Evaluation of measures intended to increase the fertilizer-N recovery by lowland rice

Red clover root material confined in mesh bags was buried in three different limed and unlimed soils and incubated for 196 days at room temperature. Remaining amounts of organic matter, as well as concentrations of C and N of the decomposing material were determined three times during the incubation and finally the concentration of soil mineral N and pH of remaining roots was also assessed. Liming only temporarily affected the decomposition rate of organic matter and N release, and at the end of the incubation no effects could be observed due to liming. A possible explanation is that the decomposing root residues provide a well buffered micro-environment for the decomposing microflora. Liming did not change the pH of the root residues even when 97–98% of dry mass had disappeared from the mesh bags. Concentrations of mineral N were higher in limed than in unlimed soils.     

Aluminum inhibition of shoot lateral branches ofGlycine max and reversal by exogenous cytokinin

Aluminum effects on the morphological development of soybean (Glycine max (L.) Merr.) were characterized in greenhouse and growth chamber experiments. An Al-sensitive cultivar, ‘Ransom’, was grown in an acid soil (Aeric Paleudult) adjusted to 3 levels of exchangeable Al. Lateral shoot development at the nodes of the main stem was extensive in the limed soil containing 0.06 cmol(+) Alkg⁻¹. However, lateral shoot length and weight were severely inhibited in the unlimed soil containing 2.19 cmol(+) Alkg⁻¹, and in the unlimed soil amended to 2.63 cmol(+) Alkg⁻¹ with AlCl₃. This inhibition by the high Al/low pH condition was reversed by the exogenous application of a synthetic cytokinin 6-benzylaminopurine (BA). The daily application of 20 μg mL⁻¹ BA applied locally to the lateral meristems of plants grown in the unlimed soil stimulated lateral shoot growth substantially, such that it was either comparable to or greater than that observed in the limed treatment without BA. Accumulation of K, Ca, and Mg in lateral shoot branches was also stimulated by the local application of BA. The inhibitory effects of Al on lateral shoot development were confirmed in solution culture. In addition, differential sensitivity to Al was evident among the primary root, first order lateral roots, and second order lateral roots. The length of the primary root was only slightly decreased by increasing concentrations of Al up to 30 μM. In contrast, the length of basipetally located first order lateral roots was restricted to greater extent; up to 50% by 30 μM Al. Second order lateral lengths were inhibited even more severely; up to 86% by 30 μM Al. Substantial evidence in the literature indicates that the root apex is a major site for the biosynthesis of cytokinin that is supplied to shoots, and cellular function and development in this region of the root are impaired during Al toxic conditions. This suggests that one mode of action by which Al may affect shoot growth is by inhibiting the synthesis and subsequent translocation of cytokinin to the meristematic regions of the shoot. The present observation of a reversal of Al-inhibited lateral shoot development by exogenously applied cytokinin supports this hypothesis. However, the inability of applied cytokinin to counter the restriction imposed by Al on total shoot dry matter production implies the impairment by Al toxicity of other root functions, such as ion and water transport, also played an important role in altering shoot morphology.     

Effect of lime,phosphorus and mycorrhizal fungi on growth,nodulation and nitrogen fixation by white clover (Trifolium repens) grown in UK hill soils

Summary The responses of white clover (cv NZ Grasslands Huia grown in four UK hill soil types) to additions of lime and P, to inoculation with Rhizobium and mycorrhizal fungi, and to differences in soil water status were assessed in pot and field experiments. With a deep peat soil in pots, shoot production, nodulation and N fixation by clover were increased by 160, 130 and 85% respectively following inoculation with mycorrhiza, but in the field, despite a doubling of root infection, there was no response in growth. On a brown earth soil in the field inoculation with one endophyte (Glomus mosseae L1) out of four tested depressed production of white clover shoots by 42% but enhanced that of leeks (Allium porrum) by 50%; the others were without effect. With dry peaty podzol and brown earth soils in pots, clover shoot production was highest with added P when a water holding capacity of 80% was maintained, but roots from the latter had only 2.6 compared to 68 nodules per plant from the former. Further work is required to explain poor nodulation in the brown earth soils.     

