A chapter from

Adaptation of Chickpea in the West Asia and North Africa Region

f+p

Edited by

N P Saxena, M C Saxena, C Johansen, 5 M Virmani, and H Harris

International Crops Research Institute for the Semi-Arid Tropics International Center for Agricultural Research in the Dry k#r Patanchew 502 324. Andhra Pradesh, India PO Box 5466, Akppo, Syrk

5.5. Symbiotic Nitrogen Fixation by Chickpea in WANA and SAT

D P Beck1 and 0 P Rupela2

Introduction

'I'ht- :ability of i hic.kl)es to lix i~t~riospht~rii. nitrogen Icssrns its dispcn- dcnce nrr soil N i d rc.inf'orc.cs its rolc in the cropping s y > t t * ~ ~ ~ s ol WANA and SKI'. 1 lowever, the crop is oft(-n considrrcd infrrior in this rt.garcl to other. I C ~ I I I I ) ~ (raps p w n in whmt-biletid rotations (I'upil- styliani>u 1987; Keiitingt* tat ill. 19HX). P~lhlishcd cstiti~atcs of N,, fisa- ticln hy uhickpra in the WANA region range from 0 to 17ti kg ha-1 per scason, with tllr propottion of total N I'ron, fixatiori virryirlg ht*tween O and 82010, depending on thr method ot' Ineasurt*mc5nt, i.11lti- KIT, prtsscncc of a ~ ~ ) r ~ t ) r i i l t ~ ' rhix~~biil, and t~nvironrnental variables (IZizk 191i6; I'apastylianou 1987: Kciltinjic ct al. 1988; Hcck ct al. 1991). Nitrogtnn lixation has a positive cl'fect on soil Nz balance and growth of thc suhscqtrcnt crop (Evilns 1982; Heichcl 1987; Kcirtingc r t al. 1')HH). 'l'hereforr, prarticivi which inr.rcasc N fixation will mini- mize t h ~ quantity of sail N utiliwd by the crop, and therehy incrmsr yields in the subsequent non-legurne crop.

l'hc main stri~tcgics for improving biological nitrogen fixation (HNF) in chickpea are similar t o those for most legumes. Btscausr the process of N, fixation is photosynthate-driven, increasing chir Lpca yield is thc simplcst and gcncrally thc 111ost successful strategy to iml~r~ovr RNF. Hrectiing for improved N2 fixation is rarely done because it gets less priority than that for yield and resistance to biotic/stiotic strcsscs, and also hrcer~sc mcasrlring NZ fixation tnay bc a difficctlt and cxpen-

... . ... ,,. I C ~ P ~ I I I ~ ~ ~ I W I I > I ' r t ~ f a l I ~ ~ lC:AMl>A, I X ) NIX 54iiii, Ah-I~UI. Svrw 2. Agronomy Dlvr'i~irn. IL'RISATArir Crntcr. I1iltanchc*ru 502 324, Andhra I'raiirsh, l n ~ t l ~ .

sivc process. 0;)tirrlizlng thr host-rh~zob~,~ ;issot littion, hv Inoc c~latinc the c.liic.kpc*a pli~nts with \c.lcc tixl rt,~zolrl;t, I \ ~ t~crr tor i , , tflt* most r.ornnion ;tppri.)ai h to improvr K,. f~x;~! ion.

'T'cchniqrlc*~ );IT rllcilsurlng S:. fixat~or~ arc* +4s.

the fixation process (Munns 1977; Streeter 1988). This factor is par- ticularly important in experiments conducted at research stations, where soil fertility (N in particular) tends to be high.

Critical tolerance levcl and degree of suppression vary ~ t h I w m e species (Harper and Gibson 1984). The limited research on chickpea indicates that levels of NOsN below 10 ppm will not adversely affect N, fixation (Rawsthorne et al. 198S), but variation with cultivars is expected (Rupela and Johansen 1992a).

Chickpea also appears to be a fairly efficient scavenger of soil N, especially under conditions where sufficient rhizobia are not present for efficient symbiosis (Beck 1992). The practice of fertilizing chick- pea with 20 kg N ha,' at sowing is widely recommended in SAT, probably because it somctimes helps to negate the adversc effect of high temperature on symbiosis (R~wsthorne et al. 1985).

