NITROGEN FIXATION BY LUPINUS ARBOREUS GROWN IN THE OPEN AND UNDER DIFFERENT AGED STANDS OF PINUS RADIATA

Abstract

An intensive 3‐month study of nitrogen fixation by Lupinus arboreus grown on open sand dunes and under different aged stands of Pinus radiata in New Zealand has been carried out. Nodule samples were obtained either by digging whole seedlings or by sieving 0.01 m3 soil cores. Nitrogen fixation as estimated by the acetylene reduction assay and 15N analysis agreed well with published data from biomass studies. On open dunes nodule growth and activity increased rapidly during spring and estimated nitrogen fixation reached a peak of 11.5 kg N ha‐1 week‐1 at about flowering. When fruits began to swell, both on open dunes and in forest sites, nitrogen fixation by lupins decreased. Light transmission in forest sites was not correlated with the time of pine thinning. Lupin growth, nodule biomass and nitrogen fixation in non‐flowering lupins was closely related to the amount of light transmitted. When the sand became very dry nitrogen fixation was reduced and moisture stress was considered to be a factor likely to limit nitrogen fixation in drier parts of the year. Nitrogen fixation by lupin within forest stands on sand dunes is sufficient to make a significant contribution to the nitrogen economy of the forest. Copyright © 1973, Wiley Blackwell. All rights reserved