Where Does the Nitrogen from Forest Fertilizers Go?

by Claire Payne, SRS Science Delivery Group

Pine plantations cover vast areas in the nearly flat, poorly drained high-water-table soils of the Atlantic Coastal Plain of the southeastern United States. Growing productive forests on these lands requires drainage and extra nitrogen from fertilizers. While most of the nitrogen from fertilizers remains in the plant-soil system, concerns exist that leached nitrogen could enter the areas surface waters.

Combined with the areas proximity to nutrient-sensitive streams, estuaries, and other natural areas, water quality emerges as a concern. Managing the quality and quantity of water that drains from forests is important to reduce off-site impacts of fertilizing. Additionally, land managers are increasingly interested in understanding how to control the transport and fate (basically where it ends up) of nitrogen in order to increase fertilizer use efficiency and productivity on their forest stands.  

“When we started looking at this question, there were a number of models that could be used at the plot level to look at nitrogen fate,” said Devendra Amatya, research hydrologist with the Southern Research Station (SRS) Center for Forested Wetlands Research located near Charleston, South Carolina. “But these models were inadequate at the forest stand scale. Also, catchment studies designed to measure stream water quality did not evaluate different components of nitrogen balance at a sufficient resolution to assess mechanisms controlling nitrogen cycling in a forested area.”

Amatya worked with  North Carolina State University (NCSU) assistant professor Mohamed Youssef, professor Wayne Skaggs, and then graduate student Shiying Tian to develop and test a comprehensive model—called DRAINMOD-FOREST—that could accurately determine the hydrology and nitrogen dynamics of pine stands on poorly drained soils while also simulating tree growth and related processes.

Amatya received a grant from the National Council for Air & Stream Improvement, Inc. (NCASI) to set up the modeling project, and the Forest Service provided funding to the NCSU Agricultural Research Service. Amatya, Mohamed, Tian, and Skaggs worked with NCASI to develop and test DRAINMOD-FOREST on a research site on Weyerhaeuser Company lands—a loblolly pine plantation within three 62-acre watersheds where hydrologic data collection began in 1988 when the trees were 15 years old.  

“These long-term data covering various water and silvicultural treatments  have not only provided a valuable basis for assessing how different treatments impact the hydrology, drainage outflow, and water quality of these systems but also for developing the forestry versions of DRAINMOD models, including DRAINMOD-FOREST,” said Amatya. “Furthermore, the long-term data capturing climatic variability are critical for calibrating and validating the models so they can  be used as reliable assessment tools.”

Results of the studies were recently published as an article in the Journal of Environmental Quality and were reviewed online by NCSU and publicized through the National Science Foundation Science 360 website. Tian, Youssef, Skaggs, Amatya, and Chescheir co-authored “Modeling Water, Carbon, and Nitrogen Dynamics for Two Drained Pine Plantations under Intensive Management Practices,” which is also a part of the DRAINMOD-FOREST modeling project.The journal Forest Ecology and Management published the paper.

 For more on the project, watch a video featuring Amatya.   

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