Nitrogen fixation in chickpea sectns to be morc sensitive than grain production and N assimilation (which is mainly limited by availability) to high tetnpcratures (Hawsthornc et al. 19115) and drought Wery ct al. 19811). In field studies with six chickpca cultivars in Syria (ICARDA 1992), drought strcss depressed Yfi, more than N uptake (Fig. 5.5.1). A line-source sprinkler was used over 2 seasons in thcsc studies, where Pflx increased more rapidly than yield at lower moisturc levels, indicating that fixation wes severely limited at the lower end. Values for Pfi, in different cultivars at lower moisture levels varied widcly from 15 to 38%. Fixation efficiency reached an average maximurn of 6896 at about 5 0 0 kg ha-1 dry matter produced. Nitrogen uptake from soil remained constant until moisture became sufficicnt for max- imum fixation, when soil N uptake increased (Fig. 5.5.1). Thc correla- tions between dry matter prodr~ced and N yields were high, with coefficients of 0.92 for tatal N and 0.90 for fixed N.

The high correlation between dry matter production and N yield indicates that N, fixation in chickpea, under conditions where ade- quate rhizobia are prcsent, is yield-driven, and that cnvironrnental

Q "tis m Total crop N

I 0 Fixctl N

-1 xo 150 0 0 0 0 0

Figure 5.5.1. Relationship of dry matter production with N yield and source in chickpea cultivars, northern Syria, 1987-91.

constraints on plant yield will limit N2 fixation. These rearlts may partly explain why N fertilization impraves yield of nonirrigated chickpea in low N soils, but does not affrct yield in the irrigated crop (ICRISAT 1992).

Breeding for improved N, Fixation

Agrononiic and cnvironlnental considerations often limit the biomass yield of a legume crop and therefore the capacity of that crop to fix

N,. Yicld is also determined genetically, for example, low N yield may be a characteristic of somc species. In studies ovcr a range of environ- ments and agronomic practices, N yield and N, fixation by chickpea wcrc consistently less than for the other cool-season food legumes (Rennic and Duhetz 1986; Evans and Herridge 1987; Snrith e t al. 1987; Beck ct al. 1991). Average yields wcre 100 kg N ha.' fbr chirk- pea, 196 kg N ha-1 for lentil, 185 kg N ha-1 for tirld pea, and 200 kg N ha-' for fabo bean. These studies did not indicate that the inherent capacity of chickjxa for ncdulation and N2 fixation was less than for the other sprcies. It may be concluded, therefore, that inrrrasir~g N yield of rhickpca may rcsult in increased N, fixatior~.

Plant breeders select for high yield within the constraints of local environments and crop yields largely detcrmintl thc amount of N2 that is fixed by the crop, particularly in low N soils (Hardarson c-t al. 1984; Kumar Hao and Ilart 1987). Therefore, breeders who mustly work in low N soils will tend t o sclcrt for niaterial with RCK)(I capacity for N2 fixation. Hreeding for symbiotic characteristics in chickpea is possible. Ex;u-nplcs of' possible strategies t a exploit art. nitrate tolrr- ance (i.c., the ability ot'the plant to nod~llatc and fix N2 in the prcs- encc of soil nitrate), the capacity t o fix Nz at low available moisture Icvels, and general ndulat ion capacity. Chickpea cultivars sclectcil for cirought tolcrancr seem to vary in their capacity for N2 fixation under drought stress. Natural variation for nitratc tolcrancc (Hupcli~ and Johansen 1995) and ntdulation capacity (Hupcla 1994) also rxist in chickpea. I t may be irr~possiblc to produce a Icgume that is dcpen- dcnt solcly upon N2 for growth and cannot use nitrate, but there is scope to improve Pf,, in thc presence of nitrate for chickpea.

Some argue that legumes should be ohle to tise bc>th atmospht.tic and soil N sources so that they can scavenge nitrl tr horn the soil which would otherwise be lost through leaching and denitrifi~-ation, while others argue that in many soils, nitratc is relatively stahltp over t ime and can be considered as a stable pool of N. Secondly, if dcplt*.

lion of' soil nitrate was considered rircessary, i t wol.~ld make rnore sense t o use D rrreal cmp w ~ t h a highrr tlemand for N and greater econamlc rrturn.

