Nitrogen Fixation by Ceanothus'
C. C. Delwiche, Paul J. Zinke', and Clarence M. Johnson3
Kearney Foundation of Soil Science, University of California, Davis, California
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.
Materials and Methods
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 remove small amiiounts of conltaminiatinig oxides of nitrogen. 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 laboratory, the plant tisstues Nvere (ligested by a mlicroKjeldahl l)rocedure anid the nitrogeni obtained analvzed 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 relativelv 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 intact plant unider good growing coniditionis. N'o quantitative 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. 'M ost
collectionis were mlade (Idirinig the imiolntlhs of January
Nodulated plant specimenis were collected in the
field and tissues exposed to isotopically labeled nitrogen 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 transferred 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
Received April 26, 1965.
'Present address: School of Forestry, UIniversity of
California,
Berkeley, California.
3 Present address: Departm.rent of Soils and Plant
Nutrition, University of California, Berkeley, California.
I
FTG. 1. Typical nodules of C. gloriosus Hoowell
shown with tube used for incubation.
1045
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10(4()
PLA.\N T PHY1\SIOLOGY
b r-Mioits SPccics of Ceanlothus
Table 1. Ii.raltio of A.N,',- 1,
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).
Rate of
Nitrogen
Portion
of
plant
Roots
Nodules
Species
C .lOcamlJs T
and G
Inicubation
content
period
( jmoles/g
fr x-t)
(hr)
I
5
(C.bitccrri-,mits H and A
Roots
Nodules
.,
C.jcpsonsii Greene
C.prostratas Benth
Roots
N'odule.,
C.foliosiis Parry
C.rim-cantnts (Hook) Nutt.
I
..
Roots
Nodules
C.VL'lntistS I)ougl.
(C.thyrsifloris Esch.
C.glor0iosos Howell
var. c.raltatls
(vsoredia(ts 1H ani(l A
5
I
5
I
,.
C'
II
Roots
(.divricatilts Nlutt.
C.Griscuts ( Trel.) M cM inn
N d11
Noduiles
.I
anl(I February, at wvxhich timle the moisture conitelnt of
tlle soil was sufficiently high to permit the removal
of plants from the soil ,with nodules intact. Vegetative activity at this season is consi(lerable, althou(gl
because of the relatively low temperature ( 5-100)
growth is not as vigorotus as it would be during the
spring or auttuimni seasonis provi(led imoisture supply
is adequate.
Discussion
Because it is tliffictilt to mlake an accurate determiiniation of the total amiiount of niodulated imaterial
per plant anid because annual variations in the rate of
nitrogein fixationi have not been evaluated, it is not
possible to make any but the most rough estimate
of the extent to which these niitrogen-fixinig reactions
contribute to the overall nitrogen economy of the
ecosystem. Nevertheless, assuming that a Ceanoth us
shrub occupying a projected area of 1 square meter
would have a total miass of nodtules of 100 g fresh
weight andCi that these are fixing nitrogenl at the rate
of 10 umoles/g fresh weight per hour for 50 % of
the year, some reasonlable figures canl he, obtained.
An area onie-tentlh of which wNas occupied 1y Coniothis wotil( be receivingnitr-ogen1iw the symnbiotic-
Atom %
excess
N 15
170
190
115
130
125
115
122
160
105
135
75
175
8(
1 25
145
0.0005
0.0028
0.0283
0.0055
0.0233
0.1576
0.1150
0.0001
0.0443
0.0460
0.0015
0.3047
0.1012
0.0793
88l
114
1 50
82
0.0062
0.365
0.412
0.372
0.172
fixationi
(inmoles/hr/
g fr wt)
0.34
10.6
13.0
2.9
58.3
7'2.0
56.0
93
.
93.0
0
25.0
0.45
212
32.0
39.6
1(00
0.5 5
43.3
61.8
30.4
process at the rate of approximately 60 kg ler hectare per year. Although sticll a nitrogen fixation
rate is inot spectacularly higli, tlle niitrogeni fixed is
well placed arid immediately available to the plant.
By conlparisoll with estimates of niitrogeni fixed1by
free-living organismiis, these figures are quite high.
On the basis of a rising nitrogen profile in the Nile
Valley alltivitim, calculations of the nitrogeni fixatioln rate hy free-living organismiis anlouilts to 13.5
pollids per acre (15 kg/hectare) per year (2). Ohservilig successioni rates Oil Sotitherni AMichigan sandl
(Itliles, the fixation rate is estimated as 4.03 kg per
lhectare annually (4).
The sl)ecies of Ccaniothus exailiieed ranged froi
small prostrate plaaits (such as Ccn aiotsilS prostrahils)
rarely reaching a height of more than 10 cml Zand
formiing thick mnats oni the ground, to large erect anld
almost tree-like plants (such as Ceaitothuis zelutithus)
reaching- heights tip to 4 ni. The extent to wvhich
these plants contribute fixed nitrogen to the ecosystelll, besides being a ftinictionl of poptilation derisitv
alndl plant size. undotubtedly also lhas somiie species
variationis which as vet have not been evaltiated.
The fact that noine of the species examilned fail to fix
ilitrogenl indicates tilat this -.property probably is
characteristic of the genus Ccanlotlhuls ill genleral.
The Nx ide distribtition of these pl)lants iakes, tlhei
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DEL\WICIHE ET AL.-NITROGEN FIXATION BY CEANOTHt S
significanit contributors of fixed mitrogeni
to
native
1047
Literature Cited
associations.
Although a number of workers have succeeded in
obtaining nodulation of Ceaniothuls planits by suspensionls of organisms obtained from crushed nodules,
the enidophyte has not been studied. In their gross
morphology the nodules are niot unilike those of leguminous species (see fig 1). The evenitual isolation
of pure cultures of the endophyte will make possible
their characterization as well as the determination
of any species specificity and cross ilnoculation characteristics.
Summary
By direct application of isotopic tracer technliques in the field it has been possible to examine
niodulated specimens of the genus Ceaniothuis for the
ability to fix nitrogen. All species examined were
found to fix nitrogen at rates which are ecologically
signiificant.
1. BOND, G. 1957. Isotopic studies of nitrogen fixation in nonlegume root nodules. Ann. Botany,
N.S. 21: 513-21.
2. JENNY, H. 1962. Model of a rising nitrogen profile in Nile Valley alluvium, and its agronomic and
pedogenic implications. Soil Sci. Am. Proc. 26:
588-91.
3. JEPSON, W. L. 1936. In: The Flora of California,
Vol. II. University of California Press, Berkeley.
p 460-62.
4. OLSON, J. S. 1958. Rates of succession and soil
changes on Southern Lake Michigan sand dunes.
Botan. Gaz. 119: 125-70.
5. QUICK, C. R. 1944. Effects of Snow Brush on the
growth of Sierra gooseberry. J. Forestry 42:
827-32.
6. SNYDER, R. M. 1925. Nitrogen fixation by nonileguminous plants. Mich. State Agr. Expt. Sta.
Quart. 8: 34-36.
7. VLAMIS, J., A. M. SCHULTZ, AND H. H. BTSWELI.
1958. Nitrogen fixation by Deerbrush. Calif.
Agr., Jan. issue. p 11.
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