Isotopic compositions of S, N and C in soils and vegetation of three forest types in Québec, Canada

The concentrations and the isotopic compositions of S, N and C were studied in soils and in the dominant plant species of three forested watersheds (Québec, Canada) located along a latitudinal and atmospheric deposition gradient. Large increases in S, N and C isotope ratios (up to 3.9‰, 10‰, 2.6‰, respectively) were observed with increasing soil depth at the three watersheds. These increases were accompanied by a strong decrease in elemental concentrations resulting in a strong negative relationship between these two variables. Both S and N concentrations throughout the soil profile and δ34S and δ15N in the mineral soil appeared to increase with increasing S and N deposition rates and decreasing latitude. A strong positive linear relationship was found between δ34S and δ15N (R2 = 0.72) values and between organic S and N concentrations (R2 = 0.96) in soils. The slope of the linear relationship between δ34S and δ15N (δ34S = f(δ15N)) indicated that isotopic fractionation was almost 4 times higher for S than for N during transformations that occurred in soil. However, this difference might reflect a higher degree of openness of the S cycle compared to the N cycle rather than an isotope effect per se. Overall, the results suggest that N and S inputs significantly impact the isotope ratios and the concentrations of N and S in the soils, and that S and N were closely associated and subject to similar processes with the same isotopic effects throughout the soil profile. Contrary to most studies, δ34S-SO4 in stream water of the most northerly site with the lowest S deposition rate was significantly higher than δ34S-SO4 in atmospheric depositions but similar to the δ34S of the bulk mineral soil. It suggests that the mineral soil actually contributes a large portion of the stream S-SO4 for this site.