1. The role of nitrogen (N) in limiting herbage production was assesse
d in an altitudinal sequence of Agrostis-Festuca grasslands in the Moo
r House National Nature Reserve. 2. For most of the growing season the
concentration of ammonium-N found in the soils at the times of sampli
ng greatly exceeded that of nitrate-N. 3. Laboratory determinations of
soil N mineralization rates at field growing season temperatures show
ed that mineral N accumulated almost entirely as nitrate in samples fr
om lower altitudes and as ammonium in soil samples taken at the highes
t altitude. 4. A linear relationship between the mean annual concentra
tion of N in the herbage and primary production in the four grassland
sites suggests that soil N supply was a limiting factor. 5. Applicatio
n of a N-15 root uptake bioassay to excised roots of Agrostis capillar
is and Festuca ovina, collected from the study sites, indicated that t
he demand for N was inextricably linked with the demand for phosphorus
(P). At the lowest altitudinal site (480 m) N supply was adequate. Ho
wever, the bioassay data indicate that the demand for N was high at th
is site, probably due to interactive effects of enhanced P status rela
tive to swards at higher altitudes, i.e. lack of P stress induced bett
er 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 precipitati
on 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 prod
uction in the Moor House National Nature Reserve.