665
INTO THE MINERAL
INVESTIGATIONS
EXPERIMENTAL
NUTRITION OF SEVERAL GRASS SPECIES
IV. NITROGEN
LEVEL
BY A. D. BRADSHAW, M. J. CHADWICK,* D. JOWETTt
R. W. SNAYDON +
AND
CollegeofNorthWales,Bangor
Botany,University
of Agricultural
Department
INTRODUCTION
(Bradshaw,Lodge,
withgrassspeciesin thisseriesofinvestigations
Previousexperiments
& Snaydon1960b)
Lodge
Jowett& Chadwick1958,1960a; Bradshaw,Chadwick,Jowett,
in sand
determined
have shownthatresponsesto variationin individualsoil nutrients,
these
of
behaviour
and waterculture,can throwconsiderablelighton the ecological
nitrate
to
in
response
variation
alreadyinvestigated,
species.In additionto thenutrients
nitrogenseemedespeciallyworthyof investigation.
recognizedas
to be specifically
Nitrogenappearsto havebeenthefirstmineralnutrient
thatnitrogen
accepted
it
is
now
generally
and
1961),
for
(Russell
necessary plantgrowth
nutrient
factors
limiting
soil
most
and
deficient,
most
generally
of
important,
is one the
of
levels
of
sub-optimal
The
prevalence
general
of
1957).
thegrowth cropspecies(Black
complant
and
to
semi-natural
also
natural
in
situations
applies
nitrogen agricultural
of thesecommunities,as is shownby the numerousrecordsof increasedproductivity
Voisin
&Lessells
1959,1960).
1954;
in
Boyd
nitrogenfertilization(e.g.
munities responseto
naturalhabitatsis knownto varywidely
The nitrogencontentofthesoilsofcontrasting
be expectedthatat least some of the
(e.g. Olsen 1921; Millar 1955).It might,therefore,
of thesecontrasting
variationin botanicalcompositionbetweenthe plant communities
habitatsis attributableto the variationof soil nitrogencontent.This in turnimplies
variationbetweenplant species in responseto nitrogensupply.Because of the comeven
petitiveconditionsexistingwithinnaturaland semi-naturalplant communities,
betweenspeciesin responseto nitrogenmightlead to large
relativelysmall differences
small
in responseto relatively
in botanicalcompositionbetweencommunities
differences
in soil nitrogenlevel.Thereis, in fact,a certainamountofevidenceindicating
differences
in responseto nitrogenand thatthe botanicalcompositionof plant
that speciesdiffer
levelsof soil nitrogen.This evidence
variesgreatlyin responseto different
communities
is briefly
surveyedbelow.
(i) Some wild specieshave been shownto possess definite'nitrophilous'habits,e.g.
and Rubusidaeus(Hesselman1917).
Urticadioica(Olsen 1921),Epilobiumangustifolium
in responseto nitrogenbetweenUrticadioica and otherspecies
Recentlythe difference
affinities
withnitrophilous
(Filipendulaulmaria,Deschampsiacaespitosaand Mercurialis
(Taylor1963; Pigott& Taylor1964)as has therangeof
perennis)has been demonstrated
reactionexhibitedby a numberof otherwildspecies(Mayser1954).At theotherend of
* Now at: SchoolofAgriculture,
ofCambridge.
University
ResearchOrganisation,
and Forestry
Serere,Uganda.
t Now at: East AfricaAgriculture
ofReading.
ofAgricultural
Botany,University
I Now at: Department
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666
Mineralnutrition
ofgrasses.IV
the scale speciessuch as Callunavulgaris(Gessner1932) and Nardusstricta(de Coulon
1923)have thereputationofbeing'nitrophobes',
and Atriplexhastatahas been shownto
growwell underconditionsof nitrogendeficiency
(Rees & Sidrak 1956).
(ii) Differences
in responseto nitrogenbetweenvariouscrop plantshave long been
realized(e.g. Russell 1961; Crowther,Tomforde& Mahmoud 1937; Crowther& Yates
1941).In vegetablessuchdifferences
have been shownto,be verymarkedin experiments
in sand culture(Woodman 1944). An awarenessof differences
in responseto nitrogen
betweeneconomicallyimportantgrass species underliesmuch presentday grassland
management,and certainspecificexperiments
(Remy & Vasters1931; Lovvorn 1945)
suggestthatsuchdifferences
betweenspecies,inresponseto nitrogen,
maybe considerable.
