CALCULATIONS AND ASSUMPTIONS INVOLVED IN THE USE OF THE A-VALUE AND N-15 ISOTOPE-DILUTION TECHNIQUES FOR THE ESTIMATION OF THE CONTRIBUTION OF PLANT-ASSOCIATED BIOLOGICAL N2 FIXATION

In a paper published recently in this Journal a new equation was presented for the calculation of the proportion of plant nitrogen derived from plant-associated biological nitrogen fixation (BNF) using the 'A-value' technique. To apply this technique it is assumed that the specific availability ('A-value') of soil N is constant regardless of the quantity of added labelled N fertilizer. The authors stated that it follows from this assumption that the proportion of plant N derived from the N-15-labelled fertilizer (%Ndff) is proportional to the rate of addition of the labelled fertilizer. In this present paper we show that this is not the case, and furthermore data in the literature indicate that the 'A-value' of soil N is often not constant at different applied N levels. This technique, in common with the isotope dilution (ID) technique, relies on the assumption that the specific availability of soil N is equal for different plants (both 'N2-fixing' and 'non-N2-fixing'). However, as in addition the assumption is made that the 'A-value' of soil N is constant at different rates of applied N, we argue that the ID technique is more reliable, particularly when BNF contributions are small.