Potential of a root bioassay for determining P-deficiency in high altitude grassland.
The significance of P deficiency in limiting sward production was assessed in an altitudinal sequence of Agrosto-Festucetum grasslands at the Moor House National Nature Reserve, Cumbria, UK. A 32P root uptake bioassay was applied both to sand-culture grown plants of Agrostis capillaris and Festuca ovina and to roots of tillers collected from field experiments, to indicate the potential value of this method for determining the P status of the hill grasslands. The 2 species grown in sand-culture showed negative relationships between 32P uptake in pg P/mg root over a 15-min period and (a) a range of P concn (0.4-40 mg P/litre) supplied in the culture solution, and (b) the total P content (mg P/plant). Total annual DM production (g/m2) in four altitudinal grassland sites and total P in the annual sward production (mg/m2) were negatively and asymptotically related to 32P uptake by excised roots of F. ovina, the most common component species in the swards. Sward production differed significantly in the 2 years of observations relating to the degree-day values above 6°C for the respective growing seasons. The 32P root uptake values derived from the bioassay reflected the differences in the P demand by the swards between these 2 years with the results forming a single regression. In a field fertilizer experiment, the bioassay could detect the effects of site, applied P fertilizer and site × fertilizer interactions only 2 weeks after fertilizer application. The results of the bioassay appeared to provide integrated assessments of the demand for P, the P supply in the soil, and likely responses of sward production to fertilizer application in high-altitude grasslands.