Improved growth of white clover in hill grasslands by mycorrhizal inoculation

Field trials in a hill grassland area of mid-Wales showed that growth of white clover could be improved by inoculation with selected vesicular-arbuscular (VA) mycorrhizal fungi. After one growing season mycorrhizal inoculation had approximately doubled shoot dry weight, runner number and nodulation at two sites previously improved by adding lime and fertiliser and by replacing the native vegetation mainly with rye-grass and white clover. At a previously unimproved site, limed at the time of planting, mycorrhizal inoculation increased shoot dry weight by up to two and a half times, almost doubled runner formation, and increased nodulation up to five-fold where 90 kg P/ha as basic slag had been applied. Responses to mycorrhiza at this site were smaller with 22-5 kg P/ha as basic slag and were absent where no P was added. After two growing seasons inoculation responses were even more pronounced at this previously unimproved site; where clovers were being overgrown by adjacent native grasses only those given both 90 kg P/ha and mycorrhizal inoculation survived well.     

Role of magnesium in combination with liming in alleviating acid-soil stress with the aluminium-sensitive sorghum genotype CV323

An experiment to study the effects of Mg nutrition on root and shoot development of the Al-sensitive sorghum (Sorghum bicolor (L.) Moench) genotype CV323 grown in pots of sandy loam under different acid soil stress is reported. This experiment had a factorial design: four rates of liming were combined with four rates of Mg fertilization. When no Mg was added, the pH of the soil solutions (collected in ceramic cups) increased from 4.0 (unlimed) to 4.2, 4.7 and 5.9 at the increasing rates of liming. After 30 days of growth dry matter yields of the limed treatments were 40%, 115% and 199% higher than that of the unlimed treatment. Without liming and at the highest liming rate, adding Mg did not affect plant biomass significantly. At the two intermediate levels of liming, however, 11.3 mg extra Mg per kg soil increased dry matter yield to the same levels as found at the highest liming rate. Concentrations of Mg in the soil solution rose after Mg was added and fell when lime was added, but adding both Mg and lime increased Mg concentrations in the plant shoots. In plants of the limed treatments, dry matter yield was correlated closely with the Mg concentration in the shoot. This was not so in the unlimed treatment. Furthermore, in the unlimed treatments root development was inhibited, but reduced Mg uptake by the plants resulted mainly from the direct effect of Al- (or H-) ions in the soil solution rather than from impaired root development. It is concluded that Mg fertilization counteracted the interfering effects of Al- and H ions on Mg uptake.     

The effects of lime and phosphorus on the function of wheat roots in acid top soils and subsoils

Two wheat varieties with differing aluminium tolerance were grown in pots of acid soil. Liming did not change significantly the amounts of chemically extractable P and K, but caused improved vegetative growth, increased inflow of P and K and reduced uptake of Al. Without lime, roots had a higher content and concentration of P than shoots; liming reversed this. Without lime the sensitive variety with a shorter root length had an Al inflow ten times that of the tolerant one: tolerance involves a mechanism for exlcuding Al. The inflow of P per unit inflow of Al (mol ratio) without lime was three times greater for the tolerant variety which therefore has more P to counteract the effects of Al. The same varieties were grown in two-layer soil columns, with a low P status and a limed topsoil and acid subsoil. Liming the subsoil improved plant growth but this was still restricted by low P availability. Addition of P to the topsoil caused good growth regardless of subsoil acidity: root growth increased in both layers and P (labelled with³²P) taken up from the topsoil was translocated to roots in the subsoil. This P inactivated root Al and allowed the roots to grow and take up more P from the acid subsoil with however a reduction in inflow. The sensitive variety was affected more by the acid subsoil and low P availability, had a similar ability to translocate P to subsoil roots but could not attain the growth rate of the tolerant wheat even with P and lime.     

Leaf phosphorus fractionation and growth responses to phosphorus of the forage legumes Trifolium repens, T. dubium and Lotus pedunculatus

'Grasslands Huia' white clover ( Trifolium repens L.), 'Grasslands Maku' lotus ( Lotus pedunculatus Cav.) and suckling clover ( T. dubium Sibth) were grown in a controlled environment at various levels of P supply. Dry weights and the concentration of inorganic-, lipid-, ester- and residual-P in trifoliate leaves were measured. Lotus grew better than white or suckling clover at low P. White clover and lotus responded steeply to increased P and had similar shoot dry weights at high P. Suckling clover had lower shoot weights than the other species at all P levels. The superior growth of lotus at low levels of p was probably due to better root growth and P uptake. Lotus had higher shoot P concentrations at low levels of P but lower concentrations than the others at high levels. White clover and suckling clover had similar shoot P concentrations at all levels of supply. In white and suckling clover total leaf P concentration rose with P supply. Of the P fractions, inorganic-and residual-P showed the largest rises in concentration. The increases in lipid- and ester-P were smaller. Increases in lotus leaf P were small, primarily because of the relatively small rises in inorganic- and residual-P. White clover is a vigorous species but requires high levels of P for best growth. Suckling clover has a relatively small response to improvements in P availability. The behaviour of the various P fractions is similar to that in white clover. Lotus grows well at low P but also shows rapid growth at high P supply. Whether efficiency is defined as the ability to extract P from the environment or to maintain low internal P concentration, lotus makes efficient use of P over the whole range of P supply.     