Recacise N yield and dry rnattcr productlirn art1 gcnrrally h i~h ly correlated (Mytton 1983), thta f;)lIowiny: proc.c.c.iure could be followed to enhance N-, fixallon in ch~ckpca:

I . Scrren a large and divcrsc jirrr~ipli~sm (500-IOCW genotypes) of c.hickpra, inc)c~ihtcti with highly effective rhlxohia. For production 01' dry mtlttrr uridrr low N conditions (prcfr*r;~bly in thc field, hut ~t could be donc in a grrrnhousr).

2. Sclrct silpcrivr genotypes ( c , ~ . , top lo'%,) fur flirther cvnluatlon. 'The sccond round of' screening is ideally done In th r f~eld on low N-f'ertility soil, ajiain with a mixture of h~ahly rf'tectivr rh~zobia. Asscssn~ents should includr measurements of grain yirld and total N yield.

3. Cotlipare elite grrrotypes over a rarlgr of edi~phic (particularly soil N fertility) and environmental (including dlvcrsr rhl~obial popula- tion) conditicrns for grilirj yield, N yirld, arrd NL, fi~ation, thc latter tising ''IV nlcthcds.

C*;t*notypes that arc identified at this stage RS s ~ i p ~ r i o r In all three attributes and actepted to the soils and crrvironnrrnth for which they iilc likely t o bc tised wo~ild h;rvr a~rmrdratc comxr~crt-ial application. thjih N,,-fixing gtmotypes that produce low grain yields or grain of low quality could hc rised as donor parents in a brrrdlng prngrdrrr.

It is also important to rcmovc t l ~ r vff~~i- t of crop duration un N yield. Increased N yirld due LCI high gcr\\-th and ~ s s i ~ n i l ~ t i i r n rates is nwre uscftll ht.catlse it rean bc rsprt.ssc*~i in any ~ r ~ v i r o n ~ i l ~ n t ; whcrcas incrcastd cmp N ciur t o Icinger crnp iiur,rtirtn ~,;ln rrnl!. he esprcsscd if thc dur i~t i i~n crf the scascrn in 3 parti&-uldr cnvirnnment or ~.ropping system is suffic.it*ntly Iona. In cvmmer~.1;11 agri~.\llti~rt, individual crops 111ust fit into cropping systcnrs \vtiir.h arc rit.tcrlliint*d hy seasc3nal

clianges in tctnpcraturr, moistt~rc availability, ri~diation, i~voilahility of' land and rcsotlrccs to grow and harvest thc crop, n~arketing nrringr- ments, etc.. 'The optitilurn driri~tion of' any crop is thcrcfure dctcr- mined hy several f i~~tors, tlic. least important of whir11 is N yirlii or N;, fixatiorr.

Inoculation

lacal production or itnport ol' inocr~lants for i'armrrs can only be justified if thc* Ity,unic benetits from inoculation ilrr showr~ by in- crcilses in yicld or it\ N, fixation in tirld trials ancl hrmrrs' fields. It is essential to detcrrrrint* thc need for inoculation before initiating any program on inoc~lilrlt dt*vrloptllmt, production, distrihr~tion, or usc. Response to inoculation by Iegun~cs has hrcn shown to he influcnccd mainly hy cropping history (Rrockwrll cr al. I!)HZ), soil N :ivailahility (Somascgaran arid Rt>hlool 1900), and most important l y, tlic indigc- nous popr~liltion of rllizohia that nodcllate thc host (Thies et al. 1991). Various methods to dctcrrninc the need for inoi-ulatiort arc. describcd in detail by Beck ct al. (1993).

'Thc introdt~ction of cold-tolcrunt, ascochyta blight resistant lines for wintcr sowing into new, drier production arcits of WANA has been accon~~anicd by nodtilation dcfic.iency in scvcral arcas (M Solh snd S P S Hrniwal, personal co~nmunication). In these new production areas, soils are lcss likely to contain adequatc populations of the C'iccr- spccific rhimbia than traditional c:hirkpea arcns, and crops tnay show significant yield incrcascs when seeris are inoculated with selected rhizobial strains. Extensive surveys of native r11i;~obia-nodulating chickpca have been reccntly conducted in Syria and Turkey, where symbii>tic effectiveness and size of native populations wcre measured (Kcatinge et al. 1995). It was found that even within the major chick- pea-growing regions, many soils contained rhizobiel populations eithcr at very low levels or with low symbiotic effectiveness on the cultivars

tested. It has h e n sr~ggested that this deficiency may bc one rcason for the grncrally low average chickpea yiclds from these arcns.

