One of the major challenges facing large-scale delta restoration is confidence that the trajectory of delta growth of these newly emergent landscapes will form ecosystems as predicted in mathematical and physical models, with biogeochemical functions that reduce nutrient loading and promote carbon sequestration. A DELTA Observatory has been developed at the Wax Lake Delta (WLD) where active land building in the face of sea level rise and subsidence has occurred since 1973, providing an analog for engineered diversions of the Mississippi River, and an ideal natural laboratory to understand the ecogeomorphic evolution of growing delta lobes. Delta restoration requires calibrated predictive models for design and scenario analysis that are grounded in comprehensive field-based data sets. The DELTA LAB research observatory at WLD provides critical data to research community focusing on evolution and restoration of prograding delta environments, with experimental questions focused on plant community development, soil development, and nutrient biogeochemistry along chronosequence of delta formation. We predict shifts in N:P ratio follow both age, elevation and disturbance gradients driving by ratio of mineral sediment deposition and net primary production. Soil surveys, vegetation analysis, biogeochemical processes are sampled along chronosequence and elevation around flood events to test these hypotheses.
Results/Conclusions
We found that high peak discharge river floods in 2008 and 2011 resulted in the greatest net elevation gain of 5.4 to 4.9 cm over each flood season, respectively. Strong river flood events build elevation that controls colonization of forested wetlands, Salix nigra. Thus allogenic and autogenic controls on elevation change and vegetation dynamics establishes gradients in carbon and nitrogen sequestration, controlling N:P ratios. An extensive soil survey was conducted across different elevation and age gradients in delta islands using a Russian corer to a depth of 30 cm. There is a shift in carbon and nitrogen content, and N:P ratio, from < 4 to >8, at about 20 yr age class as function of increased NPP and decreased sediment deposition. We used a combination of 15N tracers with a modified continuous flow-through (FT) system to discover a clear shift in efficiency of nitrate uptake and direct denitrification with increased organic matter content in soils with increased age of delta development (increase from 50 to 400 µmols m-2 hr-1). These patterns will be summarized to describe how flood pulses, vegetation colonization, and carbon accumulation modify ecosystem properties with age of delta development.