POTENTIAL OF A ROOT BIOASSAY FOR DETERMINING P-DEFICIENCY IN HIGH-ALTITUDE GRASSLAND

(1) The significance of phosphorus deficiency in limiting sward production was assessed in an altitudinal sequence of Agrosto-Festucetum grasslands at the Moor House National Nature Reserve. (2) A P-32 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. (3) The two species grown in sand-culture showed negative relationships between P-32 uptake in pg P mg root-1 15 min-1 and (a) a range of P concentrations (0.4-40 mg P litre-1) supplied in the culture solution, and (b) the total P content (mg P plant-1). (4) Both the total annual dry matter production (g m-2) in the four altitudinal grassland sites and total P in the annual sward production (mg m-2) were negatively and asymptotically related to P-32 uptake by excised roots of Festuca ovina, the most common component species in the swards. (5) Sward production differed significantly in the 2 years of observations relating to the degree day values above 6-degrees-C for the respective growing seasons. The P-32 root uptake values derived from the bioassay reflected the differences in the P demand by the swards between these two years with the results forming a single regression. (6) In a field fertilizer experiment, the bioassay was able to detect the effects of site, applied P fertilizer and site x fertilizer interactions only 2 weeks after fertilizer application. (7) The results of the bioassay appear to provide integrated assessments of (i) the demand for P, (ii) the P supply in the soil, and (iii) likely responses of sward production to fertilizer applications, in high-altitude grasslands.