Strontium isotope constraint on the provenance of basic cations in soil water and stream water in the Kawakami volcanic watershed, central Japan

Positive correlations among the concentrations of Sr, Ca and Mg in soil water and stream water in a forested volcanic watershed at Kawakami, central Japan indicate that the Sr isotope can be used as a tool to elucidate the source and transport of the essential terrestrial ecosystem nutrients, Ca and Mg. The Sr-87/Sr-86 ratio in surface soil water varied widely along a 74 m slope; it was almost identical to the Sr-87/Sr-86 ratio of exchangeable soil and existing plants with different rooting zones (Japanese oak and bamboo grass), but differed significantly from that of soil minerals. In contrast, the Sr-87/Sr-86 ratios of stream water at baseflow periods were uniform regardless of location and time, and were indistinguishable from those of the weathered andesite bedrock. The Sr isotopic and elemental data indicate that Sr, Ca and possibly other cations (e.g. K, Mg) in the plant cycle efficiently with exchangeable soil through soil water and without a large supply of cations from secondary soil minerals, while cations in stream water are controlled mainly by exchange reactions with bedrock derived secondary minerals occurring in the lower soil horizon to water saturated zones. It is calculated that about 10% of Sr and Mg and 20% of Ca in stream water are derived from wet precipitation, while the contribution of atmospherically derived Sr and Ca in the exchangeable soil pool increases from 22 to 72% and from 42 to 87% respectively, with increasing distance from the valley. This increase of Sr and Ca in the exchangeable soil pool coincides with rock derived, low Ca materials dominating in the surface soil of the upper elevation in the slope. Copyright (C) 2001 John Wiley & Sons, Ltd.