PARENT SESSION
Oral Session #98: Carbon and Nitrogen Cycling: N-addition, N-flux.
Presiding: C. Hoover
Friday, August 9. 9:00 AM to 12:00 PM. Greenlee Meeting Room, TCC.

Threshold responses of N₂O flux to added N in intensively managed ecosystems.

McSwiney, Claire^*,1, Robertson, G.¹, ¹ Kellogg Biological Station, Hickory Corners, MI

ABSTRACT- The most important proximal controls on N₂O production in soils are N availability (as NO₃^- and NH₄⁺), organic carbon, and oxygen levels. In current models, N₂O flux is related linearly to N-availability, which may not be an accurate representation of the way that ecosystems respond to N. In order to better understand how N availability relates to N₂O production, we conducted an N rate study at the Kellogg Biological Station in southwestern Michigan. Ammonium nitrate was added to 4 replicate fields in continuous corn at 9 levels, ranging from 0-260 kg N ha^-1 yr^-1. We measured surface N₂O fluxes before and after fertilizer was added to the soils to determine the threshold where N₂O fluxes began to increase. Corn yields were determined at harvest and soil mineral N was determined after harvest. The threshold for N₂O flux occurred at 134 kg N ha^-1 yr^-1, which is just above the rate at which corn is fertilized for normal yields at KBS. Gains in grain yields decreased at N addition rates above 101 kg N ha^-1 yr^-1. Soil inorganic N pools increased at the point along the N-availability gradient where increases in yield declined and N₂O fluxes increased. If these patterns are representative of other crops and growing seasons, N₂O mitigation in cropping systems could be achieved without yield penalty by adjusting N fertilizer additions to crop N needs.

KEY WORDS: Nitrous oxide