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Vertical structure and diurnal variability of ammonia exchange potential within an intensively managed grass canopy
1Agroscope Reckenholz-Tänikon Research Station ART, Zürich, Switzerland
2Plant and Soil Science Laboratory, Faculty of Life Sciences, University of Copenhagen, Copenhagen, Denmark
3Institut National de la Recherche Agronomique (INRA), UMR Environnement et Grandes Cultures, Thiverval-Grignon, France
4Natural Environmental Research Council, Centre for Ecology and Hydrology, Edinburgh Research Station, Midlothian, UK
*now at: Section for Economy and Technology, Halmstad University, Halmstad, 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 concentrations (χa) measured by micrometeorological techniques.
Leaf apoplastic NH4+ did not significantly differ between intact leaves from different heights above the ground. 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 χa 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, especially during the night. Neither χs nor χa could be measured close to the soil surface, the litter NH4+ material indicated a high potential for NH3 emission tough.
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 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.
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Citation: Herrmann, B., Mattsson, M., Jones, S., Cellier, P., Milford, C., Sutton, M. A., Schjoerring, J. K., and Neftel, A.: Vertical structure and diurnal variability of ammonia exchange potential within an intensively managed grass canopy, Biogeosciences Discuss., 5, 2897-2921, 2008. Bibtex EndNote Reference Manager
