Biological nitrogen fixation estimated by 15N dilution, natural 15N abundance, and N difference techniques in a subterranean clover-grass sward under Mediterranean conditions.

摘要

In an Eutric Leptosol of southern Portugal, N2 fixation by subterranean clover (Trifolium subterraneum) mixed with grasses (Lolium perenne and Dactylis glomerata) was evaluated over 2 years by 15N dilution (ID), natural 15N abundance (NA) and N difference (ND) techniques. The amount of fixed N2 in subterranean clover determined by natural 15N abundance with correction for isotopic fractionation ranged from 32 kg N ha-1 in 1992/93 to 37 kg N ha-1 in 1993/94. Assuming that no isotopic fractionation occurred during fixation, fixation capacities at around 80% of nitrogen derived from the atmosphere (Ndfa) were found by the natural 15N abundance technique. However, with correction for isotopic fractionation during N2 fixation, fixation capacities closer to 50% Ndfa were obtained, similar to the value obtained in the second-year experiment with the isotopic dilution method. In year 1, the fixation capacity, as estimated by the isotopic dilution method, was about 37% Ndfa. The nitrogen difference (clover N-grass N) calculations underestimated the amount of N2 fixed in 1992/93. This method assumes that both legumes and non-legumes absorb the same amount of N from the soil, which may not be true. Also, only the above-ground legume herbage was analysed inthis experiment. It is suggested that either ID or NA methods, particularly with a correction for the B-value, can be used to estimate N2 fixation in mixed pastures, rather than the ND method. The natural 15N abundance (NA) technique can be more versatile than the ID technique, allowing frequent sampling in undisturbed grassland ecosystems with reduced costs. Nitrogen fixation decreased in June, in both years, probably due to a lower soil water content and higher soil temperature.