Seasonal variability of apoplastic NH4+ and pH in an intensively managed grassland

The stomatal compensation point of ammonia (chi(s)) is a major factor controlling the exchange of atmospheric ammonia (NH3) with vegetation. It is known to depend on the supply of nitrogen and to vary among plant species, but its seasonal variation has not yet been reported for grassland. In this study, we present the temporal variation of apoplastic NH4+ concentration ([NH4+](apo)) and pH (pH(apo)) measured in leaves of Lolium perenne L. in a grassland, through two periods of cutting / fertilisation, followed by a livestock grazing period. The total free NH4+ concentration measured in foliage ([NH4+](fol)), and soil mineral NH4+ and NO3- concentration are also presented. The value of [NH4+](apo) varied from less than 0.01 mM to a maximum of 0.5 mM occurring just after fertilisation, whereas the apoplastic pH ranged from pH 6 to 6.5 for most of the time and increased up to pH 7.8, 9 days after the second fertilisation, when grazing started. [NH4+](fol) varied between 20 and 50 mug N-NH4+ g(-1) f.w. The compensation point at 20degreesC, ranged from 0.02 mug NH3 m(-3) between the fertilisations to 10 mug NH3 m(-3) just after the second fertilisation. The reasons for these seasonal changes are discussed, with respect to plant metabolism and the concentration of ammonium and nitrate in the soil.