Determination of soil exchangeable-cation loss and weathering rates using Sr isotopes

Abstract

To assess the response of forests to a changing chemical environment, a means is needed for separating the total cation export from the watershed into a component derived from mineral weathering reactions and a component due to the removal of exchangeable (plant-available) cations in the soil^1–3. We show that this separation may be possible by using ⁸⁷Sr/⁸⁶Sr ratios as a tracer of cation sources in stream water. Our measurements from a high-elevation forest ecosystem in the Adirondack mountains, New York, indicate that mineral weathering reactions contribute about 70% and soil cation-exchange reactions about 30% of annual strontium exports. Based on these results and the ratios of major cations to strontium in the local glacial till, we estimate the release of Ca²⁺, Mg²⁺, K⁺ and Na⁺ owing to weathering. The present weathering rate seems adequate to replace annual losses of cations from the total soil exchangeable pool, suggesting that the watershed is not in immediate danger of acidification from atmospheric deposition. But as our strontium isotope data indicate that 50–60% of the strontium in the organic-soil-horizon exchangeable and vegetation cation pools has an atmospheric origin, reduction of atmospheric cation inputs⁴ coupled with continued strong-acid anion inputs⁵ may result in significant depletion of this cation reservoir.