Recently,experiments
in sand culturehave shownthatweedspeciesmaydiffer
markedly
in theirnitrogenresponse;theseresponsesare similarto thoseof the cropswithwhich
theyare oftenassociated(Chadwick& Obeid 1963).
Table 1. Composition
of theculturesolution
Nutrient
N
Ca
K
P
Mg
Mn
Cu
Zn
Mo
Fe
B
Concentration
(ppm)
1-81
243
50
78
16
37
0 55
0-064
0-065
0 019
2
0 37
Form of nutrient
NaNO3
NaNO3(180), KNO3(28), Ca(NO3)2(35)
CaCl2 (or Ca(NO3)2 at 243 ppm N)
K2SO4 (or KNO3 at 243 ppm N)
NaH2PO4
MgSO4, MgCl2 (equal amounts)
MnSO4
CuSO4
ZnSO4
(NH4)6Mo7024
Fe-citrate
H3BO4
fertilizers
have
have been carriedout in whichdifferent
(iii) A numberof experiments
for a period of years.
been applied to permanentgrasslandcommunitiescontinuously
theresultsshowthataddition
treatment
haveincludeda nitrogen
Wheresuchexperiments
of nitrogencan greatlychangethe botanicalcompositionof a sward(Brenchley1958;
Siebold 1958; Murphy 1960; Pawson 1960; de Vries & Kruijne 1960; Willis 1963).
to a saltmarshalso greatlyalteredthebotanicalcompositionof
The additionofnitrogen
the vegetation(Davies & Jones1932).
(iv) Finally,thecorrelative
approachappliedto naturalplantcommunities,
despiteits
pitfalls,indicatescorrelationsbetweenthe distributionof species and variationsin
nitrogenlevelwhichare stronglysuggestiveof causal relationships
(Olsen 1921; Ellenberg1952).
INVESTIGATIONS
as in thepreviousexperiments
The same specieswereused in thepresentinvestigations
also originating
fromthe
material
the
withphosphorus(Bradshawet a!. 1960b),
plant
also
similar
to
those
were
used
same sources.The cultureand harvestingtechniques
nutrient
solution
of
the
used
is
a!.
The
previously(Bradshaw et
1958).
composition
At
the
of
alteration
in
Table
was
varied
1. Nitrogen
NaNO3.
presented
by
highest
in orderto preventexcessively
nitrogenconcentration,
highlevelsof sodium,thepotassiumand calciumwereintroducedas nitratesinsteadof sulphateand chloride,and the
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667
A. D. BRADSHAWet al.
accompanied
levelweretherefore
The variationsin nitrogen
NaNO3 adjustedaccordingly.
of
byvariationsin sodiumlevel(and, at the highestnitrogenlevel,in theconcentrations
sulphateand chlorideions) but such variationsare unavoidablein sand culture.Since
by variationin sodium,wereall at high
whoseuptakemightbe affected
othernutrients,
at
due to thisoccurred,althoughsome effect
levelsit was unlikelythatanylargeeffects
the highestlevelcannotbe completelyprecluded.
was carriedout in an unheatedglasshouseduringMay and June1959.
The experiment
Growthtook place over8 weeks.Because ofpossiblelocalizedvariationofenvironment,
and in orderto increasethe precisionof the estimationof the species x nitrogeninterthewholeplotswithineach
action,a split-plotlay-outwas adopted,speciesconstituting
of thefourreplications.
Lolium perenne
Agrostis stolonifera
_
5.0
40 (-)
()
3-0T
220-
-
'
//
->{'
-
-
0~~~~~~~~~~~~~~~~~~~~~~~10
- 1-
C
-
-
Agrostislenuis(normal)
~Agrostis tenuis(lead resistant)
cristatus
Cynosurus
-Agrostis
-
_"j
A*
Nitrogen (ppm)(log
,
canina
t XFestuczaovina
scale)
yieldof severalgrassspeciesin sandculturein relationto
FIG. 1. Variationin dryweight
barsrepresent
L.S.D. at 500 fordifferences
Thevertical
nitrogen.
inlevelofnitrate
variation
level.
anyone species,and (ii) at anyone nitrogen
(i) within
The dryweightyieldvalues (totalsof shootplus root),on a logio basis, are givenin
in orderthattheresponsesofthespecies,
Fig. 1. The valueswereexpressedlogarithmically
greatlyin overallyield,couldbe comparedin termsofproportionalrather
whichdiffered
was thatanalysis
An added advantageofthetransformation
differences.
thanarithmetic
(Table 2), sincethere
data was possibleforall speciessimultaneously
of thetransformed
afterlogarithmic
betweenspeciesor treatments
in variability
difference
was no significant
transformation.
pointwhichmaybe notedis thegenerallevelofyieldattainedbyeach species.
