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ARS Home » Northeast Area » Beltsville, Maryland (BARC) » Beltsville Agricultural Research Center » Adaptive Cropping Systems Laboratory » Research » Publications at this Location » Publication #317906
Title: Urea hydrolysis rates in soil toposequences as influenced by pH, carbon, nitrogen, and soluble metals

Author
item FISHER, KRISTIN - University Of Maryland
item Meisinger, John
item JAMES, BRUCE - University Of Maryland

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/13/2015
Publication Date: 1/11/2016
Citation: Fisher, K.A., Meisinger, J.J., James, B.R. 2016. Urea hydrolysis rates in soil toposequences as influenced by pH, carbon, nitrogen, and soluble metals. Journal of Environmental Quality. 45(1):349-359.

Interpretive Summary: Understanding urea transformations in agricultural ecosystems is important for evaluating the potential loss of unreacted urea to nearby streams or estuaries. This study evaluated factors affecting the dynamic conversion of urea to ammonium-nitrogen in the surface and subsurface soil horizons along a field transect that included an active grain-production area, an edge-of-field grass border, and a nearby perennial riparian zone adjacent to surface waters. The research found that the conversion of urea to ammonium-nitrogen was eight times faster in the surface soil than in the subsoil. Soil pH and soil organic-matter related components, like carbon and nitrogen, were also important factors affecting urea break down. These results are important for scientists, nutrient managers, and policy makers for understanding the likely benefits that neighboring non-agricultural ecosystems can provide for reducing the potential loss of unreacted urea to nearby bodies of water.

Technical Abstract: A simultaneous increase in the use of urea fertilizer and the incidence of harmful algal blooms worldwide has generated research on potential loss pathways of unhydrolyzed urea from agricultural areas. The objective of this research was to study the dynamics of urea hydrolysis in soil profile toposequences sampled from the Coastal Plain and Piedmont regions of Maryland to understand native hydrolysis rates, as well as the controls governing urea hydrolysis both across a landscape and with depth in the soil profile. A pH-adjustment experiment was conducted to explore the relationship between pH and urea hydrolysis because of the importance of pH to both agronomic productivity and microbial communities. Soils were sampled from both A- and B-horizons along transects containing an agricultural field, a grassed field border, and a perennially vegetated zone adjacent to surface water. On average, the A-horizon hydrolysis rates were eight times greater than corresponding B-horizon rates, and the riparian zone soils hydrolyzed urea faster than the agricultural soils. The pH-adjustment of these soils indicated the importance of organic-matter related factors (carbon, nitrogen, and extractable metals) in determining rates of urea hydrolysis. These results suggest that organic-matter rich riparian-zone soils may be valuable in mitigating losses of unhydrolyzed urea from neighboring fields. Additional field-scale urea hydrolysis studies would be valuable to corroborate the mechanisms described herein and to explore the conditions affecting the fate and transport of unhydrolyzed urea in agro-ecosystems.