Bi-directional exchange of ammonia over cereals

Studies of the exchange of NH3 between cereals and the atmosphere were performed during two extended joint field experiments at Manndorf in Lower Bavaria (1990), and at Bellheim in the Palatinate (1995). Measurements of NH3 concentrations were obtained by different denuder techniques above mature wheat (Manndorf) and mature triticale (Bellheim), as well as within the canopy (Bellheim). Vertical NH3 fluxes were obtained by application of concentration gradients to aerodynamic and/or modified Bowen-ratio techniques, NH3 exchange was characterised by bi-directional fluxes during both experiments, During night and early morning, fluxes were always directed to the canopies (net deposition, up to -30 ngN m(-2)s(-1)), while during daytime (late morning to early evening) net NH3 emission from the canopies was observed (up to + 100 ngN m(-2)s(-1)). Bi-directional fluxes always point to the existence of so-called compensation points. Considering data of those periods where consistent meteorological conditions had prevailed, a ''canopy compensation point'' of 3.7 ppb could be derived from the Manndorf data, Based on our field data, an effective control of NH3 emission by physiological processes is suggested by the anticorrelated in-canopy profiles of NH3 and CO2 (''D-shaped'' (day) and ''C-shaped'' (night) for NH3 and vice versa for CO2).