Measurement of biological N2 fixation in field-grown Robinia pseudoacacia L

Quantitative estimates of biological N2 fixation (BNF) in Robinia pseudoacacia, one of the few temperate N2-fixing trees (NFTs) are lacking. A field experiment was conducted to measure N2 fixation in R. pseudoacacia over a 2-year period, using the 15N isotope dilution (ID) method. Two non-NFTs, Ligustrum vulgaris and Syringa vulgare, served as reference plants. In general, errors occurred in using the ID method to measure N2 fixation when the dilution of 15N by the unlabelled N in plants at transplanting was not taken into account. This could have serious implications for estimating N2 fixation in trees in established stands. For the first year, dissimilar values of N2 fixation were measured by the two reference plants. However, for the second year, or for the entire 2 years, similar values were obtained. From this study, we deduced that R. pseudoacacia has a high potential for N2 fixation. At the end of the first year, the percentage of N derived from fixation (% Ndfa) averaged for the two reference trees was 65%, and 90% in the second year. The overall % Ndfa for the 2-year period was 80. The equivalent amounts fixed were, 12.2 and 25.8 g N tree−1 in the first and second years, respectively. At the planting rate used, 5714 plants ha−1, this would be equivalent to about 220 kg N ha−1 within the 2 years. Although all the aboveground parts together gave % Ndfa values close to that for the whole tree, none of the individual plant parts gave an accurate estimate of total N fixed in R. pseudoacacia, and excluding the N in roots resulted in underestimates of total N fixed by about 14.7, 37.5 and 30.3% in the first, second year and the total for the entire 2 years, respectively. Thus, for N balance studies in ecosystems, it is necessary to destructively sample all plant parts in order to obtain accurate estimates of the contribution from BNF.