Biogeosciences (Jan 2009)
Vertical structure and diurnal variability of ammonia exchange potential within an intensively managed grass canopy
Abstract
Stomatal ammonia compensation points (χ<sub>s</sub>) of grass species on a mixed fertilized grassland were determined by measurements of apoplastic [NH<sub>4</sub><sup>+</sup>] and [H><sup>+</sup>] in the field. Calculated χ<sub>s</sub> values were compared with in-canopy atmospheric NH<sub>3</sub> concentration (χ<sub>a</sub>) measurements. <br><br> Leaf apoplastic [NH<sub>4</sub><sup>+</sup>] increased by a factor of two from the lowest level in the canopy to the top level. Bulk leaf [NH<sub>4</sub><sup>+</sup>] and especially [NO<sub>3</sub><sup>−</sup>] slightly increased at the bottom of the canopy and these concentrations were very high in senescent plant litter. Calculated χ<sub>s</sub> values were below atmospheric χ<sub>s</sub> at all canopy levels measured, indicating that the grassland was characterized by NH<sub>3</sub> deposition before cutting. This was confirmed by the χ<sub>a</sub> profile, showing the lowest χ<sub>a</sub> close to the ground (15 cm above soil surface) and an increase in χ<sub>a</sub> with canopy height. Neither χ<sub>s</sub> nor χ<sub>a</sub> could be measured close to the soil surface, however, the [NH<sub>4</sub><sup>+</sup>] in the litter material indicated a high potential for NH<sub>3</sub> emission. <br><br> A diurnal course in apoplastic [NH<sub>4</sub><sup>+</sup>] was seen in the regrowing grass growing after cutting, with highest concentration around noon. Both apoplastic and tissue [NH<sub>4</sub><sup>+</sup>] increased in young grass compared to tall grass. Following cutting, in-canopy gradients of atmospheric χ<sub>a</sub> showed NH<sub>3</sub> emission but since calculated χ<sub>s</sub> values of the cut grass were still lower than atmospheric NH<sub>3</sub> concentrations, the emissions could not entirely be explained by stomatal NH<sub>3</sub> loss. High tissue [NH<sub>4</sub><sup>+</sup>] in the senescent plant material indicated that this fraction constituted an NH<sub>3</sub> source. After fertilization, [NH<sub>4</sub><sup>+</sup>] 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<sub>4</sub><sup>+</sup>] was even more pronounced after fertilization and calculated χ<sub>s</sub> values were generally higher, but remained below atmospheric [NH<sub>3</sub>].
