About

Gary Feyereisen is an adjunct associate professor in the Department of Bioproducts and Biosystems Engineering at the University of Minnesota. His areas of interest include biogeochemistry, contaminant hydrology, and water quality, with a focus on crops. His work involves studying the interactions between biogeochemical processes and water systems, contributing to the understanding of water quality and environmental contamination related to agricultural and natural systems.

Research topics

• Biology
• Environmental science
• Engineering
• Environmental engineering
• Pulp and paper industry
• Ecology
• Chemistry
• Botany
• Agricultural economics
• Animal science
• Soil science
• Agricultural science
• Agronomy
• Geography
• Economics

Selected publications

• Modeling nitrogen fluxes in a tile-drained cropping system in the Midwest using an enhanced SWAT model

  Agricultural Water Management · 2026-02-17

  articleOpen access

  Artificial (tile) drainage systems are extensively implemented across the U.S. Midwest to enhance crop production in poorly drained soils; however, they also pose environmental challenges by significantly altering nitrogen fluxes within agricultural landscapes. In response, sustainable intensification strategies seek to increase agricultural productivity while reducing environmental impacts, often through improved management practices such as cover cropping and conservation tillage. Effectively evaluating the trade-offs and synergies of agricultural management practices demands advanced modeling tools capable of representing coupled biogeochemical and hydrological processes across diverse spatial and temporal scales. This study presents the first application of an enhanced version of the Soil and Water Assessment Tool (SWAT), integrated with Century/DayCent-based biogeochemical modules, to simulate both nitrate (NO3⁻) loss and nitrous oxide (N2O) fluxes in a tile-drained corn-soybean system. The model was applied to long-term field data (2004–2010) from an Iowa site with two treatments: with and without winter rye cover crops. With careful calibration, the model reproduced tile discharge and crop yields well and captured the direction and magnitude of cover-crop reductions in NO3⁻ losses. However, interannual variability in NO3⁻ export and event-scale N2O peaks remained difficult to reproduce, likely due to limited sampling frequency and structural constraints in soil hydrology, solute transport, and vertical resolution. The model simulated a ∼41 % reduction in NO3⁻ leaching with cover crops, close to the observed ∼50 %. In contrast, effects on average daily N2O flux varied by year and conditions, ranging from −30–67 % (observed: −24–28 %). These results support the model’s use for assessing long-term nitrogen-loss responses to cover crops in tile-drained systems, while highlighting priorities for improving event-scale biogeochemical simulations.

  PublisherDOI

• Rye performance in central Iowa under different seeding and nitrogen fertilizer rates

  Agronomy Journal · 2025-07-01 · 1 citations

  articleOpen access

  Abstract The inclusion of a harvested winter cereal before soybean [ Glycine max (L.) Merr.] can increase productivity and sustainability of a cereal‐soybean rotation. Still, literature disagrees on proper seeding and fertilizer rates of the winter cereal for efficient production of both crops in the rotation. This study aimed to evaluate rye ( Secale cereale L.) spring green ground cover, biomass production, and nitrogen (N) accumulation in a rye‐soybean rotation, using three seeding rates (20, 40, and 60 kg ha −1 ) and three N rates (0, 30, and 60 kg N ha −1 ) from 2021 to 2023 in central Iowa. Remote sensing was used to assess rye growth. Green ground cover was up to 42% higher in the highest seeding rate (60 kg ha −1 ). Conversely, rye biomass production was not affected by seeding or N rates and averaged 3.0 ± 1.1 and 4.9 ± 1.3 Mg ha −1 in the first and second growing seasons, respectively. Rye N accumulation increased 0.13 and 0.51 kg N ha −1 for each kg N applied in the first and second growing seasons, respectively. Soybean yield was similar among treatments but 19%–38% lower than the county average production, especially in the second growing season due to limited precipitation. This study provides new evidence that rye biomass production can be optimized with low inputs, whereas higher seeding rates increase spring green ground cover. However, because of reduced soybean yields, further studies are needed to evaluate rye harvest time and alternative soybean varieties to optimize soybean production after rye for central Iowa conditions.

