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 Watersheds
Intensively managed watersheds retain a large share of their net nitrogen inputs (often more than 70%), yet where that nitrogen comes from, where it goes, and how it moves remains hard to quantify. The difficulty lies in the nonlinear interactions between hydrological and biogeochemical processes at the watershed scale.
In the intensively tile-drained Upper Embarras River watershed in east-central Illinois, we combine continuous in-stream sensing, water and nitrate isotope analyses, and advanced transport modeling to examine how nitrogen is transported and transformed under variable weather and hydrological conditions. Together, these tools allow us to better characterize flow generation pathways and to understand how soil properties, watershed functions, and climate variability control nitrogen cycling and export across scales.
Our results show that water transit time (water age) offers a concise description of both watershed hydrological function and nitrogen biogeochemical cycling, and that a shallow groundwater table and subsurface tile drains are the dominant controls on flow generation in this lowland watershed. Combining water age modeling with isotope analysis revealed coherent variations among discharge, water age, and nitrate isotopic signatures. This offers a new lens for examining how shifting flow paths interact with subsurface nitrate storage and reactivity.
We are now incorporating coupled simulations of water and nitrate isotopes into conceptual and process-based models, with the goal of strengthening how water age and nitrogen transformation are represented from the field to the watershed scale.
Conceptual diagram illustrating the variable source zone concept explaining coupled flow generation and nitrate transport from an intensively tile-drained lowland watershed (see Sha et al. 2025 and Miller et al. 2026 for more details).
Biogeochemical and Agronomic Drivers of Nitrogen Loss from Row Crop Systems
Subsurface tile drainage has greatly increased the productive capacity of corn-soybean systems in the U.S. Midwest, but it has also had unintended environmental consequences, including degraded soil and water resources. We combine isotope tracers with replicated field trials to quantify the biogeochemical and agronomic drivers of nitrate loss from tile-drained agricultural fields.
The isotopic composition of nitrate, the major form of nitrogen lost to aquatic systems, is a powerful tool for “fingerprinting” the nitrogen sources and biogeochemical transformations behind hydrological nitrate loss. Using these fingerprints, we show that substantial amounts of fertilizer nitrogen are lost through drainage by way of soil preferential flow. Leveraging the strength of isotopes in differentiating nitrogen sources and pathways, we also developed complete nitrogen balances that link hydrological and gaseous losses to nitrogen inputs from fertilizer and soybean biological nitrogen fixation.
These results place new constraints on the sources and fate of nitrogen in row crop systems and provide a foundation for evaluating how management practices affect nitrogen loss. Current fertilizer strategies focus on the timing, placement, and rate of application, but they rarely consider how soil physical conditions shape nitrogen use efficiency and nitrogen loss potential. That is the gap our recent work addresses.
We now use water isotopes as hydrological tracers to examine how management systems influence macropore development and nutrient loss through preferential flow paths. We have also integrated nitrate isotope tracers into process-based agricultural system models (e.g., RZWQM2) to untangle the factors controlling coupled water transport, nutrient dynamics, and crop growth in agroecosystems.
(right) Nitrate source characterization in dual isotope space for tile drainage from a corn-soybean field. (left) isotope-based complete nitrogen balances for a tile drained corn-soybean rotation field in east-central Illinois (see Hu Y. et al. 2024 and Hu Y. et al. 2026 for more details)
Indirect Stream Nitrous Oxide Emissions across Local and Regional Scales
Nitrous oxide (N2O) is a powerful greenhouse gas that comes primarily from agricultural activities. Yet bottom-up and top-down estimates of N2O emissions still diverge widely across heterogeneous agricultural landscapes. One major reason is that emissions are not limited to agricultural soils (direct emissions). They also arise when volatilization, leaching, and erosion move reactive nitrogen to ecosystems downwind and downstream, where it re-concentrates and is converted to N2O (indirect emissions).
Driven by the hypothesis that nitrous oxide emitted from different sources has distinct isotopic signatures, we measure the isotopologues and isotopomers of N2O emissions in drainage network using novel nitrous oxide laser and mass spectrometry. These measurements showed that riverine N2O is a mixture of soil- and groundwater-derived N2O, and that nitrification contributes substantially to stream N2O supersaturation and emissions, especially in watersheds with a high percentage of agricultural land. The results highlight the role of hydrologic connectivity in delivering soil-produced N2O to river networks, a pathway that regional N2O budgets have largely overlooked.
To examine indirect emissions and hydrologic connectivity at larger scales, such as the contiguous U.S., we combine extensive field observations with machine learning frameworks. Our findings indicate that seasonal increases in hydrologic connectivity, such as late-winter thaws and postharvest rainfall, can raise river N2O emissions substantially relative to direct soil emissions.
In collaboration with the Biometeorology Group at the University of Minnesota, we combine N2O isotopic fingerprinting with isotope-aided atmospheric inverse modeling at a very tall tower in the U.S. Corn Belt. The goal is to quantify the magnitude, pathways, and climate sensitivity of regional N2O emissions and, together with our field-to-regional measurements and modeling, to produce a more robust N2O inventory for the Corn Belt and beyond.
Predicted spatial patterns of riverine N2O across the contiguous U.S. (see Hu M. et al. 2026 for details)