The effect of source,concentration and time of application of nitrogen on the growth,nodulation and nitrogen fixation ofLotus pedunculatus andTrifolium repens

Summary Studies under growth cabinet conditions investigated the effect of source and concentration of nitrogen and timing of nitrogen application on the growth and nitrogen fixation byLotus pedunculatus cv. Maku andTrifolium repens cv. S184. KNO₃, NaNO₃ and NH₄NO₃ were added at transplanting at the following rates: 3.33, 7.78 and 13.33 mg N/plant. KNO₃ was added at 3.33 and 7.78 mg N/plant at 0, 6, 12, 18, 24 or 30 days after transplanting.Lotus shoot weight increased with all increasing nitrogen sources but clover only responded to KNO₃ and NaNO₃. The root weight of both species increased with increasing KNO₃ and NH₄NO₃. The percentage increase in lotus and clover shoot growth was greater than that of root growth when KNO₃ was added within a week of transplanting. Increases in growth by both species resulted from added nitrogen except with lotus when NaNO₃ was applied where increased nitrogen fixation also contributed to increased growth.Weight and number of effective nodules on both species were increased with 3.33 mg N per plant as KNO₃ but nitrogen fixation was not affected. Addition of 13.33 mg N as NaNO₃ reduced weight and number of effective nodules in both species and also nitrogen fixation by lotus.KNO₃ increased growth and nodulation of both species when applied within one week after transplanting. Nodulated lotus plants responded to KNO₃ by increasing growth but not nodulation.KNO₃ appeared to affect infection and development of nodules on lotus and may affect the growth of existing nodules on clover.     

Carbon flow in an upland grassland: effect of liming on the flux of recently photosynthesized carbon to rhizosphere soil

The effect of liming on the flow of recently photosynthesized carbon to rhizosphere soil was studied using ¹³CO₂ pulse labelling, in an upland grassland ecosystem in Scotland. The use of ¹³C enabled detection, in the field, of the effect of a 4‐year liming period of selected soil plots on C allocation from plant biomass to soil, in comparison with unlimed plots. Photosynthetic rates and carbon turnover were higher in plants grown in limed soils than in those from unlimed plots. Higher δ¹³C‰ values were detected in shoots from limed plants than in those from unlimed plants in samples clipped within 15 days of the end of pulse labelling. Analysis of the aboveground plant production corresponding to the 4‐year period of liming indicated that the standing biomass was higher in plots that received lime. Lower δ¹³C‰ values in limed roots compared with unlimed roots were found, whereas no significant difference was detected between soil samples. Extrapolation of our results indicated that more C has been lost through the soil than has been gained via photosynthetic assimilation because of pasture liming in Scotland during the period 1990–1998. However, the uncertainty associated with such extrapolation based on this single study is high and these estimates are provided only to set our findings in the broader context of national soil carbon emissions.     

Characterization of the anomalous infection and nodulation of subterranean clover roots by Rhizobium leguminosarum 1020

Anomalous nodulation of Trifolium subterraneum (subterranean clover) roots by Rhizobium leguminosarum 1020 was examined as a model of modified host-specificity in a Rhizobium-legume symbiosis. Consistent with previous reports, these nodules (i) appeared most often at sites of secondary root emergence, (ii) were ineffective in nitrogen fixation and (iii) were as numerous as nodules formed by an effective Rhizobium trifolii strain. R. leguminosarum 1020, grown on agar plates or in the clover root environment, did not bind the white clover lectin, trifoliin A. This strain did not attach in high numbers, and did not induce shepherd's crooks or infection threads, in subterranean clover root hairs. However, R. leguminosarum 1020 did cause branching, moderate curling and other deformations of root hairs. The bacteria probably entered the clover root through breaks in the epidermis at sites of lateral root emergence. The anomalous nodulation was inhibited by nitrate. Only trace amounts of leghaemoglobin were detected in the nodules by Western blot analysis. The nodules were of the meristematic type and initially contained well-developed infection, bacteroid and senescent zones. Infection threads were readily found in the infection zone of the nodule. However, the bacteroid-containing tissue senesced more rapidly than in the effective symbiosis between subterranean clover and R. trifolii 0403. This anomalous nodulation of subterranean clover by R. leguminosarum 1020 suggests a naturally-occurring alternative route of infection that allows Rhizobium to enlarge its host range.     

