Investigating in-field practices to mitigate nitrate loss from subsurface drainage in Iowa in a corn/soybean rotation

Abstract

This dissertation evaluates various in-field management practices intending to quantify their impact on nitrate-N loss via subsurface drainage in corn and soybean systems. The focus is nitrogen retention in two ways: fertilizer timing and cover crops. The first chapter is a general introduction to nutrient management in agriculture. The second chapter investigates fertilizer timing and concludes that there is not a benefit to applying anhydrous ammonia in the fall vs. spring, so long as the fall-applied N is applied with a nitrification inhibitor, when the soil is 10 degrees C and cooling, and at the maximum return to nitrogen. The third chapter presents data from a cover crop mixture study with lysimeters, exploring cover crop options beyond winter cereal rye. Oats before corn had no negative impacts on corn yield while improving spring water quality. There was no water or soil quality benefit to adding more species with winter cereal rye which grew successfully even in the northern half of the state. The fourth chapter reports on a cover crop management study including both no-tillage and conventional tillage. At this particular site in north-central Iowa, winter cereal rye was more successful at improving water quality with tillage compared to no tillage. However, after 11 (in the conventional system) or 15 (in the no-tillage system) years, there were increases in soil organic carbon content in all conservation practices. Switching to broadcast seeding improved cover crop growth in both tillage treatments. The fifth chapter includes general conclusions and recommendations for future research. The general conclusions of this dissertation are that the management of nitrogen in Iowa is complex due to its mobility in water and weather-dependent transformations. The most consistent way to improve water quality is to increase the months that living crops are growing. General recommendations include more research on the potential of hairy vetch in Iowa, more long-term studies to better understand soil health and its influences on water quality, and research on the benefits of combining both edge-of-field and in-field practices.