Urea Transformation Under Varying Cover Crops.

Ammonia volatilization (AV) is the dominant loss pathway for cropping systems where urea-based nitrogen (N) fertilizers are surface applied. The objective of this study was to quantify AV under controlled laboratory conditions from two soil types where cropping system field experiments were implemented in the previous year. The first laboratory trial evaluated summer annual cropping systems on a Eunola loamy fine sand which included: corn, cotton, full season soybeans, sorghum, peanuts, and double cropped winter wheat/soybeans. The second study evaluated different crops on a Bojac sandy loam which included no crop residues, corn, wheat, and the cover crops cereal rye, hairy vetch, and a high diversity mixture. Intact soil cores were taken from each site within cropping system plots and allowed to equilibrate to 26 C^o overnight and then 62 mg N as urea per chamber was surface-applied. Losses of N via AV ranged from 52-71% of the applied N on the Eunola loamy fine sand across cropping systems. Sorghum produced the greatest AV losses at 71% and this was significantly higher than corn with 52% losses of applied N as ammonia. Ammonia loss from corn was also significantly lower than peanut (67% loss of applied N) and double-cropped winter wheat/soybean (65% loss of applied N). In the cover crop study no differences in cumulative AV were found for different cover cropping systems. Cumulative ammonia losses ranged from 48-64 % of the applied N for the two week trial. Hairy vetch had the greatest ammonia losses (64% of applied N) when the cover crop mixture had the lowest cumulative ammonia loss with 48% of applied N. These initial findings indicate that annual cropping systems from the previous summer had a greater influence on AV losses of N than cover cropping systems. One explanation for this is that plant bound urease has been released by degradation whereas cover crops had recently been terminated and the contents of the cells may have not been incorporated into the extracellular urease component of soils.