While crop yield has increased in many parts of world since the 1960s, this trend has not occurred in sub-Saharan Africa. Dr. Tully is working in Kenya and Tanzania to determine what changes can improve yield without excessive fertilizer use. Africa as a whole plans to increase nitrogen-based fertilizer use, but there is an upper limit to the amount plants can use before the rest is lost to the environment.
To determine an optimal amount of fertilizer, Dr. Tully planted corn in Kenya and Tanzania, fertilized them with inorganic nitrogen (N) treatments ranging from 0-200 kg N/ha, and assessed the ultimate fate of the nitrogen—was it stored in the soil, incorporated into the plant, or washed out in groundwater?
The effect and destination of the nitrogen depended on the climate, particularly precipitation patterns, and soil conditions in the field. The soil in the Kenyan plots contained high amounts of clay, slowing the loss of nitrogen in the soil and increasing nitrogen availability for the crops. Moderate amounts of nitrogen doubled yield, as most of the nitrogen was directly taken up by the plant. In Tanzania, the soil in the plots had a much higher proportion of sand, resulting in a build-up of nitrogen as ammonium in the soil and very little incorporation by the corn.
Back in the U.S., Dr. Tully turned her attention to soil nutrient dynamics on the Eastern Shore, Maryland. The dominant agricultural crops feed—corn and soybeans support their primary industry—poultry. Over the years, phosphorus from fertilizers and chicken litter has accumulated in the soil to the point of saturation. Phosphorus typically binds to soil particles with minimal nutrient leaching to the surrounding environment. Yet, as human-accelerated sea-level rise causes saltwater intrusion of coastal farmland, soil dynamics are changing. Saltwater can significantly alter soil chemistry, which may result in the release of bound soil nutrients as runoff. While nitrogen and phosphorus are essential nutrients for living organisms, too much can throw entire ecosystems off balance. Excess phosphorus in particular has led to algal blooms in fresh water estuaries. Over time algal blooms deplete life supporting oxygen resulting in ‘dead zones.’ But to what magnitude, rate, and extent are saltwater intrusions increasing the movement of phosphorus?
Dr. Tully is exploring such questions via laboratory experiments and field studies. Field collections of soil and soil water in addition to vegetation monitoring were carried out in five settings on several Eastern Shore farms: crop, crop edge, bare ground, neighboring ditches, and marshes that have been invaded by Phragmites, a large exotic perennial grass that thrives in high concentrations of nitrogen and phosphorus. Her findings show that phosphorus was released into ditch water adjacent to crops, which then flows into tidal creeks and eventually the Chesapeake Bay. Understanding effects of saltwater intrusion on nutrient release is critical to the health of the Bay, and crop productivity thus should be included in restoration models and crop planning. As in the case of Kenya and Tanzania, successful strategies for balancing complex food and environmental issues necessitate engaging all stakeholders, especially local farmers who are directly affected.
Tully K, Hickman J, McKenna M, Palm CA, Neill C. 2016. Effects of increased fertilizer application on inorganic soil N in East African maize systems: vertical distributions and temporal dynamics. Ecological Applications 26: 1907-1919. doi:10.1890/15-1518.1
Becca Eckert is a Ph.D. student in Bill Lamp’s lab. Her research examines how changes in light and nutrient availability affect macroinvertebrate growth and diversity in heterotrophic headwater streams as mediated by changes in leaf-associated algal quantity and quality.
Lisa Kuder is a Ph.D. student in Dennis vanEngelsdorp’s Bee Lab. Her research focuses on road ecology, specifically improving highway rights-of-way for pollinators.