The highly specific rtli~uhial requirement of chickpea extends to strain-cultivnr specificity for N2 fixation (Beck 1992). This inlplics that limited cffcctivcness of naturali7td rhimbial ppulations with nrwly introdt~ced ctlltivars may restrict the genetic potential for di- nitrogrn fixation. Nwcssity for inoculation may therefore also exist where introduc.cd cultivars- -selrcted for high yiclds-cannot express their ftlll capability for N2 fixation in symbiosis with native rhizobial populations that hove devclopcd in aciaptution with Iwal Iandrarrs.

In trials conducted over 4 scasons (1987/88-1990/91) in northern Syria (seasorla1 rainfall of 300-51K) tnm), variations in N2 fixation and yield of chickpcn cultivars inocttlated with srlccted Rhizobium strains wcrc evalllated. 'l'hc purposc was to establish base-line valucs for PI; , in recor~~rnendcd cultivnrs so that improvements throt~gh rhimbial strain selectiori and lcgume breeding could be quat~tified. IJsc of 1 S N methodology and nonnodulating chickpea and barlcy as refcrencr crops allowcd accurate evaluation of N2 fixation tmder a wide rangc of cnvirontnental conditions. Indigenous chickpcu rhirnbial popt~lations based on the most probable number (MPN) estimations in the field soils were low to moderate, ranging from 9.1 x 10' to 4.2 x 10" rhimbia g-I soil. Rhi.u>bial strains wcrc se1et:ted according to the N2- fixing perfonnancc in aseptic hydroponic culture in greenhouse trinls.

Inoculation had no general cffect on crop dry rnatter yields at lower rainfall sites (Pig. 5.5.2). At 340 mm rainfall, however, crrltivars began to show differential yield effects with rhizobial inoculation, ranging From no response to n 750 kg ha.' increase. IJnder conditions of higher moisture (504 mm), thc averagc inoculated cultivar yiclded about 800 kg ha-1 more dry matter than when not inoculated (Fig. 5.5.2). Cultivar yields, which differed little at low rainfall, varied widely at high rainfall; yield rcsponsc to inoculation varied from no response in cultjvar ILC: 5396 to 1.9 t ha-1 in ILC 482.

N tutal inoculated 0 A N total uninwulated o DM yield inoculated m DM yicld uninoculukd A N fixcd inoculated v N fixed uninoculated +I

Annual rainfall (mm) Figure 5.5.2. Effect of inoculation on dry matter production and N yield in chickpea, norihern Syria, 1987-91.

In uninwlated cultivars, PC, rcmains rclativdy constant at about GO'% between 2000 and 7001) kg ha-l dry matter production (Fig. 5.5.3). The effect of this constant proportion of fixed- to soil-derived N in the plant is that with increasing dry matter (and N) production, the quantities of soil N taken up by the crop increase. Fiyrr 5.5.3

shows average soil N uptake (the distance bctwcen total N and fixed N curves) increasing from 20 kg ha-1 to nearly 50 kg ha-1 over the range of dry matter produced in the trials. In contrast, the efficiency of NZ fixation has clearly increased at higher yield levels as a result of rhimbial inoculation (Fig. 5.5.4). In inoculated cultivars, Pflx increases with dry matter production, reaching a maximum of 80(#1 at the high- est yicld Icvels. Increased fixation efficiency with yield results in a high proportion of fixation-derived N in the plant and a low, relatively constant fraction of soil-derived N (Fig. 5.5.4).

1.k: 1!1,11lc*r \ tc l~ l (kt! h.1 ' Figure 5.5.3. Nitrogen yield and source in uninoculated chickpea cultivars, northern Syria, 1988-90.

I I I Ittin) ?(nu\ .zcnn) 4 n w ) s t n u ) o c r n ) 'Itnu) ntnn,

IJr) ~ i i : ~ t t \ - ~ ~ ~ r t ~ l u c t ~ ~ ~ ~ ~ ( k g I1i1 I )

Figure 5.5.4. Nitrogen yield and source in inoculated chickpea culti- van, northern Syria, 1988-90.

