FIELD ESTIMATION OF N-2 FIXATION BY ACACIA SPP USING N-15 ISOTOPE-DILUTION AND LABELING WITH S-35

Biological N2 fixation by understorey legumes Acacia melanoxylon and Acacia mucronata in a mixed eucalypt forest was estimated by both natural N-15 abundance and N-15-enriched isotope dilution methods over a period of 27 months following a prescribed fire. A native grass, Poa sieberiana, and opportunistic non-fixing species were used as reference plants. The natural N-15 abundance method was considered to provide accurate estimates of N2 fixation because of uniformity in natural N-15 enrichment and high deltaN-15 values in soil and reference plants. Estimates of N2 fixed by isotope dilution of applied N-15 were not consistent with those obtained by the natural N-15 abundance method. Estimates of N2 fixed by A. melanoxylon and A. mucronata increased from 2.0 and 1.4 mg per plant after 1 year to 26 mg per plant and 57 mg per plant after 27 months, respectively. Accretion of N to the forest ecosystem from N2 fixation by these Acacia species was low (5 and 116 g ha-1 after 27 months) due to low plant densities following prescribed fire. Labelled S (S-35) was applied to the plots at sowing with labelled N (N/S = 10/1). Legumes accumulated significantly less dry matter and total S compared with the reference plants, although total N uptake was similar. Reference plants accumulated more labelled S and generally more labelled N than the legumes, and ratios of labelled to unlabelled S for all species were higher than ratios of labelled to unlabelled N derived from soil sources. Equal ratios of labelled to unlabelled S between legumes and reference plants did not correspond with equal ratios of soil-derived labelled to unlabelled N. These results therefore cast doubt on the validity of the concept of labelling with S-35 to assess the accuracy of reference plants to estimate the isotopic composition of soil-derived legume N.