1Agroscope Reckenholz-Tänikon Research Station ART, Reckenholzstrasse 191, 8046 Zürich, Switzerland
2Plant and Soil Science Laboratory, University of Copenhagen, Faculty of Life Sciences, Thorvaldsensvej 40, 1871 Frederiksberg C, Copenhagen, Denmark
3Scottish Agricultural College, King's Buildings, West Mains Road, Edinburgh EH9 3JG, UK
4Inst. National de la Recherche Agronomique (INRA), UMR Environnement et Grandes Cultures, Thiverval-Grignon, France
5Institute of Earth Sciences "Jaume Almera", CSIC, Lluis Solé I Sabaris, 08028, Barcelona, Spain
6Natural Environmental Research Council, Centre for Ecology and Hydrology, Edinburgh Research Station, Penicuik EH26 0QB, Midlothian, UK
*now at: Section for Economy and Technology, Halmstad University, Halmstad, 30118, Sweden
Abstract. Stomatal ammonia compensation points (χs) of grass species on a mixed fertilized grassland were determined by measurements of apoplastic [NH4+] and [H>+] in the field. Calculated χs values were compared with in-canopy atmospheric NH3 concentration (χa) measurements.
Leaf apoplastic [NH4+] increased by a factor of two from the lowest level in the canopy to the top level. Bulk leaf [NH4+] and especially [NO3−] slightly increased at the bottom of the canopy and these concentrations were very high in senescent plant litter. Calculated χs values were below atmospheric χs at all canopy levels measured, indicating that the grassland was characterized by NH3 deposition before cutting. This was confirmed by the χa profile, showing the lowest χa close to the ground (15 cm above soil surface) and an increase in χa with canopy height. Neither χs nor χa could be measured close to the soil surface, however, the [NH4+] in the litter material indicated a high potential for NH3 emission.
A diurnal course in apoplastic [NH4+] was seen in the regrowing grass growing after cutting, with highest concentration around noon. Both apoplastic and tissue [NH4+] increased in young grass compared to tall grass. Following cutting, in-canopy gradients of atmospheric χa showed NH3 emission but since calculated χs values of the cut grass were still lower than atmospheric NH3 concentrations, the emissions could not entirely be explained by stomatal NH3 loss. High tissue [NH4+] in the senescent plant material indicated that this fraction constituted an NH3 source. After fertilization, [NH4+] increased both in apoplast and leaf tissue with the most pronounced increase in the former compared to the latter. The diurnal pattern in apoplastic [NH4+] was even more pronounced after fertilization and calculated χs values were generally higher, but remained below atmospheric [NH3].