The first
yields;
Loliumperenneand Agrostisstolonifera
generallyshow the highestdry-weight
of
levels
tenuis
show
intermediate
of
Cynosuruscristatusand both ecotypes Agrostis
yield.
of
low
levels
yield;A. canina,Festica ovinaand Nalrdusstrictaexhibitgenerally
The formof the speciesresponseto variationsin nitrogenconcentration
variesconnitrogen
speciesx
(P<OOO1)
siderably.This is apparentfromthe highlysignificant
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All use subject to JSTOR Terms and Conditions
ofgrasses.IV
Mineralnutrition
668
interaction
(Table 2). Inspectionof the data suggeststhatall speciesshowa responseup
linearly.Linear regressioncoefficients
to 27 ppm N levelthatcan best be characterized
foreach speciesforthe levels 1-27 ppm N foreach replicate
determined
weretherefore
and theseweresubjectto an analysisof variance.The resultsofthisanalysisare givenin
Table 3. Distinctdifferences
betweenspeciesin the degreeof responseoverthispartof
values)
Table 2. Analysisof varianceof dryweight(logarithmic
SS
Replicates
Species
Wholeploterror
level
Nitrogen
x species
Nitrogen
Sub ploterror
Total
d.off.
3
0 0018
8 6430
7
0-4531
21
5
20 3298
3-9372
35
1 1077
120
34 4707
191
*** P<0.001.
MS
F
0 0006
1-235
0-0216
4 067
0-1125
00092
57.2***
4406***
12 2***
the curveare apparent.At theselevels Cynosuruscristatusand Nardusstrictarespond
The
yield,to increasein nitrogenconcentration.
least,in termsof increaseddry-weight
show the greatest
lead resistantecotypeof Agrostistenuis,A. caninaand A. stolonifera
response,thetwoformerspeciesbeingthelowestyieldersat 1 ppm N. The otherspecies,
responsesat theselowerlevelsof nitrogen.
includingLoliumperenne,showintermediate
Table 3. Regressionof log dry weightyield on log nitrogenlevel over
therange1-27ppm(log nitrogen
levelscoded 1-4)
Replicates
Species
Error
Total
(a) Analysisofvariance
SS
d. of f.
MS
0 000655
3
0-001964
0 159667
7
0 022810
21
0-027902
0-001328
31
0-189533
F
17-176***
*** P<0.001.
forindividual
coefficients
species
(b) Linearregression
0 347
tenuis(lead resistant)
Agrostis
A. stolonifera
A. canina
A. tenuis(normal)
Loliumperenne
Festuca ovina
Cynosuruscristatus
Nardus stricta
L.S.D. at 5 % prob.
L.S.D. at 1% prob.
0-342
0 309
0-277
0-248
0-217
0 189
0-134
0-052
0.072
Over the range 27-243 ppm N even more diversepatternsof responseare evident.
Nardusstrictaand Agrostiscaninaboth producemaximumdryweightyieldat 27 ppm
N. But whereasNardusstrictashowsa highlysignificant
declinein yieldabove 27 ppm
equally over the range 27-243 ppm N. Two
N, Agrostiscanina yieldsapproximately
cristatus,
producemaximumyieldsat 81 ppm N.
species,Festucaovinaand Cynosurius
a non-significant
decline
cristatus
Festucaovinashowsa significant
decline,and Cynosurus
above thislevel,althoughthereis no significant
difference
betweentheresponseof these
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669
et al.
A. D. BRADSHAW
and
two speciesoverthewholerange1-243ppm N. Loliumperenne,Agrostisstolonifera
thetwopopulationsof A. tenuisyieldmostat 243 ppm N, thehighestlevelused. But the
less overtherange9-243 ppm N than
twopopulationsof A. tenuisrespondsignificantly
do Loliumperenneand Agrostisstolonifera.
