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

• Published in: Applied geochemistry Vol. 26; no. 12; pp. 2181 - 2190
• Main Authors: Marty, C., Houle, D., Gagnon, C., Duchesne, L.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01.12.2011; Elsevier
• Subjects: atmospheric deposition; Atmospherics; carbon; Deposition; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; forest types; forested watersheds; Geochemistry; isotope fractionation; Isotope ratios; isotopes; latitude; mineral soils; Minerals; nitrogen; nitrogen content; Pollution, environment geology; Quebec; soil depth; soil profiles; Soils; Streams; sulfur; Transformations; Watersheds; Quebec; Canada; Eastern Canada; Canada, Quebec; fractionation; isotope ratios; streams; latitude; vegetation; concentration; cycles; slopes; drainage divide; North America; atmospheric deposits; forests; drainage basins; depth; isotopes; soil profiles; transformations; soils
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2011.08.002

► Isotopic compositions of S, N and C in the soil and the vegetation of three boreal forests were studied. ► They increased with soil depth and decreased with latitude and atmospheric inputs. ► Isotopic fractionation in ecosystem processes was higher for S than for N. ► Almost all S in the stream of the site with the lowest atmospheric inputs originated from the mineral soil. 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 δ 34S and δ 15N 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 δ 34S and δ 15N ( R 2 = 0.72) values and between organic S and N concentrations ( R 2 = 0.96) in soils. The slope of the linear relationship between δ 34S and δ 15N ( δ 34S = f( δ 15N)) 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, δ 34S-SO 4 in stream water of the most northerly site with the lowest S deposition rate was significantly higher than δ 34S-SO 4 in atmospheric depositions but similar to the δ 34S of the bulk mineral soil. It suggests that the mineral soil actually contributes a large portion of the stream S-SO 4 for this site.