  PublisherOA PDFDOI

• Combining soil conservation with phosphorus drawdown can confront legacy phosphorus accumulation and transfer

  Journal of Soil and Water Conservation · 2025-05-04 · 3 citations

  articleOpen access

  Legacy phosphorus (P) in agricultural soils (i.e., P that derives from historical human activities) can resist conventional nutrient management strategies to improve water quality (e.g., placement, rate, source, and timing of application). Further, soil conservation practices such as reduced tillage, while potentially beneficial for improving soil health and minimizing erosion, can promote dissolved P loss. Comprehensive legacy P management requires targeted mitigation strategies that consider the sources and processes involved in P mobilization and transport. We modeled trade-offs and interactions of nutrient management and soil conservation strategies in legacy P mitigation efforts at three key sites in the northern United States where legacy P contributions to water quality are a concern. The Annual Phosphorus Loss Estimator (APLE) model was used to simulate generalized management scenarios at each site: current site-specific practices, conventional conservation practices (no-till and manure injection), and P drawdown (curtailing fertilizer P additions and extracting P from soils via crop uptake and harvest). Modeled results highlight that the effects of legacy P are not always obvious; even at sites near the range of agronomic optimum, losses of legacy P in runoff can be significant. Phosphorus drawdown via crop uptake and removal offers the potential to deplete legacy P stores but requires dedication and time. In model simulations, no-till reduced total P losses due to reductions in sediment transport. Coupling drawdown strategies with appropriate conservation management to avoid inadvertent P losses can reduce both dissolved and particulate P losses. Focusing on either soil conservation or soil P drawdown alone is insufficient to meet water quality goals. Phosphorus drawdown strategies must be accompanied by practices supporting soil conservation to ensure that legacy P management benefits water quality in the short and long term.

  PublisherDOI

• Quantifying microplastics in environmental waters: Mass concentrations are superior to abundance

  Agricultural & Environmental Letters · 2025-10-07

  articleOpen access

  Abstract Microplastics are contaminants of global concern that are primarily studied in marine and urban environments. Understanding of microplastics in drained agricultural watersheds is lacking. We aimed to evaluate microplastics in ditch and tile drainage water through periodic sampling. Water samples were filtered to capture particulates that were digested to remove organics, then stained and evaluated using fluorescence microscopy and image analysis. Further, we compared and contrasted microplastic abundance, the current reporting standard, with microplastic mass concentration, often unreported, to determine the most accurate assessment. Open‐ditch drainage had greater microplastic contamination than drainpipe outlets. Agricultural drainage contained 2–6 orders of magnitude less mass concentrations of microplastics than sampled urban surface waters and laundry graywater. However, when evaluated by abundance, the difference was not apparent. These findings improve our understanding of microplastics in agricultural watersheds and demonstrate the importance of evaluating microplastic contamination based on mass concentrations for accurate assessments. Core Ideas Mass concentration (ng/L) is a better predictor of microplastic contamination than abundance (counts/L). Agricultural drainage water had lower microplastic mass concentrations than surface water or laundry graywater. Open‐ditch drainage had greater microplastic contamination than drainpipe outlets. Mass of microplastic pieces from smallest to largest: drainage water < river water < lake water = laundry water.

  PublisherDOI

• Nitrogen and phosphorus removal from agricultural drainage water by a modular bioreactor

  Journal of Environmental Management · 2025-06-09 · 2 citations

  article
  PublisherDOI

• Phosphorus lability across diverse agricultural contexts with legacy sources

  Journal of Environmental Quality · 2024-09-29 · 6 citations

  articleOpen access

  Abstract The buffering of phosphorus (P) in the landscape delays management outcomes for water quality. If stored in labile form (readily exchangeable and bioavailable), P may readily pollute waters. We studied labile P and its intensity for >600 soils and sediments across seven study locations in the United States. Stocks of labile P were large enough to sustain high P losses for decades, indicating the transport‐limited regime typical of legacy P. Sediments were commonly more P‐sorptive than nearby soils. Soils in the top 5 cm had 1.3–3.0 times more labile P than soils at 5–15 cm. Stratification in soil test P and total P was, however, less consistent. As P exchange via sorption processes follows the difference in intensities between soil/sediment surface and solution, we built a model for the equilibrium phosphate concentration at net zero sorption (EPC 0 ) as a function of labile P (quantity) and buffer capacity. Despite widely varying properties across sites, the model generalized well for all soils and sediments: EPC 0 increased sharply with more labile P and to greater degree when buffer capacity was low or sorption sites were likely more saturated. This quantity–intensity–capacity relationship is central to the P transport models we rely on today. Our data inform the improvement of such P models, which will be necessary to predict the impacts of legacy P. Further, this work reaffirms the position of labile P as a key focus for environmental P management—a view Dr. Sharpley developed in the 1980s with fewer data and resources.