In mast cultivars tested, inoculation did not increasc the amount of crop N per unit dry rnattcr produced. The proportion of crop N derived from fixation was, however, often increased by inoculation. The effect of this impravcment-that can be detected only with N, fixation mcasuremcnt techniques such as thosc incorporating 'SN-is improved soil fertility. Although the cffccts of inoculation on yicld arc limitcd, the quantities of soil N preserved could be significant in a systems context. Farmers, howevcr, will not adopt inoculant technol-

ogy it' they do not get as a result of applying the technology, increa.wd yiclds ot' thc legume or ol' thc subsequent c.crral crop.

The itrterat:tion ktwccrr strains and rultivars for N2 fixation effi- cicncy, in addition to a similiir interaction Ihr cor~ipetitiorr and nodule formation, coniylicates the approach t o wide-scalc intwulation of chickpea cultivars, csyccially wticre ncw improved cultivars arc being rcleasrd on a rrgulur basis. Two strategirs triay hc used to increasr N fixcd hy tlic chicklwa crop. Sclecrion of cultivars for high N;, fixation with a broad mnge of rhiwbia rcduccls the nerd for inoctrlation with specific strains. 'l'liis, howcvcr, may fail whcrc nativr strains arc absent or incffcctivr. Alternativrly, mixtures of highly effective strains may hc used as inoculants. This works with some cultivars, but is dcpcn- dent on strain-cultivar interaction for competitiveness in nodule for- mation, and on tho sucr:essfttl use of inoculant technology by farmers.

Evvm whcrr ir~oculatio~~ can irrcreasr yiclds, its ellictivcness is heavily dctwndent 011 the qt~ality of the inoculant and the way the prt)dt~ct is apl~lird. Exprriencr has shown that successful transfer of i n t ~ d a n t tcchnology to farmers for improvement of BNF is difficult at bcst (Thompson 1991). Rhiznhium incwulants arc biologiral produrts and thrrcfore s~rsceptiblr to major problcms with ~nanufacturing (quality control), distribution (loss of viability during transport and distribution), and extension (Roughley 1988). Slistribution of poor quality inoculants is not uncommon, and is g~ncrally followed quickly by fanner disinterest in inoculation.

Contribution of N, Fixation to Cropping Systems Results from legume-bascd rotation cxperimcnts in rainfed cropping arcas of many countrics havc been published in recent years (e-g., Evans and Taylor 1987; Evans a al. 1989). 'fie* cxpcriments reflect the growing concern of scientists and farmers in thosc areas about declining levels of N fertility in the soils and rrduccd production of

tion and its valt~c t o soil N incrcascs in lupin, field pca and other legumcs in south-eastern Australia. Australian Jot~rnal of Agric\~ltwol Rcscarch 40:791-805.

Evans, J., and Taylor, A. 1987. Estimating dinitrogen (",I fixation and soil accretion of nitrogen hy griiir~ Ichwmcs. .lournal of the k ~ s t r a - linn Institute of Agrirulturnl Science 53:78-82.

Hadarson, G., Zayata, P., and Danso, S.K.A. 1984. Effects of plant genotypes and nitrogcn fertilizer on symbiotic nitrogcn fixation by soyhean c~lltivars. Plant and Soil 82:397-405.

Harper, J.E., and Gibson, A.13. 19H4. 1)iff'crential ntdulatiorr toler- ance lo nitratr anlong legume slxcics. Crop Science 24:797-801.

Hrirhel, G. 1987. I.rgumr nitro~en: sytnbiotic fixation and recovery by sr~bsr~qurnt crops. Pages 63-80 in Energy in plant nutrition and pcst control (I jelscl, Z.R., ed.). Amsterdam, Netherlands: Elsevier.

f4erridge, D.F., O.Y. R~~pela, R. Scrroj, and k k , I1.P. 1992. Screening trchrriqtles and improvrd biological nitrogcn fixation in cool season food legumcs. Pagcs 472-4!12 in Exysnding the productivn and USP of' cool spason fcn~d 1cgtlmt.s (Meuhlhaurr, F.J., and Kaiscr, W.J., eds.), Dortirecht, Netherlands: Kluwer Aridemir Publishers.