Differential response of six cowpea (Vigna unguiculata (L.) Walp) cultivars to liming in an ultisol

Summary The differential response of six cowpea (Vigna unguiculata (L.) Walp) cultivars to liming was assessed in a greenhouse trial using an Ultisol (Typic Paleudult) from southeastern Nigeria. Lime rates from 0 to 5.0 t/ha were applied to give a range of soil pH, determined in 11 soil/water from 4.25 to about 7.0. Without liming, relative dry matter yields of tops ranged between 46.6 and 76.8 percent. Significant yield responses to the first lime increment (0.5 t/ha) were observed particularly in the less tolerant cultivars and maximum yields were generally obtained with application of 1.6 to 2.5 t/ha lime.Among the six cultivars, TVu 4557 and Ife Brown were most tolerant, Vita-1 and Vita-3 were moderately tolerant, and TVu 1977-OD and TVu 4552 were least tolerant to soil acidity. The latter two cultivars were the most poorly nodulated when grown in the unlimed soil. The first lime increment enabled excellent nodulation to be achieved in all cultivars. Nitrogen analyses on tops confirmed the visual symptoms of nitrogen deficiency in two cultivars (Vita-1, TVu 1977-OD) grown in the unlimed soil. Analysis of data on chemical composition of plant tops and chemical soil characteristics gave indications that aluminium toxicity is the major growth limiting factor for cowpeas in this soil.This study, albeit with a limited range of cowpea germplasm clearly indicates that cowpea possesses considerable potential as a grain legume on strongly acid soils (pH<5.0), even in circumstances where lime is not available. Low rates of lime (0.5 t/ha or less) will greatly expand the available choice of cultivars.     

Influence ofRhizobium leguminosarum biovartrifolii host specific nodulation genes on the ontogeny of clover nodulation

Summary The infection of white clover seedlings byRhizobium strains with different host range properties was assessed using various microscopic techniques. Several wild-type andRhizobium leguminosarum biovarvicias hybrid strains containing definedR. l. bv.trifolii host range genes were used. The morphological changes in the root tissue of uninoculated and rhizobia inoculated white clovers were identified and compared. In particular, changes were observed in the induction of inner cortical cell division, alterations to nodule development and lateral root formation. The responses of the infected roots and the types of structures formed support the hypothesis that lateral roots and nodules may be physiologically homologous structures. To establish a normal pattern of nodulation on white clover roots, both sets of known host specific nodulation genes (operonsnod FERL andnod MNX) ofR. l. bv.trifolii were required. However, some nodule development occurred when only thenod FERL genes were present in the hybrid strain.     

Sugar-Binding Activity of Pea Lectin Expressed in White Clover Hairy Roots

Introduction of the pea (Pisum sativum L.) lectin (PSL) gene into white clover (Trifolium repens L.) hairy roots facilitates nodulation by the nitrogen-fixing bacterium Rhizobium leguminosarum biovar viciae, which normally nodulates pea and not white clover (C.L. Diaz, L.S. Melchers, P.J.J. Hooykaas, B.J.J. Lugtenberg, and J.W. Kijne [1989] Nature 338: 579-581). Here, we show that PSL is functionally expressed in transgenic white clover hairy roots transformed with the PSL gene. PSL could be isolated from these roots by affinity chromatography. Immunoanalysis of PSL showed the presence of polypeptides corresponding to the PSL precursor and its [beta] subunits. In addition, we developed a highly sensitive localization technique based on specific binding of a glycan moiety of rat IgE to PSL. Similar to the situation in pea roots, PSL appeared to be localized on the external cell surface of elongated epidermal cells and on the tips of emerging and growing root hairs of transgenic white clover hairy roots. PSL was not observed on normal white clover roots and on hairy roots without the PSL gene. These results show that (a) in transgenic white clover hairy roots, PSL is correctly processed and targeted to root cells susceptible to rhizobial infection, and (b) like in pea roots, PSL is surface bound with at least one of its two sugar-binding sites available for (rhizobial) ligands.     

