IBL Tropical Forest Research Program Sources of Nutrient Inputs Controls over nutrient supply are key to understanding the structure and function of terrestrial ecosystems. Conceptual models once held that in situ mineral weathering was the primary long-term control over the availability of many plant nutrients, including the base cations calcium (Ca), magnesium (Mg), and potassium (K). Recent evidence has shown that atmospheric sources of these ‘‘rock-derived’’ nutrients can dominate actively cycling ecosystem pools, especially in systems on highly weathered soils. Such studies have relied heavily on the use of strontium isotopes as a proxy for base-cation cycling. Our recent work on Osa Peninsual forests, led by graduate student Carl Bern, showed that vegetation and soil-exchangeable pools of strontium in forests on on highly weathered soils are still dominated by local rock sources. This pattern exists despite substantial atmospheric inputs of Sr, Ca, K, and Mg, and despite nearly 100% depletion of these elements from the top 1 m of soil. We believe a dynamic landscape on the Osa, caused by both high rates of tectonic uplift and erosion, can maintain parent-material dominance of actively cycling Sr even in soils that are largely depleted in rock-derived nutrients. Recent data on sulfur isotopes also suggest that the balance between local weathering and atmospheric sources of nutrients in such forests is also likely to vary considerably among major nutrients; unlike Sr, atmospheric inputs appear to be the major source of actively cycling sulfur in Osa forests on upland Ultisols.