Christina Tonitto

Application of the DNDC model to tile-drained Illinois agroecosystems: Model comparison of conventional and diversified rotations

Using the DeNitrification–DeComposition (DNDC) model we compare conventional, fertilizer-driven corn–soybean rotations to alternative management scenarios which include the management of cereal rye cover crops and corn–soybean–wheat–red clover rotations. We conduct our analysis for tile-drained, silty clay loam soils of Illinois. DNDC simulations suggest that, relative to conventional rotations, a nitrate leaching reduction of 30–50% under corn and of 15–50% under soybean crops can be achieved with diversified rotations, an outcome which corroborates results from a quantitative literature review we previously conducted using a meta-analysis framework. Additionally, over a 10-year simulation, legume-fertilized systems are predicted to result in 52% lower N2O gas flux relative to fertilizer-driven systems. We identify soil organic carbon storage, legume N-fixation rate, and cereal rye cover crop growth as areas requiring further development to accurately apply DNDC to diversified cropping systems. Overall, DNDC simulation suggests diversified rotations that alternate winter and summer annuals have the potential to dramatically increase N retention in agroecosystems.

Carbon and energy fluxes in cropland ecosystems: a model-data comparison

Croplands are highly productive ecosystems that contribute to land–atmosphere exchange of carbon, energy, and water during their short growing seasons. We evaluated and compared net ecosystem exchange (NEE), latent heat flux (LE), and sensible heat flux (H) simulated by a suite of ecosystem models at five agricultural eddy covariance flux tower sites in the central United States as part of the North American Carbon Program Site Synthesis project. Most of the models overestimated H and underestimated LE during the growing season, leading to overall higher Bowen ratios compared to the observations. Most models systematically under predicted NEE, especially at rain-fed sites. Certain crop-specific models that were developed considering the high productivity and associated physiological changes in specific crops better predicted the NEE and LE at both rain-fed and irrigated sites. Models with specific parameterization for different crops better simulated the inter-annual variability of NEE for maize-soybean rotation compared to those models with a single generic crop type. Stratification according to basic model formulation and phenological methodology did not explain significant variation in model performance across these sites and crops. The under prediction of NEE and LE and over prediction of H by most of the models suggests that models developed and parameterized for natural ecosystems cannot accurately predict the more robust physiology of highly bred and intensively managed crop ecosystems. When coupled in Earth System Models, it is likely that the excessive physiological stress simulated in many land surface component models leads to overestimation of temperature and atmospheric boundary layer depth, and underestimation of humidity and CO2 seasonal uptake over agricultural regions.

Planning and implementing small dam removals: lessons learned from dam removals across the eastern United States

We review and build on a growing literature assessing small dam removal outcomes to inform future dam removal planning. Small dams that have exceeded their expected duration of operation and are no longer being maintained are at risk of breach. The past two decades have seen a number of small dam removals, though many removals remain unstudied and poorly documented. We summarize socio-economic and biophysical lessons learned during the past two decades of accelerated activity regarding small dam removals throughout the United States. We present frameworks for planning and implementing removals developed by interdisciplinary engagement. Toward the goal of achieving thorough dam removal planning, we present outcomes from well-documented small dam removals covering ecological, chemical, and physical change in rivers post-dam removal, including field observation and modeling methodologies. Guiding principles of a dam removal process should include: (1) stakeholder engagement to navigate the complexity of watershed landuse, (2) an impacts assessment to inform the planning process, (3) pre- and post-dam removal observations of ecological, chemical and physical properties, (4) the expectation that there are short- and long-term ecological dynamics with population recovery depending on whether dam impacts were largely related to dispersion or to habitat destruction, (5) an expectation that changes in watershed chemistry are dependent on sediment type, sediment transport and watershed landuse, and 6) rigorous assessment of physical changes resulting from dam removal, understanding that alteration in hydrologic flows, sediment transport, and channel evolution will shape ecological and chemical dynamics, and shape how stakeholders engage with the watershed.