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

Published in Applied Geochemistry 26: 2181-2190

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 d³⁴S and d¹⁵N 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 d³⁴S and d¹⁵N (R² = 0.72) values and between organic S and N concentrations (R2 = 0.96) in soils. The slope of the linear relationship between d³⁴S and d¹⁵N (d³⁴S = f(d¹⁵N)) 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, d³⁴S-SO₄ in stream water of the most northerly site with the lowest S deposition rate was significantly higher than d³⁴S-SO₄ in atmospheric depositions but similar to the d³⁴S of the bulk mineral soil. It suggests that the mineral soil actually contributes a large portion of the stream S-SO₄ for this site.

Forests

Scientific Article

Ecosystems and Environment, Forest Ecology, Forestry Research, Forests

Duchesne, Louis, Houle, Daniel

MARTY, C., Daniel HOULE, C. GAGNON and Louis DUCHESNE

2011

Paper

Available at the Direction de la recherche forestière

azote, soufre, carbone, forêt, écologie écosystèmes et environnement, isotope, écosystèmes et environnement, article scientifique de recherche forestière, ecosystems and environment, nitrogen, sulfur, carbon, forest