Agrostis stolonifera
5
Nardus stricta
()
4
o
0
11
(ii'X)
3_
/I
<
/
o
Cl
X
Nf
jou
/
Agrostistenuis(normal)
ovina
-.,_Festuca
. . .
Lolhum perenne
>,Agrostls tenuis(lead resistant)
,,
/
, .
Agrostis canmna
ynosu rus c rista tus
~~~~~~~~~~~~~~~C
0
0-
1
9
3
27
81
243
Nitrogen (ppm) (log scale)
FIG. 2. Variation in shoot root ratio of several grass species in sand culturein relationto
variationin levelof nitratenitrogen.The verticalbars representL.S.D. at 50 fordifferences
(i) withinany one species, and (ii) at any one nitrogenlevel.
Table 4. Analysisof varianceofshoot: rootratio
Replicates
Species
Whole plot error
Nitrogenlevel
Nitrogenxspecies
Sub plot error
Total
SS
MS
d. off.
4-5619
3
1P521
4 909
7
34-3667
7-0005
21
0-333
26 647
133-2361
5
35
0-846
29-5988
0-272
32 6702
120
241-4342
191
* P<o-05,
*** P<o0oo1.
F
4-56*
14-73***
97 90***
3-11***
Since the plants were grownin sand cultureit was possible to harvesttheirroots
completelyand calculateshoot: root ratiosbased on dryweightvalues. The values are
in this
givenin Fig. 2 and theanalysisofvariancein Table 4. As in previousexperiments
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670
ofgrasses.IV
Mineralnutrition
inthesevaluesbutcertaintrendscan be
series,thereareconsiderablerandomfluctuations
Cynosuruscristatushas the lowestoverallshoot: rootratioand Agrostis
distinguished.
level.
thehighest.Nardusstrictashowsa veryhighratioat thehighestnitrogen
stolonifera
of
The generaleffectof increasingnitrogenis to increaseshoot: root ratioirrespective
the effecton yields.
(arc sin) and subjectedto an analydryweightvaluesweretransformed
The percentage
reducedpercentagedry
sis of variance(Table 5). Increasingnitrogenlevel significantly
interaction,
(P
0f01)speciesx nitrogen
weightin all species.Therewas also a significant
some speciesshowinga more markedreductionin percentagedryweightthan others.
Nardus strictaand Lolium
The greatestreductionwas shown by Agrostisstolonifera,
perennewhileAgrostistenuisand Festucaovinashowedscarcelyany reduction.
Table 5. Percentagedryweightof shoots(arc sin transformation)
1
27.7
Loliumperenne
stolonifera 29-6
Agrostis
26-2
A. tenuis(n.)
25 9
A. tenuis(l.r.)
cristatus 27-7
Cynosurus
canina
26-4
Agrostis
29-3
Festucaovina
34 8
Nardusstricta
3
26 3
26-8
26-4
26 9
26-8
27-0
29-8
33 2
Nitrogen(ppm)
27
9
25 6
24-9
25-9 25-6
26-6 26-0
26 7
26-8
26-4 25 8
26 9
26-5
28 3
28 9
32-5 32-0
81
24-8
23-5
25-7
25 4
25-5
25-3
27.7
30 8
243
23-8
22 4
23-8
24-4
24 4
24-7
27.8
309
L.S.D. fordifferencesbetweenN levelswithinspecies 1-30
betweenspeciesat anyone N level1-50
DISCUSSION
show verydifferent
It is evidentfromthe resultspresentedthatthe speciesinvestigated
patternsof responseto nitratenitrogenin sand culture,and, apart fromFestuca ovina
no two speciesreactsimilarly.The least responsivespeciesis
and Cynosuruscristatus,
Nardusstricta;thisproducesmaximumyieldat 27 ppmN, showsonlya twofoldincrease
in yieldovertherange1-27 ppm N, and yieldsless at 243 ppm N thanat 1 ppm N. In
producesmaximumyieldat 243ppmN and showsa twentycontrastAgrostisstolonifera
fold increasein yield over the range 1-243 ppm N. Because of the greatdiversityof
patternsof responseit is not easy to rankthe speciesin orderof responseto nitrogen;
criteriamightbe used, such as: optimumnitrogenlevel,magnitudeof
severaldifferent
levels,response
yield),responseat low nitrogen
maximumresponse(maximum/minimum
used. The order
accordingto thecriterion
at highlevels,etc. The orderof speciesdiffers
for optimumlevel is: N. stricta= A. canina< C. cristatus F. ovina< A. tenuis
(normal)= A. tenuis(lead resistant) A. stolonifera L. perenne.The order for
response1-27 ppm N is: N. stricta< C. cristatus< F. ovina< L. perenne< A. tenuis
= A. tenuis (lead resistant).The order for
(normal)< A. canina< A. stolonifera
maximumresponseis: N. stricta< C. cristatus< F. ovina< A. canina A. tenuis
Anydecisionis made
(normal)< L. perenne< A. tenuis(lead resistant)< A. stolonifera.
