Although nitrogen is an essential building block of life, in excess nitrogen is a major factor in the degradation of water quality and climate system stability, with important implications for ecosystem and human health. Excess nitrogen in water can contribute to algal blooms and “dead zones” in streams, rivers, and coastal waters, while emissions of nitrous oxide (N2O) – a potent greenhouse gas with warming potential 300 times higher than carbon dioxide – exacerbate climate change and destroy the stratospheric ozone layer. On the other hand, loss of nitrogen from agricultural landscapes represents a substantial economic loss for farmers.
Our research program examines the transport and transformation of reactive nitrogen and the underlying hydrological, biogeochemical, and management controls in hydrologic systems. These coupled processes and interactions are being investigated using novel stable isotope techniques and hydrological and transport modeling to determine how best to manage inputs of nitrogen to protect ecosystem and human health, water quality, and climate system stability.
Current Projects
Reactive Transport of Nitrogen in Lowland Agricultural Watersheds
While intensively managed watersheds are known to retain a high percentage of net N inputs (e.g., >70%), accurately quantifying the sources, fates, and transport of reactive nitrogen in heterogeneous watershed systems continues to be a conundrum due to the nonlinear interactions between hydrological and biogeochemical processes at the watershed scale.
In the intensively tile-drained Upper Embarras River watershed, east-central Illinois, we are combining StorAge Selection (SAS) functions, water and nitrate isotope analyses, and continuous nitrate sensing to examine the transport and transformation of nitrogen under variable hydrological regimes. This combination of novel analytical and modeling tools will allow for better characterization of flow generation pathways and elucidate how nitrogen cycling and export are controlled by watershed functions and climate variability across different scales.
The results from this research show that time-variant water age is a concise description of watershed hydrological transport, highlighting the dominant controls of a shallow groundwater table and subsurface tile drains on flow generation in this lowland watershed. Combining water age modeling with isotope analysis revealed coherent variations between discharge, water age, and nitrate isotopic imprints, which offer a novel lens through which to examine flow paths variations and their interactions with subsurface nitrate storage and reactivity.
We are currently extending the SAS approach by coupling it with biogeochemical and isotopic modeling to quantify nitrogen cycling rates and source/sink strengths at the watershed scale.
Temporal dynamics of and relationships between discharge, nitrate concentration, water age, and the oxygen isotopes of water and nitrate measured in the Upper Embarras River watershed, Illinois.
Biogeochemical and Agronomical Drivers of Nitrate Loss from Tile-Drained Row Crop Systems
Intensification of subsurface tile drainage has greatly increased the productive capacity of corn-soybean systems in the U.S. Midwest but has had unintended environmental consequences including degradation of soil and water resources. Our current work is combining isotope tracers with replicated field trials to better quantify the biogeochemical and agronomical drivers of nitrate loss from tile-drained fields.
This work demonstrates that the isotopic composition of nitrate – the major form of nitrogen being lost to aquatic systems – is a powerful tool for “fingerprinting” nitrogen sources and biogeochemical transformations that contribute to nitrate loss via tile drainage. Based on these isotopic fingerprints, we provide unequivocal evidence for the substantial loss of nitrogen fertilizers in tile drainage mediated by soil preferential flows (i.e., water flows through soil macropores associated with root channels, earthworm burrows, and desiccation cracks).
This research provides important new constraints for the sources and fate of nitrogen in tile-drained agricultural fields and provides a strong foundation for examination of tile nitrate loss in response to different management practices. For example, while existing nitrogen fertilizer management strategies place a strong focus on the timing, location, and amount of fertilizer applications, they typically do not consider how soil physical conditions modulate nitrogen use efficiency and nitrate loss potential in tile-drained systems.
Given this knowledge gap, our most recent research is using water isotopes as a hydrological tracer to constrain the effects of tillage systems on the development of soil macropores, as well as nutrient loss via preferential flow pathways. We have also initiated new projects to integrate isotope tracers into process-based agricultural system models (e.g., RZWQM2) to unravel the myriad factors that control coupled water transport, nutrient dynamics, and crop growth in tile-drained agroecosystems.
(Upper right) Conceptual schematic of hydrological flows at the tile-drain scale.
(Lower left) Long-term nitrate concentrations in tile drainage from measurement plots with different agricultural management. Shaded boxes denote corn growing seasons and triangles on the top axis indicate the timing of nitrogen fertilizer applications.
(Lower right) Nitrogen and oxygen isotopes of nitrate in tile drainage showing a salient denitrification trend and a mixed contribution of nitrogen fertilizer and soil organic nitrogen mineralization to nitrate loss from the tile-drained system.
Isotopic Fingerprinting of Indirect Nitrous Oxide Emissions in Agricultural River Network
Nitrous oxide (N2O) is a powerful greenhouse gas primarily sourced from agricultural activities. However, large divergence between bottom-up and top-down nitrous oxide estimates persists across heterogeneous agricultural landscapes. This is mainly due to the fact that nitrous oxide emissions are not restricted to agricultural soils (i.e., direct emissions), but arise also from volatilization, leaching, and erosion processes that re-concentrate reactive nitrogen in natural ecosystems downwind and downstream of agricultural activities (i.e., indirect emissions).
Driven by the hypothesis that nitrous oxide emitted from different sources has distinct isotopic signatures, we are measuring the isotopologues and isotopomers of N2O emissions in agricultural drainage network using novel nitrous oxide laser and mass spectrometry. These measurements show that the isotopic signature of stream-emitted N2O is within the range of soil and groundwater N2O isotopic signatures, suggesting that river N2O was a mixture of N2O from these two sources. Moreover, isotope-based N2O source partitioning revealed significant contributions of nitrification to stream N2O supersaturation and emissions, especially in watersheds with high percentage of tile-drained land use.
This highlights the important role of hydrologic connectivity in regulating the delivery of soil-produced N2O to river network.
(Upper right) Isotopic fingerprinting of riverine N2O emissions between two sampling seasons with different hydrological conditions (wet vs. dry).
(Lower right) Isotope-based N2O source partitioning showing the fraction of nitrification to N2O production and the percentage of complete denitrification that reduces N2O concentrations and emissions in the river network.
In collaboration with the Biometeorology Group at the University of Minnesota, we are combining N2O isotopic fingerprinting with isotope-aided atmospheric inverse modeling at a very tall tower within the US Corn Belt to quantify the magnitude, pathways, and climate sensitivity of N2O emission at the regional scale. Collectively, these novel measurements and modeling conducted across field to regional scales will contribute to a more robust quantification of the N2O inventory of the US Corn Belt.
They will also serve as a spatiotemporal bridge between the numerous extant surface data, atmospheric observations, and ice core and firn air records, with important implications for understanding the emerging feedback between agricultural N2O emissions and climate change.