Foliar absorption of atmospheric ammonia by ryegrass in the field

The dry deposition of ammonia (NH3) in the field was measured simultaneously at increasing distances from a point source (a dairy farm) using a biomonitor. The biomonitor was Italian ryegrass (Lolium multiflorum Lam.) grown in pots and supplied with 15N-labeled N. The use of 15N improved the precision and lowered the detection limit of the method compared with calculating the NH3 deposition using the difference method (N-balance method). Atmospheric NH3-concentrations were measured by drawing air through traps containing sulfuric acid. At the end of a 6-wk period in the growing season, the deposition of N was 3.0 g N/m2 and 0.7 g N/m2 at average atmospheric concentrations of 89 and 6 μg NH3-N/m3, respectively. Estimated deposition velocities ranged from 0.7 to 3.4 cm/s with an average of 1.6 cm/s (s.d. 1.2 cm/s).