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Humus layer is the main locus of secondary SO4 production in boreal forests

Summary

Published in Geochimica et Cosmochimica Acta 126: 18-29

Identifying the sources of S exported from catchments and the reactivity of the large soil organic S pool is crucial to understand the mid- or long-term response of forested catchments to decreasing atmospheric S deposition and global warming. Sulfur fluxes as well as S and O isotopes of SO4 were measured in precipitation, throughfall, soil solutions and streams at two boreal forest catchments respectively dominated by black spruce (BS) and balsam fir (BF) in Quebec, Canada. Overall, d34S-SO4 signature showed relatively small variations among various solution types. However, at both sites, d18O-SO4 in precipitation (averages of 10.5–11.1%o) was decreased by 3.5–3.6%o in throughfall because of the production of secondary SO4 through oxidation of SO2 deposited on the canopy. Throughfall d18O-SO4 was decreased by a further 5.4–6.6%o in the solution leaving the humus layer which was attributed to the production of secondary SO4 under the action of soil microorganisms through the oxidation of organic S during which the S atom acquired O from water and gaseous O2 present in the soil. A mixing equation based on known isotopic signature of each source suggested that  ~67–81% of the S-SO4 leaving the catchments had interacted with the canopy and the humus layer. The stability of d18O-SO4 in the mineral soil solution and in the stream of both sites, suggests that SO4 does not undergo  reduction–oxidation cycles after its passage through the humus layer. Despite its huge size, the organic S reservoir within the mineral soil would be largely inert. Given the chemical nature of SO4 transformation in the canopy, the humus layer would be responsible for nearly 100% of the biological production of secondary SO4 in the whole watershed at both sites. Taking into account the substantial production of dissolved organic S in the humus layer further emphasizes the crucial importance of the latter in the S cycling of boreal forests.

Sector(s): 

Forests

Categorie(s): 

Scientific Article

Theme(s): 

Ecosystems and Environment, Forestry Research, Forests

Departmental author(s): 

Duchesne, Louis, Houle, Daniel

Author(s)

HOULE, Daniel, Charles MARTY, Louis DUCHESNE and Christian GAGNON

Year of publication :

2014

Format :

Paper

Keywords :

pluies acides, sols forestiers, sulfates, fertilité, écosystèmes et environnement, humus, article scientifique de recherche forestière, ecosystems and environment, Acid rain, forest soils, sulphate, fertility

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