Dry matter yield,root traits,and nodule occupancy of lucerne and Caucasian clover when grown in acidic soil with high aluminium concentrations

Aims and methods

Lucerne and Caucasian clover dry matter were measured in response to recommended lime and capital P inputs for six years in an acidic soil in the New Zealand high country. The initial three years of the field experiment indicated successful establishment and persistence of both legumes. Lucerne dry matter (DM) yield was up to 4 t/ha/yr in this period and higher than Caucasian clover yields. However, a lack of persistence of lucerne was apparent from this point forward compared with Caucasian clover which produced 7.7 t DM/ha in Year 6. An experiment using tubes of soil was used to investigate whether differences in root traits, nodulation and nodule occupancy were responsible for the differences observed in field persistence over time.

Results

These showed that when rhizobia inoculant was added, the fine root length of Caucasian clover was unaffected (R² = 0.14) by aluminium (Al) content of the soil. In contrast, fine root growth of lucerne was suppressed (R² = 0.79) by the soil Al content. Nodulation of Caucasian clover was unaffected by soil pH or Al when the rhizobia inoculant was provided which suggests the viability of the commercial genotype ICC148 in this soil with a pH of 5.5 and Al ca. 7 mg/kg soil. For lucerne, the maximum nodulation score of 7.3 occurred with 2 t/ha of lime added (soil pH ca.6, Al ca. 0.3 mg/kg) plus inoculant.

Conclusions

This suggests an Al toxic threshold of <1.0 mg Al/kg soil for effective lucerne nodulation. From the lucerne nodules, eight naturalized strains of Ensifer meliloti were identified. In contrast, only one R. leguminosarum strain was detected in the Caucasian clover nodules. The competition between those rhizobia genotypes may negatively affect the efficiency of biological nitrogen fixation in lucerne. Therefore, the lack of genetic diversity of R. leguminosarum bv. trifolii in New Zealand soils might be an advantage especially if the commercial strain is acid soil tolerant.

     

Transient susceptibility of root cells in four common legumes to nodulation by rhizobia

Root cells of four common legumes were found to remain susceptible to nodulation by rhizobia for only a short period of time. Delayed inoculation experiments conducted with these legume hosts indicated that the initially susceptible region of the root became progressively less susceptible if inoculations were delayed by a few hours. Profiles of the frequency of nodule formation relative to marks indicating the regions of root and root hair development at the time of inoculation indicated that nodulation of Vigna sinensis (L.) Endl. cv California Black Eye and Medicago sativa L. cvs Moapa and Vernal roots was inhibited just below the region that was most susceptible at the time of inoculation. This result suggests the existence of a fast-acting regulatory mechanism in these hosts that prevents overnodulation. Nodulation in white clover may occur in two distinct phases. In addition to the transient susceptibility of preemergent and developing root hair cells, there appeared to be an induced susceptibility of mature clover root hair cells. A cell-free bacterial exudate preparation from Rhizobium trifolii cells was found to render mature root hair cells of white clover more rapidly susceptible to nodulation.     

Flux and turnover of fixed carbon in soil microbial biomass of limed and unlimed plots of an upland grassland ecosystem

The influence of liming on rhizosphere microbial biomass C and incorporation of root exudates was studied in the field by in situ pulse labelling of temperate grassland vegetation with (13)CO(2) for a 3-day period. In plots that had been limed (CaCO(3) amended) annually for 3 years, incorporation into shoots and roots was, respectively, greater and lower than in unlimed plots. Analysis of chloroform-labile C demonstrated lower levels of (13)C incorporation into microbial biomass in limed soils compared to unlimed soils. The turnover of the recently assimilated (13)C compounds was faster in microbial biomass from limed than that from unlimed soils, suggesting that liming increases incorporation by microbial communities of root exudates. An exponential decay model of (13)C in total microbial biomass in limed soils indicated that the half-life of the tracer within this carbon pool was 4.7 days. Results are presented and discussed in relation to the absolute values of (13)C fixed and allocated within the plant-soil system.     

