Variations of bioavailable Sr concentration and Sr-87/Sr-86 ratio in boreal forest ecosystems - Role of biocycling, mineral weathering and depth of root uptake

• The mean depth of Sr and water uptake in mixed Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) stands was investigated, using natural variations of Sr-87/Sr-86 and O-18/O-16 in soils in relation to depth. Three spruce-pine pairs were studied on a podzol and a peat site in Northern Sweden. Tree leaf and wood, as well as soils, soil solutions and roots below each tree were analysed for Sr and Ca concentrations and Sr-87/Sr-86 ratio. The O-18/O-16 ratio was also determined in xylem sap and soil solutions in relation to depth. Soil solution O-18/O-16 decreased in relation to depth. Comparing with xylem sap O-18/O-16 data indicated a deeper uptake of soil water by pine than spruce on the podzol site and a superficial uptake by both species on the peat. The Sr-87/Sr-86 ratio of bioavailable Sr generally increased in soils in relation to depth. Contrastingly, the Sr-87/Sr-86 ratio in spruce wood was generally higher than in pine wood suggesting a deeper uptake of Sr by spruce. But the Sr-87/Sr-86 ratio and concentrations of bioavailable Sr were systematically higher below spruce than below pine. In order to explain these unexpected results, we built a simple flux model to investigate the possible effects of interspecific variations in Sr cycling, soil mineral weathering and depth of Sr uptake on soil and tree Sr-87/Sr-86 ratio. At the study sites, spruce cycled in litterfall up to 12 times more strontium than pine. The use of the model showed that this difference in Sr cycling could alone explain higher isotopic signatures of trees and topsoils below spruce. Besides, high isotopic signatures of roots in the A/E horizons below spruce led us to hypothesise a species-specific weathering process. Finally, the comparison between the Sr-87/Sr-86 ratios in wood and root or soil solutions below each species suggested that the average depth of Sr and water uptake were close, but irregular variations of the Sr isotopic ratio with depth reduce the accuracy of the results. Tree species strongly influence Sr isotopic ratios in boreal forest soils through differences in Sr cycling, and possibly through specific mineral weathering.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Environmental Sciences (hsv//eng)

Nyckelord

biocycling

boreal forest

Ca

Picea abies

Pinus sylvestris

Sr isotopes

Sweden

uptake

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