cannot
be
relatedback to any
more difficult
by the factthat the nitrogenlevelsused
notpossibleto ascertainwhether
and itis therefore
known'normal'levelofsoil nitrogen,
thehighestand lowestlevelsusedmightnotbe outsidetherangeofnormallyencountered
soil nitrogenlevels.It is, however,possibleto determinea generalorderof responseby
thevariouscriteria,thisorderis: N. stricta< F. ovina C. cristatus< A.
considering
canina< A. tenuis(normal)< A. tenuis(lead resistant)< L. perenne< A. stolonifera.
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A. D. BRADSHAW
et al.
671
In viewof thelargedifferences
betweenspeciesin responseto nitratenitrogenin sand
culture,it would be interesting
to comparethe responseof thevariousspeciesas determinedin sand culturewiththeresponseas determined
in fieldconditions.Severalsources
of fielddata are available for such comparisons,particularlyinvestigations
into (a)
changesin thefloristic
compositionofpermanent
grasslandsas a resultoftheapplication
of nitrogenousfertilizers
and (b) correlationsbetweenthe distributionof species in
permanent
grasslandand nitrogen
levelsin thesoil. Unfortunately
ofthemanypublished
resultsunderthe category(a) above, manyincludeonlyone of the speciesstudied(e.g.
Brenchley1958; Remy & Vasters1931) and comparisonsare thereforenot possible;
onlya fewreportsincludemorethanthreeof the speciesstudied.A further
difficulty
is
that oftenno distinctionis drawnbetweenecologicallydiversespecieswithina single
froma number
genus,e.g. Agrostisspp. and Festucaspp. Nevertheless,
usinginformation
of sources,comparisonscan be made.
Firstlythose species occurringnaturallyon soils of low fertility
can be considered.
Fenton(1936) and Davies & Jones(1932) recordthat,in theabsenceof 'lowland'species,
Agrostistenuisand Festuca ovinaincreaseand Nardusstrictadecreasesin responseto
fertilizers
whilstMilton(1950) recordsthatsuch fertilizers
nitrogenous
increaseAgrostis
spp. and decreaseFestucaspp. Siebold (1958) recordedthe floristic
changesinducedby
fertilizers
at a numberof sitesand concludedthatthe orderof responseto
nitrogenous
nitrogenwas: N. stricta< C. cristatus< F. ovina< A. tenuis.
Field resultsare also availableforthe speciesshownhereto have a highresponseto
nitrogen.Many workers(e.g. Stapledon 1932; Murphy1960) have shownthatapplicaare necessaryforthe successfulestablishment
of Lolium
tions of nitrogenousfertilizers
Jones(1934) has shownthatwhilstnitrogenous
perenneon soilsof low fertility.
fertilizers
ofAgrostis
increasedtheproportionofL. perennein thesward,itdecreasedtheproportion
spp. and greatlydecreasedCynosurus
cristatus.De Vries & Kruijne(1960) collatedthe
resultsof a largenumberof fertilizer
experiments
and concludedthatin almostall cases,
fertilizers
increasedthe proportionof Loliumperennein the sward,caused
nitrogenous
no consistent
and caused theproportion
changein theproportionof Agrostisstolonifera,
cristatusand Agrostistenuisto decline.
of Cynosurus
all oftheresultsquotedabove,itwouldappearthattheorderofresponse
Summarizing
to nitrogenunderfieldconditionsis: N. stricta< C. cristatus< F. ovina< A. tenuis
< A. stolonifera
< L. perenne.Ellenberg(1952), using correlativemethods,classified
the species underconsiderationhere as: L. perenne-highrequirement;A. stolonifera
and C. cristatus-mediumto highrequirement;
A. tenuis-mediumto low requirement;
from
A. canina,F. ovinaand N. stricta-lowrequirement.
His conclusionstherefore
differ
theorderderivedfromfieldexperiments
onlyin thepositionof Cynosurus
cristatus.
