NITROGEN FIXATION BY NODULATED ALNUS RUGOSA

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Abstract
Quantitative data from solution cultures of Alnus rugosa are presented which indicate facultative independence of this species regarding combined nitrogen supply and its close similarity to A. glutinosa in efficiency of N-fixation and physiology of nodule symbiosis.Volume–weight measurements and chemical analyses are given for soils and soil – surface organic matter in stands representative of succession involving nodulated A. rugosa, confirming that this species is capable of contributing greatly to nitrogen levels of the soil. Total seasonal litter-fall from woody species in several stands was collected and analyzed for nitrogen. Annual amounts of litter-N returning to the ground appear to be substantially greater under alder and alder–forest phases than under those dominated by Salix and Salix–forest vegetation. On favorable sites, soil nitrogen may accumulate under Alnus rugosa thickets at rates in excess of 150 lb/acre per year.However, no excess nitrogen accumulation was found in sandy soil supporting the Alnus shrub phase of a dry upland "old-field" succession and an explanation for this has been advanced.The past and present ecological position of Alnus species in northeastern North America is related to their capacity to fix nitrogen.