Variations of bioavailable Sr concentration and 87Sr/86Sr ratio in boreal forest ecosystems

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 ⁸⁷Sr/⁸⁶Sr and ¹⁸O/¹⁶O 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 ⁸⁷Sr/⁸⁶Sr ratio. The ¹⁸O/¹⁶O ratio was also determined in xylem sap and soil solutions in relation to depth. Soil solution ¹⁸O/¹⁶O decreased in relation to depth. Comparing with xylem sap ¹⁸O/¹⁶O 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 ⁸⁷Sr/⁸⁶Sr ratio of bioavailable Sr generally increased in soils in relation to depth. Contrastingly, the ⁸⁷Sr/⁸⁶Sr ratio in spruce wood was generally higher than in pine wood suggesting a deeper uptake of Sr by spruce. But the ⁸⁷Sr/⁸⁶Sr 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 ⁸⁷Sr/⁸⁶Sr 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 ⁸⁷Sr/⁸⁶Sr 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.     

A comparison between Ca and Sr cycling in forest ecosystems

In favourable conditions, the ⁸⁷Sr/⁸⁶Sr isotope ratios of the Sr delivered by rain and soil mineral weathering differ. Assuming that Ca and Sr behave similarly in forest ecosystems, several authors have used the ⁸⁷Sr/⁸⁶Sr variation in forest compartments to calculate the contribution of rain and mineral weathering to Ca fluxes and pools. However, there are a number of experimental reports showing that Ca and Sr may behave differently in the soil and in the plant. We have tested this Ca–Sr analogy in the field by measuring the variation of Sr and Ca concentrations, fluxes and pools in spruce, beech and maple stands on granite, sandstone and limestone. Results show that (1) variations of Ca and Sr concentrations are generally correlated at each level of the ecosystems. (2) In spruce on acid soils, a preferential uptake of Ca over Sr occurs (Aubure spruce Sr/Ca = 0.8×10⁻³; soil exchangeable Sr/Ca between 2 and 6×10⁻³). On calcareous soils, a preferential uptake of Sr over Ca by spruce may occur. (3) In spruce and beech on acid and calcareous soils, a preferential translocation of Ca over Sr from roots to leaves occurs ((Sr/Ca) in leaves was between 10 and 90% of that in roots). (4) The biological cycling of Ca and Sr leads to an enrichment of the upper soil layers in Ca and Sr. Compared to Sr, Ca accumulates in the upper layer of acid soils because Ca cycling through litterfall is favoured over Sr cycling, and possibly because of the selectivity of acid organic exchangers for Ca. This revised version was published online in June 2006 with corrections to the Cover Date.     

Do deep tree roots provide nutrients to the tropical rainforest?

The contribution of deep tree roots to the nutrition of a tropicalrainforest were studied along an edaphic transect in French Guyana. Soil typeswere mapped in relation to the texture of the upper horizons and the depth ofoccurrence of the loamy saprolite. The position of mature individuals of fourcommon species, differing by they rooting depth, was identified and tree leaveswere analysed for major nutrients and strontium (Sr) isotopic ratios.On average, the range of leaf isotopic ratio (⁸⁷Sr/⁸⁶Sr= 0.714–0.716) was narrow compared to that of bulk soils(⁸⁷Sr/⁸⁶Sr = 0.72–0.77). Steep gradients ofincreasing ⁸⁷Sr/⁸⁶Sr in roots with soil depth were foundin all investigated profiles, which indicated that the flux of Sr deposited inrain and leached from the litter layer was tightly retained in the upper soillayers. Over the whole of the site, as well as within each soil unit, tree⁸⁷Sr/⁸⁶Sr ratios were very similar whatever the species,and close to litter and near-surface roots ⁸⁷Sr/⁸⁶Srratios, suggesting no or very little Sr contribution from deep tree roots.Variations of Ca and Sr concentrations in leaves were strongly correlated butnot with leaf ⁸⁷Sr/⁸⁶Sr ratios. These results support thetheory that Sr and Ca uptake and cycling are mostly superficial in tropicalrainforests.