Comparisonof theresultsobtainedby thesand culturetechniquewiththeabove field
data showsa veryclose correspondence
betweenthetwo setsof resultsobtainedby such
methods.Two discrepancies
thesimilarresponse
verydifferent
onlyare apparent.Firstly,
of Loliumperenneand Agrostisstolonifera
in sand cultureis at variancewiththe lower
responseof A. stolonifera
in the field.Secondly,althoughA. tenuis(normal)showed a
smallerresponsethanLoliumperenneand Agrostisstolonifera
significantly
at the higher
nitrogenlevelsin sand culture,thiswas less thanmightbe expectedfromthefieldresults
and A. tenuisactuallyshowed a slightlygreaterresponsethan Loliumperenneat low
nitrogenlevels.One otherapparentlycuriousresultconcernsthe lead resistantpopulationofAgrostistenuis,whichshoweda greaterresponseto nitrogenthandid the'normal'
thewholerangeand especiallyat low nitrogenlevels.At firstsight
populationthroughout
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672
ofgrasses.IV
Mineralnutrition
thisis unexpectedsincethelead minespoil,fromwhichthelead resistantpopulationwas
obtained,was highly'infertile'.There are, however,no data concerningthe available
nitrogencontentof the mine soils; sincethe lead resistantplantsgrewon the spoil as
fromotherplantsand a largevolumeofpotentially
scatteredplants,withno competition
exploitablesoil per plant,completelyadequate nitrogensupplymightbe available for
theseplants.Whatevermaybe thetrueexplanation,theresultpointsto theoccurrenceof
variationin responseto nitrogenbetweenpopulationswithina species.This is confirmed
by the recentworkof Crossley(1963) on Loliumperenneand Dactylisglomerata.
In a previouspaper(Bradshawet aL. 1960b)theresponseto phosphorusof thespecies
studiedherewas comparedwiththeircalciumresponse(Bradshawet al. 1958).Although
a fairlyclose correlationwas foundbetweenresponseto phosphorusand responseto
responses
cristatus)
showedmarkedlydifferent
calcium,individualspecies(e.g. Cynosurus
thatcan nowbe madebetweenphosphorusresponse
to thetwonutrients.
The correlation
and nitrogenresponseis closerthan thatbetweenphosphorusand calciumresponses;
seriesof species,founda similarclose correlation.The
Crossley(1963),usinga further
basis of this correlationof responseto phosphorusand nitrogenis not immediately
apparent; it mightbe the resulteitherof a close relationshipbetweennitrogenand
phosphoruslevelsin soils,or to a close relationshipbetweenthe uptakeand/orutilizaand phosphorusbytheplant.Thereis,indeed,evidenceofa closecorrelationofnitrogen
tion betweennitrogenand phosphoruscontentof soils (e.g. Black & Goring 1953;
also existbetweennitrogen
Thompson,Black & Zoellner1954). But close relationships
ofproteinsandtheir
and phosphoruswithintheplant.Theyarebothessentialconstituents
and thereforesome correlationin theirutilizationmightbe expected;in
derivatives,
additionNielsen,Carson & Hoffman(1963) foundthat additionalnitrogenincreased
boththeuptakeand percentageof phosphorusin maize,whilstSlater& Goodall (1957)
that the
in lettuce.It would appear,therefore,
founda similar,but mutual,synergism
of responseto nitrogenand phosphorusmightbe due to both environmental
similarity
and physiologicalphenomena.
ofnitrogensupplyon theshoot: rootratioofspeciesfollowsthegeneral
The influence
speciesx
patternreportedbyotherworkers(Russell1961; Black 1957),butthesignificant
nitrogeninteractionemphasizesthatall speciesdo not behave similarlyin thisrespect,
effectof nitrogen
and confirms
the resultsof Chadwick& Obeid (1963). This differing
For example,the
to interpret.
