Field estimation of N 2 fixation by Acacia spp. using 15N isotope dilution and labelling with 35S

Abstract

Biological N 2 fixation by understorey legumes Acacia melanoxylon and Acacia mucronata in a mixed eucalypt forest was estimated by both natural 15N abundance and 15N-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 15N abundance, method was considered to provide accurate estimates of N 2 fixation because of uniformity in natural 15N enrichment and high δ 15N values in soil and reference plants. Estimates of N 2 abundance method. Estimates of N 2 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 N 2 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 ( 35S) 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 35S to assess the accuracy of reference plants to estimate the isotopic composition of soil-derived legume N.