Nitrogen Fixation by Ceanothus

Abstract

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-improve-ment effect and implies the fixation of nitrogen. Nodulation of some species of Ceanothus was ob-served as early as 1912 (3). Bottomley and also Petrie, according to Snyder (6), isolated a micro-organism from the nodules of C. americanus and conl-sidered 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 nitro-gen 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 in-vestigated 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 tis-sues and other plant materials. Materials and Methods Nodulated plant specimenis were collected in the field and tissues exposed to isotopically labeled nitro-gen as soon after collectionl as practicable. Nodule material was placed in the test atmosphere within 15 to 20 minutes after removal from the intact plant. Suitable nodule clumps of approximately 2 g fresh weight were excised from the plant. Adhering soil was washed off with water an(l the tissues trans-ferred to a glass-stoppered tube of the type showni in figure 1. As controls. unnodulated root segments were placed in similar tubes and in some cases leaf tissue from the same plant or the blades of Gramiiinae were used as additional controls. The tubes were then evacuate(d to the vapor pressure of Nvater and I Received April 26, 1965. an atmosphere conisistinig of 5 cmi of N.,)'-' and(20 cm of O., were introducedl. The N'-used had been previously passed throughl anl ascarite colunin to re-move small amiiounts of conltaminiatinig oxides of ni-trogen. The tubes Nvere thleni closed off excepting those which xwere zero time controls inlto which air was admitted to atmiosplpheric pressure. After anl incubationi period of 5 houtrs the reactioni was stopped by the a(dmission of air. Uponi returln to the labor-atory, the plant tisstues Nvere (ligested by a mlicro-Kjeldahl l)rocedure anid the nitrogeni obtained ana-lvzed mass spectrometrically for its N", conitenit. Results As cal l)e seeln b referenice to tal)le I. quianitities of niitrogell as great as 200 nianiomiioles/g fresh tissue were fixe(d in a 1-hotur l)erio(l. These rates are rela-tivelv smiiall in termiis of the nitrogeni requiremiienlts of the planit l)tlt conditions probably were not ol)timiial and muitich higher rates undoubtedly occur in the in-tact plant unider good growing coniditionis. N'o quan-titative signiificance is attached to the fact that the amounit of fixationi observed (liffere(l greatly froml one species to aniother. TI'he niodulate(d tisstues uised were obtainied fronm widely varying enivironmllenits and fronl plants in various stages of development as well as at (lifferent times in the growiing -easoni. 'Most collectionis were mlade (Idirinig the imiolntlhs of January FTG. 1. Typical nodules of C. gloriosus Hoowell shown with tube used for incubation. 1045 Table 1. Ii.raltio of A.N,',-1, b r-Mioits SPccics of Ceanlothus All speciimens inicubated for tille showxn in atlmiosphere of N.,' and O., (see text). All species except sor(diatffs uind dizaricatus collecte(d durinig imionitlh of January; latter 2 earlv February. N'5 used: 52 atom percenit excess N' excel)t for sorcdiutos anid divarafiatus mxhichl was 96 atomlpercent excess '5. The small fixation observed for somlle salllples of uninodlulated root segments p)robably represents inadvertent conitaminiationi with niodules. Fixation of miore tlhani I nimole N per g per hour is cn5sidered sigllificanit (P < 0.1). Species C .lOcamlJs T and G