I

Peoples, M.B., Vaizoh, A.W., Rcrknsc:ni, R., and I lerrid#e, Il.1:. 1989. Methods for evaluating nitr;)k(:ti 1ix;rtion hy nocluli~tcd Irgurr~c~s in the field, ACIAR Monograph no. 1 I , Chnherra, Ai~strsliil: Arlhrrilli;rn C:rntrc for Intc.rnatic~nal Agric.ultur;~l Resc.i~rr.h.

Rawsthome, S., Hsdley, P., Roherts, E.11., nil Su~i,riic*rliekl, H..I. I!)H5. Effiacts of supplemental nitriltv 3rd th(.rln;ll r r . ~ i ~ n c on thc ni- trt1gc.n nutrition nf'ch~ckpc-a (Cicrr trriitrinw~~r I..), 11. Sy~rlhiotic dvsc.l- opment ;and nitrogen i~ssitltililtiorr. I'1;lnt :lnci Soil H3:27!J ?!I:{.

Hcnrric, R.J., and I)ulwtn, S. 1 9 H t i . "N-dc.trrn~irli~i nitrogrrr tisiltion i r r field-grown (.hickpea, lentil, f i ~ h i ~ bcii~i anil lieltl pco Agronomv Iour~ral 78:654 660.

Ilixk, S.G. 19fj1i. Atmospheric nitrogcn flx;~tior~ hv 11-jir~~nc.h rlnclcr Egyptian conditions. 11. C';rairr Ic*gutnc*s. Jollrnal of' M i c r o b ~ o l o ~ y ot ' t l i i~ IJnitcd Ar ib Httpuhlic 133-15.

Roughley, R.J. 1988. lnhgurnt* 1nuiwl;rnts: thcir trcllrrr>logy irncl irppl~ cation. Pages 259 268 iw Nitrogc-n fixation by I t~fi t~mes it1 Mcditcrr;l- ncan agriculture (Heck, I I.P., and M i ~ t ~ r o n , L,A., rcls.). The* I l,~r;tri~, Nrtherlnnds: Martinus NijhoSf/l)r W 111nk I)tlblihlii*rs.

Hupele, O.P., 1994. Scrt*c*ning f i ~ r intracultivaral variability of'nt)(itllo tion in chirkpca and pigeonpea. t'agcs 7 ,HR in Linking l ~ i o l t \ ~ i ~ . ~ I nitrogrn fixntion rrsearrh in Asia: report of il nic.cting of the Asia Working Group on Hiological Nitrogen Fixatiot~ in Ir#urni-s, ti..-# I lec, I(::RISA'I' Asia Crn te r , Inclia. (Rt~pels, O.l'., Kr~mar Kilo, J.V,I).K., Wani, S.P., and Johanscn, C., eds.). Patancllc.*ru 502 324, A.l'., India: International Crops Research l n s t ~ t u t c for the Srml-Arid Tropics.

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I-urbl: in thr, tropic.s c~n(.l s t~h - \ rop~ i , \ , ~ j r i ~ t,*c.c.l~nq\ ot t l ~ t . Intcsr-( (lrl:!r(*\\ (:r)nl'c.rt~nt.c! 1 1 t C:onlrnls\ion IV, 1-3 TI(.( I ~ I I , I ~ , I , lL~r\~!liu!f-,t~ (1 IIISS:IIII, ,M.S., IIII,IIIIII! 11~11, !- ,%I, , /Int\.,lr Iqlr,11, A!., , I ~ I I ~ Kl.~:lr:, ' I . I I (*(Is,]. IIhakii, N;lnt:l,1tlt4i: l l .~rri :I , icl t~\t i 1I2r1i I I I ~ I . I T ; I I I ~ i * ~ c ~ r ~ r t i : (. : o1111r 1 1

S:lxcbrte, M.(:., Silim, S.N., ,rrrcl Singh, K.13. I ( + l ( r 1:ttt.i I c ~ f \itpplta. rTIC1ltilry irrigiitlo~l cluring r t ~ l ) r ~ ~ ~ l ~ ~ ~ . t i ~ t ~ c r o ~ . : l ~ t)n \ \ . ~ r i ~ t ~ r \111i.i ! I I I 07:3-. 13.

