MCA: Crossing Disciplines to Link Nutrients in Tree Canopy, Soils, and Stream Water to Mineral Weathering in a Tropical Forest

University Of Maine, Orono ME

Investigators

Abstract

Although forested tropical watersheds cover just 13 percent of Earth’s ice-free land mass, they contain approximately half of the carbon that is stored in plants on Earth’s surface, making them disproportionately important in storing carbon. Increased carbon dioxide in the atmosphere has been shown to increase plant biomass, thereby removing carbon dioxide from the atmosphere and sequestering it in the biosphere. However, plant growth can be limited by nutrient limitation, preventing plants from converting higher carbon dioxide levels into biomass through photosynthesis. One way to increase nutrient availability to ecosystems is to mobilize nutrients from bedrock through weathering, or the breaking down of rocks and minerals at Earth’s surface. This research activity will directly address these issues, while contributing to the career development of a mid-career researcher. Increased atmospheric CO2 can increase photosynthesis, but other factors, including nutrient limitation, drought, and changes to carbon dynamics in soils may affect tropical forests’ abilities to take up CO2, thereby turning tropical forests from a carbon sink to a carbon source. This study will address one potential limiting factor (nutrient limitation) and assess whether increased weathering can provide bedrock-derived nutrients at a fast enough rate to affect forest productivity. The researchers will test the hypothesis “Do plants growing on nutrient-poor serpentinite rely on rapid mineral weathering to offset the low nutrient content of the bedrock?,” at four sampling sites at two NEON observatories in southwestern Puerto Rico. Sampling locations across a climate gradient at Rio Cupeyes will test the effect of increasing temperature and rainfall on nutrient acquisition via chemical weathering in a low nutrient ecosystem on serpentinite bedrock. The fourth site at Rio Guilarte will serve as a control as well as allow predictions about bedrock chemistry and nutrient release. The researchers will address this hypothesis through three tasks: (i) measure the nutrient, major element, and trace element chemistry of all relevant reservoirs including bedrock, soil, soil water, leaves, leaf litter, atmospheric deposition, and stream water at three sites along a climate gradient at Rio Cupeyes as well as a control site at Rio Guilarte; (ii) quantify chemical weathering rates at Rio Cupeyes and Rio Guilarte using soil pits, soil water samples, and total export via streams; and (iii) model total aboveground biomass at all three Rio Cupeyes sites and one Rio Guilarte site using remote sensing imagery and compare these across the climate gradient to test the effect of increasing temperature and precipitation on ecosystem productivity. By calculating nutrient release via weathering using whole ecosystem fluxes calculated from stream chemistry and time-integrated, long term weathering profiles from forest soils, the researchers will compare nutrient release and chemical weathering occurring over two temporal and spatial scales. These weathering models will then be compared to landscape productivity estimates from remote sensing data to examine whether weathering rate is correlated to biomass production by trees. This project is jointly funded by the Geobiology and Low-Temperature Geochemistry Program in the Division of Earth Sciences and the Established Program to Stimulate Competitive Research (EPSCoR). This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.