Stormflow drives rock-derived nutrient loss from tropical forest ecosystem on terra firme fluvial terrace in Amazonia

Tropical fluvial terraces are intensely weathered environments as a result of high mean annual precipitation and temperature. Although it is well understood that climate controls ecosystem nutrient status, the impact of individual hydrologic events and fluid flow paths on ecosystem nutrient balance remains poorly resolved. Previous studies have shown that tropical environments exhibit vertical stratification of rock-derived nutrients: biogeochemical cycling concentrates elements near the surface. Tropical environments also commonly exhibit near-surface lateral flow paths due to high content of clay minerals, extensive plant roots and bioturbation. The combination of elevated nutrient concentrations in surface soils and the propensity for shallow, near-surface flow paths primes tropical fluvial terraces to exhibit nutrient loss during storms. Understanding the linkages between hydrology and ecosystem nutrient status is critical as climate change leads to an intensification of the hydrologic cycle, with more extreme events predicted. Here, we present the results of a three-year hydrochemical monitoring campaign (approximately twice monthly sampling for the duration of the study period and two multi-day high resolution sampling campaigns) on a tropical terra firme fluvial terrace in the Madre de Dios watershed in southern Peru. We use the stable isotope composition of oxygen in water to show that the watershed is characterized by two dominant flow regimes. During baseflow conditions, streamflow consists of relatively old water: less than 5% of river water is comprised of water that recently fell as rain (young water fraction sensu Kirchner 2016a, b). During storms, the percentage of streamflow consisting of recent precipitation ranges from 30-70%. Major element analyses show elevated concentrations of rock-derived nutrients during storms, suggesting storms play a key role in the export of these elements. The combination of stable isotope and major element data is strong evidence for storm water taking near-surface flow paths that leach key nutrients from the ecosystem. We also present a budget to quantify the total calcium losses associated with storms and compare this to estimates of bio-available calcium within the study watershed.