Calcium and forest systems: Diffusion from deep sources

Input-output budgets of temperate forest systems frequently indicate calcium depletion with time. Anthropic disturbances such as acidic deposition and forest harvesting are presumed to accelerate this loss, yet few studies have shown significant change in ecosystem Ca, including vegetation, surface soil, and forest floor. We propose that diffusion of exchangeable Ca from deep zones of higher concentration associated with less-weathered or unweathered materials to near-surface zones of low Ca concentration can explain some of the enigmatic nutrient budgets. We used data from Walker Branch Watershed, Tennessee (WBW), to explore this hypothesis. Radiotracer data were used to determine a diffusion coefficient for Ca empirically. A numerical model using this coefficient and reasonable estimates of the source concentration and depth yielded both a vertical gradient of soil Ca and a rate of Ca movement to the surface that were consistent with published observations. Diffusion has not previously been recognized as important for ion movement at these larger scales of time (e.g., years) or space (e.g., meters). Although our data were from WBW, the mechanism is universal and we can extend our results to nearly all forest systems with low Ca or other ions in the surface soil and higher concentrations with depth.