Calcium, strontium and barium biogeochemistry in a forested catchment and insight into elemental discrimination

Elemental ratios of calcium (Ca), strontium (Sr) and barium (Ba) in plant tissue have been used to help identify sources of Ca in trees. However, recent work has clearly shown that ratios of Ca:Sr or Ca:Ba in plant tissues differ from soil values and discrimination factors (alteration of elemental ratios of Ca, Sr and Ba) have been established to account for differences between soil and foliar values. In this study, concentrations of Ca, Sr and Ba (and molar ratios) in bulk deposition, throughfall, soil water and stream water were measured along with tree chemistry and biomass (foliage, branches, bark, wood, roots and litter) at a forested catchment in central Ontario, Canada. Concentrations of Ca, Sr and Ba vary greatly among the eight dominant tree species and plant tissue and molar ratios of Ca:Sr or Ca:Ba vary more than tenfold. In general Ca:Sr ratios were highest in foliage (up to 3,175) and lowest in wood (low as 91) and the mass-weighted Ca:Sr ratio of the total tree biomass was approximately 340. The molar ratio of Ca:Sr in bulk precipitation was similar to 820, increased to over 1,000 in throughfall and then decreased steadily through the various soil horizons, falling to 320 in the B horizon. Conservative mass balances for the forest floor for Ca, Sr and Ba suggest that more than half the Ca, Sr and Ba annual uptake by trees is obtained from mineral soil, which is consistent with the observation that the weighted-average ratios of Ca:Sr, Ca:Ba or Ba:Sr in the forest biomass fall between values in water draining the forest floor and water draining the rooting zone (B horizon). This work shows that discrimination between Ca, Sr and Ba is primarily internal to trees and discrimination factors between soil and foliage should be interpreted with caution.