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Special Issue "Processes controlling the exchange of ammonia between grassland and the atmosphere (GRAMINAE", Editor(s): K. Pilegaard, J. K. Schjoerring, and M. A. SuttonStomatal ammonia compensation points (Xs) of grass species on a mixed fertilized grassland were determined by measurements of apoplastic [NH4+] and [H>+] in the field. Calculated Xs values were compared with in-canopy atmospheric NH3 concentration (Xa) 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 Xs values were below atmospheric Xs at all canopy levels measured, indicating that the grassland was characterized by NH3 deposition before cutting. This was confirmed by the Xa profile, showing the lowest Xa close to the ground (15 cm above soil surface) and an increase in Xa with canopy height. Neither Xs nor Xa 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 Xa showed NH3 emission but since calculated Xs 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 Xs values were generally higher, but remained below atmospheric [NH3

Vertical structure and diurnal variability of ammonia exchange potential within an intensively managed grass canopy