Sor t ln s r~ i tn~n , P., ;lnd Rohlool, H.M. I i I ( d ( I C;inclt-.~tra~r: : ( * I -1.1. 1n11lt1 stri~in II IOC ~ ~ l i ~ t i r m : i4fi*c t of soil I I I I ~ I : ~ , I ~ S ~ ~ ~ , i i ~ . ~ l ~ i ~ ~ t ~ . t t r r r l i i~ r~ l>~a l Strillrl i $ l l ~ i tivcrlinh\ ill^! ii S,. f ~ s a t l o r l by rritrirtc. ( :riili ill Kc\ it.\\.!- 111 Plant Si.~t*llt t o -:I : !

'Ihics, .l.E., Singleton, I".\\'., ;1111l Bohlool, H.U. lt.)'!1 I r i t l ~ r ~ ~ n i t s ijt tlrc ~1x1: ot' i r ~ i i i g r n i ) ~ ~ ~ rhiLirbl.tl pirp~rI;~trorr\ tjn t.>t.lhl~shr.ncnt :irlri s ~ ~ r l h i i j . tii. pc.rt;irrnanc.t. (if' ititrodur~i~tl rh~zc~hi;i vrl tli.lcl.~:r(j\vrl I ~ * ~ \ I I I I c ~ ~ . A!)- pliivl Errviron~rrct~ti~l X l i k r v h i i r l o ~ .?-: 19.-75.

' f ionipson, .l,A, l!)!ll. l , t * g ~ ~ ~ i i t ~ i n i ~ l - ~ ~ l ~ r ~ t l i r i x l i ~ t ~ t ~ ~ ~ n ;~ tx i ~111.1ltt\, t t ~ i - tri11. PagcsI15 32 ill Espcrt i .c)nvll t ,~t~o~i ern I t - p ~ ~ r r c Inoc rll'rnt I>r , t l~ lu~. . titrn irnd r l t~a l~ty ~ .unt ra l , Htwtlc*, It,lly: FnoJ .ttlrl 4grir,ultirrt* 0rgnrriz:ltion of thc I inltcd N.1ticln5.

M'c*ry, J., Desc.hurrrps, hi., ; ~ n d I~lgrr-C:'rcsstm, 6. 1!1$S. 1ntlucnc.c vt somc agroclimiitir fdLtors .irlil dglc~lli~llllr' pr~itit.t*h on nitrogen nutri-

t i c z n c > f c h i c - k p r - a ( C i c . - e r arirrtinum t .-). P a g ~ s 2S7- 30 1 in N i t r r > g c n fixation by I r g u r r ~ e s in Mcciitrrrst-rran ; r g r i s . . t r l t t a r t - CBccrk , i-3.1'.. and M a t e - r o r r , 1,-A., t-ds-), ' l ' l r c H r a g x r c , N e t l x e r l a n d s : Martinus Nijho+f-/13r W Junk I ' u l > l i s t r c r s .

W i a a y , J-t:, l9S3- E n t . i x x r a i i t r g N;, f i x a t i c y n i t - thc - field using 'SN-labcllcd fe-rtil izx-r: s c > r x ~ c - ~ - r c > l > l c - ~ ~ ~ s ar1r-4 s r > l ~ a t i c ~ n s - S o i l E&ic>lc -gy and 1 % ~ - c - l a c - m i s r r y 1 5 z f 5 3 1 - 639- Wirty, J,12,, and Minc:hin, Iz,K, 198s- Mt-asurc'anc-nt 4-1' n i t r o g e r r fix;*- tion by the a c - e t y l c t ~ r = r r d x s r - t i o n irssay: z n y t h s R ~ C I rnystc-rics- Pages 3 3 1 -344 im N i t r c ~ g t - r - f i x a t i c - n by l c g u t n l - s i n M c d i t c r r a n r a n a g r i c x s l - ture ( H e c k , L3-I)., a r a c i M a t r r r m r , LA. , c * c t s . ) . 'l'hr- 1 lsgnc, Nrthcrlands: Martin~rs NijhofT/ I 3 r W Junk P~rhlishtrrs.

Witty, 1 , Rennie, R-J-, a d Arkins, C 3 - A - 19SH. N addition r r > c t h o d s f i x assc-ssing Nz fixation under f ie ld c:-nndirions. Yagcs 7 1 S- 730 in World crops: c o o l s c a s r - n f c x d 1 e g u r x . r e - s CSumn~t-rfic-ld, R - . I - , ed-). L 3 o r d r e ~ - h t , Netherlands: Kltawer Acadcmir P x a b l i s h r r s .

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