  PublisherOA PDFDOI

• Fungal degradation of complex organic carbon supports denitrification in saturated woodchip bioreactors

  Bioresource Technology · 2024-11-20 · 4 citations

  article
  PublisherDOI

• Fifty years of environmental progress for United States dairy farms

  Journal of Dairy Science · 2024-01-11 · 19 citations

  articleOpen access

  Dairy farms in the United States have changed in many ways over the past 50 yr. Milk production efficiency has increased greatly, with ∼30% fewer cows producing about twice the amount of milk today. Other improvements include increases in crop yields, fuel efficiency of farm equipment, and efficiency in producing most resources used on farms (e.g. electricity, fuel, fertilizer). These improvements have led to changes in the environmental impact of farms. Through simulation of representative dairy farms in 1971 and 2020, changes in nutrient losses and farmgate life cycle assessments of greenhouse gas (GHG) emissions, fossil energy use, and blue (ground and surface) water use were determined for 6 regions and the United States. For all environmental metrics studied, intensities expressed per unit of fat- and protein-corrected milk produced were reduced, but the total effects over all farms or milk produced increased for 5 of the 13 environmental metrics. Reductions in the impacts of dairy farms in the eastern United States were offset by large increases in western regions because of a major increase in cow numbers in the West. The national average intensity of GHG emissions decreased by 42%, which gave just a 14% increase in the total GHG emissions of all dairy farms over the 50-yr period. The intensity of fossil energy use decreased by 54%, with the total for all farms decreasing by 9%. Water use related to milk production decreased in intensity by 28%, but due to the large increase in dairy production in the dry western regions that have a greater dependence on irrigated feed crops, total blue water use increased by 42%. Major pathways of nitrogen loss included ammonia volatilization, leaching, and denitrification, where total ammonia emissions related to US dairy farms increased by 29%, while leaching losses decreased by 39%, with little change in nitrous oxide emissions. Simulated nitrogen and phosphorus runoff losses totaled for all dairy farms decreased by 27% to 51% through more efficient fertilizer use, reduced tillage, and greater use of cover crops. Emissions of methane and reactive non-methane volatile organic compounds increased by 32% and 53%, respectively, due to greater use of long-term manure storage and silage stored in bunkers and piles. Although much progress has been made in improving production efficiency, continued improvements with new strategies and technologies are needed to meet the demand for dairy products and mitigate total environmental impacts, particularly in view of projected climate variability.

  PublisherOA PDFDOI

• USDA LTAR Common Experiment measurement: Discharge from artificial subsurface drains v1

  2024-03-11 · 1 citations

  preprintOpen access1st authorCorresponding

  Subsurface drain discharge, sometimes simply referred to as drainage, is a process by which water is removed from a soil profile or area (ASABE, 2015). On agricultural lands with intermittent high-water tables due to precipitation or melting snow, water is removed using artificial (generally referred to as tile) drains or ditches to allow timely field operations and protect growing crops from being waterlogged. This discussion is limited to artificial subsurface (tile) drain discharge. In agricultural regions where crop production requires artificial subsurface drainage, drain discharge can represent a substantial portion of the annual water budget. Thus, measuring this water flow is important in understanding the water use of crops and potential impacts on downstream water movement. The measurement of subsurface drain discharge, together with a measurement of sediment and water pollutants, provides an assessment of the losses of these constituents from agriculture to the environment. Tracking them over time quantifies the beneficial effects of improved agricultural practices.

  PublisherDOI

• Nitrogen and Phosphorus Removal from Agricultural Drainage Water by a Modular Bioreactor

  SSRN Electronic Journal · 2024-01-01

  preprintOpen access
  PublisherDOI

Frequent coauthors

• Michelle L. Soupir

  41 shared

• Ulrike Tschirner

  University of Minnesota

  39 shared

• Natasha Hoover

  Iowa State University

  39 shared

• Christopher Hay

  39 shared

• Niranga M. Wickramarathne

  University of Illinois Urbana-Champaign

  37 shared

• Keegan Kult

  Iowa Soybean Association

  36 shared

• Peter J. A. Kleinman

  Agricultural Research Service

  26 shared

• John M. Baker

  United States Department of Agriculture

  24 shared