Biological nitrogen fixation of some legumes at a coastal sand dune forest site in New Zealand

Field experiments were conducted at Woodhill Forest, near Auckland for a period of one year (July 1994 to July 1995), in a clearfelled, first-rotation replanted Pinus radiata stand. The study was aimed at screening three legumes namely: Maku lotus (Lotus pedunculatus Cav. "Grasslands Maku"), hairy canary clover (Dorycnium hirsutum (L) Ser.), and everlasting pea (Lathyrus latifolius L.) (Experiment 1), as possible replacement for the yellow tree lupin (Lupinus arboreus Sims.). The seasonal biological N₂ fixation (BNF) of the legumes was monitored using the ¹⁵N isotope dilution technique and the acetylene reduction assay (ARA). Biological N₂ fixation was also monitored in an operational area sown with a mixture of serradella (Ornithopus sativus Brot. "Grasslands koha") and Lotus pedunculatus (Experiment 2). Sampling was carried out in winter (July) and spring (November) of 1994, and summer (February) and winter (July) of 1995. Dorycnium showed highest biomass which did not differ significantly between seasons. This species also accumulated the highest dead dry matter yield. The accumulation was probably due to its relatively low N concentration in the dead dry matter (0.6% N) compared to those of Lotus and Lathyrus (1.3 and 1.4% N, respectively). Dorycnium and Lathyrus derived on average, 98 and 95% of their annual N from the atmosphere respectively, which was not significantly affected by season. Lotus on the other hand showed a significantly lower proportion of nitrogen derived from the atmosphere (%Ndfa) between spring and summer than between winter and spring. This was probably a result of high inorganic soil N derived from the decomposing plant material as well as the high temperatures and dry conditions experienced during summer, all of which led to low biological N₂ fixation (BNF). Using the ¹⁵N isotope dilution technique, Lathyrus was estimated to have fixed significantly high annual amounts of N (214 kg N ha⁻¹ yr⁻¹), compared to 55, 71 and 133 kg N ha⁻¹ yr⁻¹ fixed by Lotus, Dorycnium and legumes in experiment 2, respectively. The annual amounts of N₂ fixed by Lotus and Dorycnium were not significantly different. For all the legumes, most of the N₂ was fixed in the period between winter and spring probably due to more dry matter yield increment and the favourable weather conditions during spring. Due to its high N₂ fixation, low accumulation of dead dry matter and high dry matter increment, Lathyrus seems the most suitable species for the replacement of the lupin. In both experiments no N was found to have accumulated in the mineral soil after 3-4 years of legume growth. The variability in such a site and the difficult in the detection of total N makes N accumulation studies unsuitable. The ARA as applied in this study showed trends that could only be explained on the basis of the prevailing weather conditions during the assay. In most cases the method also showed lower N₂ fixation values when compared to the ¹⁵N method. The under-estimation of N₂ fixation in the ARA was probably due to errors introduced as a result of extrapolation after a short period of incubation, diurnal variations in light, temperature and moisture in the period prior to sampling and the failure to have the conditions of the assay matching carefully with those of N₂ fixation at the time of sampling. The ¹⁵N isotope dilution technique was better placed for field estimation of N₂ fixation due to its ability to give integrated values.