Nitrate from Field to Stream: Characterization and Mitigation

Crop production plays a vital role in ensuring global food security. Nitrogen (N), an essential nutrient for crop growth, is often applied to agricultural production systems as inorganic fertilizer and/or manure to increase crop yields, and/or is added to the soil via legume fixation. However, excess N, in the form of nitrate, readily leaches from crop production systems and contributes to water quality impairments of groundwater, rivers, streams, and estuaries. Despite significant efforts to develop improved practices for agricultural N management, groundwater nitrate concentrations continue to surpass the drinking water guidelines for human health in many areas. High nitrate is a top-ranking reason for violations at regulated U.S. drinking water utilities, and in the EU, a recent report revealed that 14% of drinking water wells exceed the nitrate concentration limit. Excessive leaching of nitrate in the Corn-Belt region of the U.S. has been linked to the development of annual large-scale hypoxic zones in the Gulf of Mexico, and approximately one-third of coastal waters and rivers. Moreover, lakes in the EU are considered eutrophic due, in part, to excessive nitrate losses from agricultural areas. In Canada, nitrate contamination related to agriculture is pervasive in many aquifers including Abbotsford-Sumas, Assiniboine Delta, Annapolis Valley, and Prince Edward Island aquifers.

Nitrate is leached through soils and primarily follows subsurface hydrologic pathways via artificial drainage systems, deep vadose zone and groundwater flow to receiving surface water bodies. The amount of leachable nitrate depends on the intricate interaction of various processes within the carbon and nitrogen cycles found in soils. These soil processes, which govern nitrate availability, correlate with nitrate transport processes within the soil profile. Understanding the impacts of these processes on nitrate leaching and effectively characterizing it at the field scale is crucial for both mitigating nitrate leaching and promoting crop production enhancement. Once leached beyond the soil profile, characterizing nitrate transport pathways and maximizing opportunities for mitigation become paramount. Assessing factors contributing to high nitrate leaching losses and subsurface transport to wells and streams is critical for identifying and developing effective nitrate mitigation strategies.

This Research Topic focuses on studies aimed at deepening our understanding of the processes that control nitrate leaching and transport in the subsurface, ultimately to drinking water wells or surface water receptors. It also encompasses investigations into experimental and monitoring methods, techniques, and modeling tools that aid in characterizing nitrate leaching and developing strategies for its mitigation.

Submissions can include a range of topics that control nitrate leaching and transport from source area production to discharge in rivers and streams such as:

• Innovative cropping and management systems that reduce nitrate leaching to the subsurface at the field scale (nitrate source reduction)
• Novel monitoring and modeling tools, including the application of isotopes to trace N-leaching sources and pathways
• Quantification of nitrate transport pathways in the subsurface, including artificial drainage, vadose zone, aquifer characterization, and groundwater-surface water interactions
• Biogeochemical processes that regulate nitrate from source to sink
• Emerging conservation practices that intercept and reduce nitrate before it discharges to surface water
• Impacts of climate change on N transport and fate in the subsurface