Compilation and Analysis of Nearly Half a Century of Drainage Nitrogen Loss Studies.

Increasing momentum surrounding meta-analytical studies within the agricultural sciences sets the context for important conversations not only about what has been established across literature, but also about what is missing from scientific understanding. As policy debate and regulatory interest related to agricultural water quality continue to grow, it is valuable to reflect upon decades of drainage research in North America to create a future vision for drained agricultural lands. In this work, the free and publically available “Measured Annual Nutrient loads from AGricultural Environments” (MANAGE) water quality database (www.ars.usda.gov/spa/manage-nutrient) was expanded to include nearly 1300 site-years of drainage nutrient load information. This quantitative review evaluated the drainage water quality and crop yield impacts across database fields such as study location, cropping management (tillage, nutrient management, crop selection), and precipitation. Across site-years, the mean and median percent of precipitation occurring as drainage were 25 and 20%, respectively, with wet years resulting in significantly greater drainage discharge and nutrient loads. This compilation allowed evaluation of climatic factors on drainage nutrient loads, for example, when a notably wet year was preceded by a drought year or having to “winterize” water quality monitoring equipment in certain geographic locations. Artificial drainage will remain a vital component of many agricultural systems, and improved understanding of management impacts, especially under variable climate conditions, can help point the way to a more sustainable future. This work provided an opportunity to assess knowledge and evaluate gaps in drainage nutrient research which will hopefully provide water quality context for other ongoing agronomic meta-analysis efforts.