N budget and NH3 exchange of a grass/clover crop at two levels of N application

Atmospheric ammonia (NH3) exchange during a single growing season was measu red over two grass/clover fields managed by cutting and treated with differ ent rates of mineral nitrogen (N) fertilizer. The aim was to quantify the t otal NH3 exchange of the two systems in relation to their N budget, the lat ter was split into N derived from symbiotic fixation, from fertilization, a nd from the soil. The experimental site was located in an intensively manag ed agricultural area on the Swiss plateau. Two adjacent fields with mixture s of perennial ryegrass (Lolium perenne L.), cocks foot (Dactylis glomerata L.), white clover ( Trifolium repens L.) and red clover ( Trifolium praten se L.) were used. These were treated with either 80 or 160 kg N ha(-1) appl ied as NH4NO3 fertilizer in equal portions after each of four cuts. Continu ous NH3 flux measurements were carried out by micrometeorological technique s. To determine the contribution of each species to the overall NH3 canopy compensation point, stomatal NH3 compensation points of the individual plan t species were determined on the basis of NH4+ + NH3 (NHx) concentrations a nd pH in the apoplast. Symbiotic N-2 fixation was measured by the N-15 dilu tion method. In the field with the lower rate of mineral N application, the clover fract ion was higher, and a higher symbiotic N-2 fixation rate completely compens ated for the lower mineral N input. During the measurement period, except a fter N fertilization, NH3 concentration ([NH3]) above the canopy was betwee n 3 and 4 mug m(-)3, which was generally higher than the measured canopy NH 3 compensation point. Thus, deposition from the atmosphere to the grass/clo ver canopy was predominant, and the system acted as net sink for NH3. The t otal amount of N emitted as NH3 was slightly higher for the high N treatmen t compared to the low N treatment but accounted for less than 1% of the N r emoved by cutting in both treatments. The results show that net NH3 emissio n from the frequently cut grass/clover field was restricted to short period s after ammonium nitrate application, and that on a seasonal basis fertiliz er N and N derived from N-2 fixation had equal effects on the exchange of N H3 between the canopy and the atmosphere.