Nitrogen Fixation by Ceanothus.

The rapidity of successioni of Ceanothus species in burned-over areas where they are indigenous and the apparent enhancing effect which these species have on the development of other shrubs and ground cover suggests that Ceanothus has a soil-improvement effect and implies the fixation of nitrogen. Nodulation of some species of Ceanothus was observed as early as 1912 (3). Bottomley and also Petrie, according to Snyder (6), isolated a microorganism from the nodules of C. americanus and conlsidered it probable that this was a nitrogen-fixing combination. The enhancing effect which Snow Brush (Ccanothus initegerri-i u(s) has onl the growtlh of Sierra Gooseberry has been reported (5). Using a direct isotopic tracer method, the fixation of nitrogen by Ceanothus a_.urcus was (lemonstrate(l (1) anld evidence has been obtained of nitrogeni fixation by Cea,nothus integerrinins (7). Using isotopic nitrogenl as a tracer, we have investigated the nitrogen-fixinig capacities of several species of Ccanothuis in their niative environlment in various locations in Northern California. WNith every species tested nodulate(d specimens were founlll. and excised nodules exhibited a strong fixation of nitrogen when compared with unnliodulated root tissues and other plant materials.