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

Abstract

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 15 N 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 (Omithopus 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 N2fixation (BNF).