 8.6%. DOC concentration, pH and length of growing season together explain 42% of the variability in formulae. The greater the circle diameter, the greater the number of molecular formulae per measurement (range of formulae per measurement: 1062 to 1334).</p

Boxplots representing the characteristics of the different groups of molecular formulae.

<p>The data are standardized to the maximum value of each parameter. Pairs with equal medians are marked *.</p

Overview of abundance of molecular formulae per month and main site.

<p>(a) Average abundance of formulae, (b) average relative abundance of nitrogen-containing compounds. Error bars indicate standard deviation between individual sampling spots or replicate measurements.</p

Median, quartiles Q1 (25^th percentile) and Q3 (75^th percentile) and minimum and maximum values of various molecular parameters.

<p>Data are shown for those molecular formulae that correlated negatively and positively with pH in the soil pore waters.</p><p>Median, quartiles Q1 (25^th percentile) and Q3 (75^th percentile) and minimum and maximum values of various molecular parameters.</p