speciesis difficult
upon theshoot: root ratioof different
two speciesAgrostisstolonifera
and Loliumperenneshowverysimilaryieldresponsesto
showsverymarkedchangesin shoot: rootratiowhilst
nitrogen;yetAgrostisstolonifera
Loliumperenneshows littlechange. On the otherhand Nardus strictaand Agrostis
have extremelydifferent
yield responsesto nitrogenand yet have similar
stolonifera
in shoot: root ratio between
changes in shoot: root ratio. The overall differences
to interpret,since it bears littleapparentrelationshipto
species is likewisedifficult
of the species.
overallyield,responseto nitrogen,or ecologicaldistribution
The sevenspeciesinvestigated
greatlyin overallyield,the orderbeing:
here differed
N. stricta< F. ovina< A. canina< A. tenuis< C. cristatus< A. stolonifera
< L.
perenne.This orderis thesame as thatreportedpreviously(Bradshawet al. 1958; Brad(Bradcristatus
in one experiment
shawetal. 1960b)exceptforthelow yieldof Cynosurus
betweenspeciesin growth
thatthedifferences
shawetaL.1958).It wouldappeartherefore
in finaldryweightyield,are inherentcharactersof the
rate,as indicatedby differences
and
have
ecologicalimplications.Comparisonof these yield differences
might
species
betweenspecies withpublisheddata on the productivity
of the plant communitiesin
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A. D. BRADSHAWet al.
673
whichtheyoccur(Kruijne1960; de Boer & Ferrari1957),showsa veryclose correlation
betweenthe yield of a species and the yield of the grasslandcommunityin whichit
is thereversalofthetwo
occurs;theone possibleexceptionto thiscorrelation
frequently
and Cynosuruscristatus.The yieldof each speciesbears an
speciesAgrostisstolonifera
levelof itsnativehabitatsas determined,
withmean soil fertility
evencloserrelationship
thatoverallyieldis indeeda
forexample,by de Vries(1954). It would appear,therefore,
characterof ecological importance.It is generallyassumed by agronomiststhat high
yieldis a characterofstrongselectiveadvantage,and thathighyieldmaybe equatedwith
This may well be trueof plant communitieson highly
successin a givenenvironment.
potentialis high,but the resultsof
fertilesoil, and in generalwherethe environmental
competitioneven on relativelyfertile
of inter-varietal
and inter-specific
investigations
soils generallyshow a verypoor correlationbetweencompetitiveabilityand overall
yield(Suneson & Weibe 1942; Sakai 1955; Knight1960; Lampeter1960; van Keuren
potential,specieswith
soils, withlow environmental
1961). In fact,on highlyinfertile
low yieldsmightbe at a selectiveadvantage,since theydo not outstripthe available
metabolizenormallyunderevenquitelow fertility
nutrient
supply,and would therefore
conditions.Indeed this appears to be the patternof behaviourdemonstratedin the
oflow productivity
and
wherespeciesfromcommunities
presentseriesofinvestigations,
soils,suchas Nardusstrictaor Festucaovina,have low overallyieldsand
highlyinfertile
tested,whilstspecies,suchas Loliumperenneand Agrostis
low responseto thenutrients
and highlyfertilesoils, have high
fromcommunitiesof highproductivity
stolonifera,
that,although
It would appear,therefore,
overallyieldsand highresponseto nutrients.
highyieldsmay be of selectiveadvantageundersome conditions,low yieldmay be of
potential.
selectiveadvantage,especiallyunderconditionsof low environmental
Care mustthereforebe exercisedin usingyield as a criterionof potentialsuccess,
especiallyin directcomparisonsof theyieldof two speciesundera givenset of environthat,in thepresentinvestigamentalconditions.It is partlydue to theseconsiderations
tion, comparisonsof the yield of a singlespecies at variousnutrientlevels have been
level.
