Calibration of the 32P bioassay for eucalypt roots in the field

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The root bioassay based on 32P uptake is potentially useful as an indicator of phosphorus limitation in forest stands and thus future growth responses to P fertilizer application. Various conditions of the bioassay were tested and modified with the aim of calibrating standard procedures for its use in eucalypt forests in southern Australia. Variables tested included root morphology, plant species, storage time for roots, effect of roots on physical quenching of Cerenkov radiation, and concentration of P in the uptake solution. Roots were sampled from established eucalypts growing in the field that had been either untreated or fertilized with P 2 months previously. Fine (<0.5 mm dia) brown roots, which are probably mycorrhizal, are recommended for sampling as these are the most active and numerous roots in eucalypt stands. Differences among species in their uptake rate per mass or surface area of root demonstrated the importance of identifying eucalypt roots. Activity of excised roots declined with storage time, but the decrease was only significant after 5 d. Errors in counting radioactivity due to the effect of root geometry on physical quenching were small. Concentration of P in the uptake solution did not influence the effect of fertilizer treatment on measured uptake rates of 32P. Kinetic parameters for P uptake derived from a range of solution concentrations (0.1 to 100 μ m) were: I max=95 nmol g −1 f.w. s −1 and K m=58 μ m (unfertilized), and I max=33 nmol g −1 f.w. s −1 and K m=62 μ m (fertilized). Decreased uptake capacity of roots from fertilized plants has been reported for other species, however the kinetic parameters for these eucalypts were mostly higher. These higher kinetic parameters may be due to species, plant age or growing conditions. The great variation among reported kinetic parameters highlights the problems with transferring results to other systems and using them more generally for modelling. It is important that data be obtained for the field situation if nutrient uptake capacity is to be compared among eucalypt plantations.