Influence of pH,exchangeable aluminium and 0.02M CaCl2-extractable aluminium on the growth and nitrogen-fixing activity of white clover (Trifolium repens) in some New Zealand soils

The effects of soil acidity on the growth and N₂-fixing activity of white clover in seven acid topsoils and subsoils of New Zealand were investigated using a glasshouse experiment.The application of phosphate (Ca(H₂PO₄)₂) to the soils resulted in very large increases in white clover growth on all soils. The application of phosphate, as well as increasing P supply, also decreased 0.02M CaCl₂-extractable Al levels, but had little effect on exchangeable Al levels.Where adequate phosphate was applied, increasing rates of lime (CaCO₃) resulted in increased plant growth on most soils. N₂[C₂H₂]-fixing activity was increased by the first level of lime for one soil, but generally remained approximately constant or declined slightly at higher rates of lime. Up to the point of maximum yield, white clover top weight was more highly correlated with 0.02M CaCl₂-extractable soil Al than with exchangeable Al or pH. At pH values greater than 5.5, plant yield declined on some soils, apparently because of Zn deficiency. The data suggest that white clover is unlikely to be affected by Al toxicity at 0.02M CaCl₂-extractable Al levels of less than about 3.3 g g^–1. However, there were differences between soils in apparent plant tolerance to 0.02M CaCl₂-extractable Al, which appeared to be caused by differing C levels in the 0.02M CaCl₂ extracts.     

The effect of moisture stress on stolon and adventitious root development in white clover (Trifolium repens L.)

Summary Humidity, at the young nodes of white clover stolones, varied by enclosing nodes in the atmosphere above a range of saturated solutions, inhibited root initiation at 85% RH or less. The threshold humidity for root initiation increased to about 93% on young nodes subject to moisture stress or old nodes on well watered plants in which root initiation had been previously suppressed by low humidity.Roots at old nodes and at the three youngest on stolons were either subject to moisture stress or adequately watered. Growth of young roots and N₂-fixation were more adversely affected by the direct effects of drought than by subjecting old roots to drought. Although old roots under stress affected new root growth and N₂-fixation, length of roots and lateral root number were little affected. By contrast stolon growth was affected more by stress to old roots than to young nodes, although after 6 weeks the contribution made by young roots to stolon growth was almost as high as old roots.The data suggest that deep roots at old nodes will allow clover stolons to grow during drought due to the high acropetal movement of water but initiation of roots and functioning of young roots at the soil surface will be adversely affected, with possible implications on the persistence of clover.     

Influence of soil pH on the soybean-endomycorrhiza symbiosis

Summary Soybean (Glycine max {L.} Merr.) cultivars were inoculated withGigaspora gigantea andGlomus mosseae to determine mycorrhizal: cultivar relationships as affected by soil pH. The specific cultivarfungal response was dependent on soil pH. Overall cultivar responses in unlimed soil (pH 5.1) were greater forG. gigantea thanG. mosseae. The Bossier —G. gigantea combination was particularly responsive in unlimed soil and showed an increase of 10% in shoot length, 35% in shoot dry weight. 75% in root dry weight, and 397% in nodule dry weight over uninoculated controls. Little cultivar response was observed withG. mosseae inoculation in unlimed soil. In limed soil (pH 6.2), the larger responses were obtained withG. mosseae inoculated plants, although inoculation with eitherG. mosseae orG. gigantea appeared effective. In general, nodulation was greater on mycorrhizal roots than on control roots.     

The diversity of Phaseolus-nodulating rhizobial populations is altered by liming of acid soils planted with Phaseolus vulgaris L. in Brazil

PCR-mediated restriction fragment length polymorphism (RFLP) analysis of the 16S-23S rRNA internally transcribed spacer (ITS) region and the 16S rRNA gene indicated that the rhizobial populations isolated from common bean (Phaseolus vulgaris L.) nodules in the unlimed soil from a series of five lime rates applied 6 years previously to plots of an acidic oxisol had less diversity than those from plots with higher rates of liming. Isolates affiliated with Rhizobium tropici IIB and Rhizobium leguminosarum bv. phaseoli were predominant independent of lime application. An index of richness based on the number of ITS groups increased from 2.2 to 5.7 along the soil liming gradient, and the richness index based on "species" types determined by RFLP analysis of the 16S rRNA gene varied from 0.5 to 1.4. The Shannon index of diversity, based on the number of ITS groups, increased from 1.8 in unlimed soil to 2.8 in limed soil, and, based on RFLP analysis of the 16S rRNA gene, ranged from 0.9 to 1.4. In the limed soil, the subpopulation of R. tropici IIB pattern types contained the largest number of ITS groups. In contrast, there were more R. leguminosarum bv. phaseoli types in the unlimed soil with the lowest pH than in soils with the highest pH. The number of ITS ("strain") groups within R. leguminosarum bv. phaseoli did not change with increased abundance of rhizobia in the soil, while with R. tropici IIB, the number of strain groups increased significantly. Some cultural and biochemical characteristics of Phaseolus-nodulating isolates were significantly related to changes in soil properties caused by liming, largely due to changes in the predominance of the rhizobial species groups.