used,ratherthancomparisonsoftheyieldofa numberofspeciesat a givennutrient
ofspecieswithwidelydifferent
overall
The needto comparetheyieldresponseto nitrogen
theuse ofproportional(logarithmic)
comparisonsratherthan
yieldshas also necessitated
betweentheuse of comparisons
arithmetic
comparisons.As an exampleofthedifference
nutrient
levels,as opposedto comparisonsof
oftheyieldofa givenspeciesat contrasting
contrastingspecies at a given nutrientlevel, the species Loliumperenneand Nardus
strictamaybe cited.At 27 ppm N Loliumperenneyieldsfourtimesas muchas Nardus
stricta,but on the otherhand at thislevelN. strictayields100% of its maximumyield
whilstLoliumperenneyieldsonly50% of its maximum;likewiseat 3 ppm N L. perenne
yieldstwiceas muchas NardusstrictayetN. strictayields60% ofmaximumwhilstLolium
discussionon thepossible
perenneyieldsonly10% ofmaximum.In viewoftheforegoing
ofthetwospecies,
selectiveadvantageoflow yield,and theknownecologicaldistribution
levelsunder
it would appear thatthe higherabsoluteyieldofL. perenneat low nitrogen
controlledconditionsis farless importantthanits verylow yieldat low nitrogenlevels
relativeto itsveryhighyieldat highnitrogenlevels.Furtherstudiesin long termexperilevels,are obviously
ments,involvingcompetitionbetweenspecies at variousnutrient
calledfor.
A comparisonof thepresentresultswithpreviousresultsforcalcium(Bradshawet al.
exist
1958) and phosphorus(Bradshaw et al. 1960b) indicatethat largerdifferences
betweenspeciesin responseto nitrogen
thanto eithercalciumor phosphorus.Thispoints
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674
ofgrasses.IV
Mineralnutrition
the
to theprobableimportanceof nitrogenas a major ecologicalfactorin determining
of species,a conclusionalso reachedby considerationof otherecological
distribution
by otherworkers(Chadwick &
and agronomicdata (see Introduction).Investigations
in
Obeid 1963; Crossley1963; Taylor1963)indicatetheprobableimportanceofnitrogen
so farpublished
diversespecies.The investigations
ofothertaxonomically
thedistribution
responseto suchnutrient
in thepresentseriesindicatehowa knowledgeof thedifferential
mayhelpto elucidatethefactorsresponsible
factorsas calcium,phosphorusand nitrogen
resultsof
species.Preliminary
of ecologicallycontrasting
forthe ecological distribution
responseto otherfactors,e.g. aluminiumand manfurther
studiesindicatethatdifferential
ofthesespecies.
elucidatetheproblemsof theecologicaldistribution
ganese,mayfurther
ACKNOWLEDGMENTS
We are gratefulfor the supportand encouragementof EmeritusProfessorR. Alun
Roberts,C.B.E., and assistanceof various membersof the Departmentof Agricultural
We would also liketo acknowledgethehelp
Botanyin themanagementof experiments.
of thestaffof the University
of California,Davis, forthepreparationof figures.One of
us (A. D. B.) would also like to acknowledgethe supportof the LeverhulmeTrustees
and the hospitalityof the Departmentof Agronomy,Universityof California,Davis,
duringthepreparationof thispaper
SUMMARY
was
The reactionof sevengrass speciesto variationsin nitratenitrogenconcentration
studiedin sand culture.
in yield response.Loliumperenneand
The species showed verymarkeddifferences
showedthe greatestresponseand yieldedmost at the highestlevel
Agrostisstolonifera
used (243 ppm N). Agrostistenuisshoweda responsesimilarto thatofLoliumperenneat
lessresponsiveat highlevels;therewas,however,
low nitrogenlevelsbutwas significantly
difference
a significant
betweentheresponseof lead resistantand normalpopulationsof
cristatusand Festucaovinawas significantly
Agrostistenuis.The responseof Cynosurus
lessthantheabove speciesat low nitrogenlevelsand bothspecieswereadverselyaffected
by thehighestlevel(243 ppm N). Agrostiscaninawas veryresponsiveto nitrogenat the
lowerlevelsbutfailedto respondto nitrogenabove 27 ppm N. Nardusstrictaresponded
onlyweaklyup to 27 ppm N and showeda verystrongdepressionof yieldabove this
level.
The responseof the variousspeciesto nitrogenin sand cultureshowsa considerable
of nitrogenresponsemade by otherinvestimeasureof agreementwithdeterminations
and
betweenspeciesdistribution
trialsand in studiesofthecorrelation
gatorsin fertilizer
soil nitrogenlevelsin permanentpastures.It is concludedthatvariationin soil nitrogen
ofplantspeciesunder
thedistribution
factordetermining
levelsis probablyan important
naturalconditions.
in overallyieldbetweenspeciesis
of differences
The possibleecological significance
discussedin relationto levels of soil fertility.
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