Role of nitrogen in herbage production by Agrostis-Festuca hill grassland.
The role of N in limiting herbage production was assessed in an altitudinal sequence of Agrostis-Festuca grasslands in the Moor House National Nature Reserve, Cumbria. For most of the growing season the concentration of ammonium-N found in the soils at the time of sampling greatly exceeded that of nitrate-N. 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. A linear relationship between the mean annual concentration of N in the herbage and primary production in the 4 grassland sites suggested that soil N supply was a limiting factor. Application of a 15N root uptake bioassay to excised roots of A. capillaris and F. ovina, collected from the study sites, indicated that the demand for N was inextricably linked with the demand for P. At the lowest altitudinal site (480 m) N supply was adequate. However, the bioassay data indicated 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. From the western boundary of the Moor House National Nature Reserve to the summit of Great Dun Fell, where the annual input of inorganic N 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 OM at the highest study site (747 m) during the growing season. It is suggested that N is a secondary limiting factor determining limestone grassland sward production in the Moor House National Nature Reserve.