Temporal variability in bioassays of ammonia exchange potential in relation to plant and soil nitrogen parameters in intensively managed grassland

The exchange of ammonia between crop canopies and the atmosphere depends on a range of plant parameters and climatic conditions but little is known about effects of management factors. We have here investigated the ammonia exchange potential of a grass sward dominated by Lolium perenne in response to cutting and fertilization. Tall grass showed a low potential for NH3 emission before cutting. During re-growth after cutting, leaf tissue concentrations of NO3−, NH4+, soluble N and total N increased along with apoplastic NH4+ concentrations. In contrast, apoplastic pH decreased resulting in largely unaltered NH3 emission potential. A high potential for NH3 emission was shown by the plant litter. Fertilization with 100 kg N ha−1 one week after cutting caused the apoplastic NH4+ concentration of the newly emerging leaves to increase dramatically. The apoplastic NH4+ concentration peaked the day after the fertiliser was applied and thereafter decreased over the following 10 days until reaching the same level as before fertilisation. A positive correlation was found between NH4+ concentrations in leaf apoplast, bulk tissue and litter throughout the experimental period. Leaf soluble N was negatively correlated with apoplastic NH4+ concentration whereas total N was weakly correlated with NH4+ concentrations in leaf tissue and soil.