The transit of ³⁵SO₄^2- and ³H₂O added in situ to soil in a boreal coniferous forest

Published in Water, Air, and Soil Pollution: Focus 4: 501-516

A solution containing ³⁵SO₄^2- and ³H₂O was applied to four plots (5 x 5 m) in a boreal coniferous forest in the Laflamme Lake watershed, Québec, under two contrasting conditions: in summer (plots 1 and 2), and on the snowpack before snowmelt (plots 3 and 4). The transit of both these tracers in the soil solution was then followed through a network of soil lysimeters located at different depths. Four months after the summer application, ³H₂O had infiltrated the whole soil profile at plot 1, while ³⁵S0₄^2- was only observed in the LFH and Bhf horizons. A ³⁵S0₄^2- budget calculated from mid-August to November indicated that 89 and 10.6% of the added ³⁵S0₄^2- was retained within the LFH and the Bhf layers, respectively. Fifteen months later, the added ³⁵S0₄^2- was distributed in the following proportions within the soil horizons: LFH (73.7%), Bhf (11.8%) and Bf (12.8%), for a total retention rate of 98.3%. The superficial penetration of ³H₂0 at plot 2 was indicative of a major lateral water movement that prevented the calculation of a ³⁵S0₄^2- budget. This situation also was observed at plot 4 during snowmelt. At plot 3, ³H₂0 moved freely through the soil profile and a significant fraction of the added ³⁵SO₄^2- reached the B horizons, where it was presumably adsorbed on aluminum (Al) and ferric (Fe) oxides. The ³⁵S0₄^2- budget for plot 3 from March to November indicated that 87% of the added ³⁵S0₄^2- was retained within the soil profile, with most being retained in the B horizons (LFH = 33.1%, Bhf = 33.1%, Bf = 20.8%). The contrasting retention patterns of ³⁵S0₄^2- within the soil profile following the summer addition and snowmelt likely was caused by the contrasting soil temperatures and soil solution residence times within the different soil layers. The persistence of ³⁵S0₄^2- in the soil solution of the entire profile long after the initial tracer infiltration, and the relative temporal stability of specific activity of S0₄^2-, point to the establishment of an isotopic equilibrium between the added ³⁵S0₄ and the active S-containing reservoirs within a given soil horizon. Overall, the results clearly illustrate the very strong potential for ³⁵S0₄^2- retention and recycling in forest soils.

Forests

Scientific Article

Ecosystems and Environment, Forest Ecology, Forestry Research, Forests

HOULE, Daniel, Richard CARIGNAN and Jean ROBERGE

2004

Available at the Direction de la recherche forestière

pollution, soil, forêt boréale, résineux, lysimètre, activité microbienne, cycle du soufre, tritium, écologie écosystème et environnement, écosystème et environnement, ecosystems and environment, coniferous boreal forest, microbial immobilization, organics, sulfure cycling, sulphur retention, tritiated water