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

Stomatal ammonia compensation points (s ) of grass species on a mixed fertilized grassland were determined by measurements of apoplastic [NH+4 ] and [H+] in the field. Calculated s values were compared with in-canopy
atmospheric NH3 concentration (a) measurements. Leaf apoplastic [NH+
4 ] increased by a factor of two from the lowest level in the canopy to the top level. Bulk leaf [NH+4 ] and especially [NO−3 ] slightly increased at the bottom
of the canopy and these concentrations were very high in senescent plant litter. Calculated s values were below atmospheric a at all canopy levels measured, indicating that the grassland was characterized by NH3 deposition before cutting. This was confirmed by the a profile, showing the lowest a close to the ground (15 cm above soil surface) and an increase in a with canopy height. Neither s nor a could be measured close to the soil surface, however, the [NH+
4 ] in the litter material indicated a high potential for NH3 emission.
A diurnal course in apoplastic [NH+ 4 ] was seen in the regrowing
grass growing after cutting, with highest concentration around noon. Both apoplastic and tissue [NH+ 4 ] increased in young grass compared to tall grass. Following cutting, in-canopy gradients of atmospheric a showed NH3
emission but since calculated s 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 [NH+ 4 ] in the senescent plant material indicated that this fraction constituted an NH3 source. After fertilization,
[NH+ 4 ] increased both in apoplast and leaf tissue with the most pronounced increase in the former compared to the latter. The diurnal pattern in apoplastic [NH+ 4 ] was even more pronounced after fertilization and calculated s values
were generally higher, but remained below atmospheric [NH3].