ROLE OF NITROGEN IN HERBAGE PRODUCTION BY AGROSTIS-FESTUCA HILL GRASSLAND

1. The role of nitrogen (N) in limiting herbage production was assessed in an altitudinal sequence of Agrostis-Festuca grasslands in the Moor House National Nature Reserve. 2. For most of the growing season the concentration of ammonium-N found in the soils at the times of sampling greatly exceeded that of nitrate-N. 3. Laboratory determinations of soil N mineralization rates at field growing season temperatures showed that mineral N accumulated almost entirely as nitrate in samples from lower altitudes and as ammonium in soil samples taken at the highest altitude. 4. A linear relationship between the mean annual concentration of N in the herbage and primary production in the four grassland sites suggests that soil N supply was a limiting factor. 5. Application of a N-15 root uptake bioassay to excised roots of Agrostis capillaris and Festuca ovina, collected from the study sites, indicated that the demand for N was inextricably linked with the demand for phosphorus (P). At the lowest altitudinal site (480 m) N supply was adequate. However, the bioassay data indicate that the demand for N was high at this site, probably due to interactive effects of enhanced P status relative to swards at higher altitudes, i.e. lack of P stress induced better growth which in turn induced N stress. 6. From the western boundary of the Moor House National Nature Reserve to the summit of Great Dun Fell, where the annual input of inorganic nitrogen in bulk precipitation and occult deposition is positively related to increasing altitude, this source of N supply far exceeded the amount of available N being released by mineralization of soil organic matter at the highest study site (747 m) during the growing season. 7. It is suggested that N is a secondary limiting factor determining limestone grassland sward production in the Moor House National Nature Reserve.