How Sulfur Makes Its Way Into The Soil Of Forested Ecosystems

The term “acid rain” may sound like an old legend to most people. This environmental issue, which once seriously threatened the health of northeastern North-American forests, lakes, and rivers, has indeed partly been solved.

The Clean Air Act, signed in 1970, introduced stricter regulations of sulfur emissions (the principal element responsible for acid rain) which resulted in a rapid and substantial reduction of SO₂ emission (gaseous sulfur) and acidic precipitation on North-American terrestrial and aquatic ecosystems. Although stream and lake water quality has largely improved in the following decades in most ecosystems, the recovery has been slower in areas with naturally acidic soils. Moreover, forested watersheds are still releasing more sulfur in surface waters than they are receiving from atmospheric deposition despite the strong decrease in sulfur deposition.

The origin of this imbalance is unclear. This could come from an underestimation of gaseous deposition — which is harder to quantify than wet deposition — or from unknown sulfur sources, or even from an increased rate of release of sulfur from the soils. Several institutes in the north-east United States and Canada have been conducting research to better understand the fate of sulfur once deposited onto forest ecosystems. This work was pioneered by Pr. Gene Likens by establishing a long-term environmental monitoring program in 1963 at the Hubbard Brook experimental forest in New Hampshire. This work has allowed scientists to understand the mechanisms by which acid rains impact the health of forest and aquatic ecosystems, and to assess the efficiency of anthropogenic sulfur emission regulations.

The fate of sulfur in forest ecosystems

In the absence of sulfur in the bedrock (e.g., in minerals such as pyrite), the only source of sulfur in terrestrial ecosystems is atmospheric deposition, which occurs as “wet” (precipitation) and “dry” (gaseous) depositions. The main form of sulfur in precipitation is the sulfate ion (SO₄^2-), whereas gaseous sulfur deposition is mainly composed of sulfur dioxide (SO₂). As vegetation’s sulfur requirement is low, the immense majority of the sulfur in a forest is stored in the soil. For instance, only 2% of a boreal forest’s sulfur is located in the vegetation vs. 98% in the soil (90% of which is in the mineral soil beneath the humus layer)[1].

In the last few years, a team of researchers has conducted a series of studies in Canadian boreal and temperate forests to better understand the fate of sulfur in these ecosystems. They surveyed changes in the concentration and in the isotopic composition of the sulfate molecule (δ³⁴S-SO₄ and δ¹⁸O-SO₄) in precipitation, soil horizons, and stream water across seasons in boreal and temperate forests of eastern Canada. Until the ’90s, it was not clear whether sulfate exported from watersheds through streams had interacted with vegetation and soil components. Indeed, the isotopic composition of sulfur in the sulfate molecule (δ³⁴S-SO₄) does generally not vary between precipitation, soil, and stream waters, which has sometimes led to the false conclusion that sulfate does not interact with biotic components during its passage through the ecosystem. But analyses of the stable isotopic composition of the oxygen atoms of the sulfate molecule have revealed a different story (see box below).

These researchers have found that the amount of sulfur is much higher (~50%) in precipitation collected under forest canopies than in forest clearings. This enrichment in sulfur is accompanied by a strong decrease in δ¹⁸O-SO₄. This results from the dissolution and oxidation of gaseous sulfur (SO₂) at the surface of the canopy. The SO₄^2- ions produced through this reaction have typical low δ¹⁸O-SO₄ values and are then washed out from the canopy. This phenomenon is particularly significant in evergreen coniferous forests whose canopy has a large exchange surface with the atmosphere for a longer period of time. The surplus of sulfur in precipitation collected under the canopy as compared to in precipitation at clearings is an estimation of dry deposition in a forest. This amount depends on site locations and varies among vegetation types and seasons because the canopy can absorb a part of the dry deposition [2].

After its passage through the canopy, the precipitation reaches the forest floor and percolates through the soil horizons. Soil water can be collected from the soil by using lysimeters, cylindric porous apparatus positioned in the soil matrix and connected to containers with negative pressure. In all the surveyed forests, δ¹⁸O-SO₄ was lower in water collected by the lysimeters located beneath the humus layer than in precipitation collected under the canopy. This additional decrease in δ¹⁸O-SO₄ indicates that the incoming atmospheric sulfate is rapidly processed by soil microorganisms before being leached. This microbial processing corresponds to the incorporation of sulfate into organic matter followed by oxidation of this organic sulfur. In other terms, it reveals a production of “secondary” sulfate within the humus layer by soil microorganisms from organic sulfur and surrounding water and O₂.

No further decrease in δ¹⁸O-SO₄ is observed between soil water collected beneath the humus layer and stream water at the boreal forest sites, indicating that soil microorganisms do not significantly process sulfate deeper in the soil profile [3]. In contrast, δ¹⁸O-SO₄ of soil water collected 25 cm below the humus layer was even lower than in the humus at the surveyed temperate forest [4]. This is the evidence that, in contrast with boreal sites, soil microorganisms process sulfur deeper in the mineral soil in temperate forests than in boreal forests, probably due to better physico-chemical conditions (higher temperatures, better litter quality, etc.).

This phenomenon is further supported by seasonal variations in soil water δ¹⁸O-SO₄. The δ¹⁸O of sulfate in the soil solution is indeed higher in fall than in spring and summer. This results from the release of primary sulfate from snow cover in early spring, which is subsequently recycled by soil microorganisms during the growing season, resulting in a higher proportion of secondary sulfate in fall than in spring and summer.

These studies show that, in contrast with what was previously thought, atmospheric sulfur strongly interacts with forest canopies and soils before being released from forested watersheds. Their work shows that between 65% and 80% of the deposited sulfate in boreal forests and even more in temperate forests interacts with the canopy and the humus layer before leaving the catchments.

References:

1. Houle D, Carignan R. Sulfur speciation and distribution in soils and aboveground biomass of a boreal coniferous forest. Biogeochemistry [Internet]. 1992;16(1):63–82.
2. Marty C, Houle D, Duchesne L, Gagnon C. Canopy interaction with precipitation and sulphur deposition in two boreal forests of Quebec, Canada. Environ Pollut [Internet]. 2012;162:354–60.
3. Houle D, Marty C, Duchesne L, Gagnon C. Humus layer is the main locus of secondary SO4 production in boreal forests. Geochim Cosmochim Acta [Internet]. 2013
4. Marty C, Houle D, Duchesne L, Gagnon C. Evidence of secondary sulfate production in the mineral soil of a temperate forested catchment in southern Québec, Canada. Appl Geochemistry [Internet]. 2019;100:279–86.