Influence of phytoparasitic nematodes on symbiotic N2 fixation in tropical herbaceous legume cover crops

 Populations of plant parasitic nematodes and their effects on symbiotic nitrogen (N) fixation in herbaceous legumes and on some selected characteristics of other plant species associated with such cover crops were studied. Two legume species [mucuna, Mucuna pruriens (L) DC. var. utilis (Wright) Bruck and lablab, Lablab purpureus L. Sweet], one grass/weed species [imperata, Imperata cylindrica (L.) Rauschel] and a cereal (maize, Zea mays L.) were used. There were three soil treatments (fumigation, fumigation plus inoculation with Meloidogyne species, and an untreated control). Plant parasitic nematode populations in soil, roots and nodules were determined at 4, 8 and 12 weeks after planting. The response of the phytoparasitic nematodes to soil treatments varied according to the plant species present. The predominant nematodes in soils, roots and nodules of legumes were of the genus Meloidogyne, whereas other genera of parasitic nematodes dominated the fauna in soils and roots of maize and imperata. Biomass yield of mucuna was not significantly affected by either Meloidogyne spp. or the other genera of phytoparasitic nematodes. In contrast, the dry matter yield of lablab measured at 12 weeks was reduced by 16% in inoculated compared with fumigated soils. Similarly, the biomass yields of maize and imperata were reduced by 10% and 29%, respectively, in unfumigated rather than fumigated soils. The amounts of N accumulated in mucuna, maize and imperata were not significantly affected by the two groups of plant parasitic nematodes. However, at 12 weeks, lablab grown on inoculated soils accumulated only 69% of the N found in plants grown on fumigated soils. Inoculation of soil with Meloidogyne spp. significantly increased the number of nodules on lablab roots compared with the non-inoculated treatments, whereas nodulation in mucuna was not affected by soil treatment. After 12 weeks, the quantity of N₂ derived from symbiotic fixation in mucuna was not significantly affected by soil treatments whereas the amount of fixed N in lablab was 32% lower in inoculated than in fumigated soils. Possible mechanisms for the non-suppressive effect of plant parasitic nematodes on mucuna are discussed. Received: 12 March 1999     

Response of Mucuna pruriens to symbiotic nitrogen fixation by rhizobia following inoculation in farmers' fields in the derived savanna of Benin

 Leguminous cover crops such as Mucuna pruriens (mucuna) have the potential to contribute to soil N and increase the yields of subsequent or associated cereal crops through symbiotic N fixation. It has often been assumed that mucuna will freely nodulate, fix N₂ and therefore contribute to soil N. However, results of recent work have indicated mucuna's failure to nodulate in some farmers' fields in the derived savanna in Benin. One of the management practices that can help to improve mucuna establishment and growth is the use of rhizobial inocula to ensure compatibility between the symbiotic partners. Experiments were conducted in 1995 and 1996 on 15 farmers' fields located in three different villages (Eglimé, Zouzouvou and Tchi) in the derived savanna in Benin. The aim was to determine the response of mucuna to inoculation and examine the factors affecting it when grown in relay cropping with maize. The actual amount of N₂ fixed by mucuna in the farmers' fields at 20 weeks after planting (WAP) averaged 60 kg N ha^–1 (range: 41–76 kg N ha^–1) representing 55% (range: 49–58%) of the plant total N. The result suggested that mucuna in these farmers' fields could not meet its total N demand for growth and seed production only by N₂ fixation. It was estimated that after grain removal mucuna led to a net N contribution ranging from –37 to 30 kg N ha^–1. Shoot dry weight at 20 WAP varied between 1.5 and 8.7 t ha^–1 and N accumulation ranged from 22 to 193 kg N ha^–1. Inoculation increased shoot dry matter by an average of 28% above the uninoculated treatments, but the increase depended on the field, location and year. For the combinations of inoculated treatments and farmers' fields, the response frequency was higher in Eglimé and Tchi than in Zouzouvou. The response to inoculated treatments was dependent on the field and inversely related to the numbers of rhizobia in the soil. Soil rhizobial populations ranged from 0 to >188 cells g^–1 soil, and response to inoculation often occurred when numbers of indigenous rhizobia were <5 cells g^–1 soil. In two farmers' fields at Zouzouvou where extractable P was below 10 μg g^–1 soil, mucuna did not respond to rhizobial inoculation despite a higher population of rhizobia. Significant relationships between mycorrhizal colonization, growth and nodulation of mucuna were observed, and inoculated plants with rhizobia had a higher rate of colonization by arbuscular mycorrhizal fungi (%AMF) than uninoculated ones. Therefore, it was shown that mucuna will establish and fix N₂ effectively in those fields where farmer's management practices such as good crop rotation and rhizobial inoculation allow a build up of AMF spores that might lead to a high degree of AMF infection and alleviate P deficiency. Received: 14 June 1999     

Variability of nodulation and dinitrogen fixation capacity among soybean cultivars

 In order to identify soybean cultivars with higher biological N₂ fixation capacities, North American and Brazilian soybean [Glycine max (L.) Merrill] cultivars, belonging to maturity groups VI–VIII, were evaluated for nodulation parameters and N₂ fixation rates. The symbiotic performance of 152 cultivars was evaluated in pots containing 4 kg soil with an established population of the three Bradyrhizobium elkanii strains [29w (SEMIA 5019):SEMIA 566 : SEMIA 587, 22%:36%:34%] which are established in most Brazilian soils cultivated with soybean. Differences were verified among cultivars, with some accumulating up to twice as much nodule dry weight and N in tissues as others. The variability among cultivars was also confirmed when six of them were used in a field experiment, resulting in differences in nodulation, yield and total N accumulated in grains. The analysis of nodule occupancy in 12 cultivars grown either under sterile conditions and receiving a double inoculum and N-free nutrient solution, or in pots containing soil with an established population of bradyrhizobia, showed the preference of cultivars for specific strains. Received: 7 December 1998     

Soil organic carbon stocks, storage profile and microbial biomass under different crop management systems in a tropical agricultural ecosystem

 We investigated the soil organic C and N stocks, storage profiles and microbial biomass as influenced by different crop management systems in a tropical agricultural ecosystem. The different crop management systems significantly affected the C and N stocks and microbial biomass C and N at different soil depths. Amongst the systems evaluated, the rice-wheat system maintained a higher soil organic C content. Inclusion of legumes in the system improved the soil organic matter level and also soil microbial biomass activity, vital for the nutrient turnover and long-term productivity of the soil. Irrespective of the cropping system, approximately 58.4%, 25.7% and 15.9% of the C was distributed in 0–15, 15–30 and 30–60 cm depths, respectively. Received: 10 October 1999     

Arbuscular mycorrhizal fungi respond to rhizobial inoculation and cropping systems in farmers' fields in the Guinea savanna

 It has been difficult to explain the rotation effect based solely on N availability in maize-soybean cropping systems in the moist savanna zone of sub-Saharan Africa. Although arbuscular mycorrhizal fungi (AMF) can contribute to plant growth by reducing stresses resulting from other nutrient deficiencies (mainly P) and drought, their role in the maize/soybean rotation cropping systems in the Guinea savanna has not yet been determined. Pot and field experiments were conducted for 2 years using 13 farmers' fields with different cropping histories in two agroecological zones (Zaria, northern Guinea savanna and Zonkwa, southern Guinea savanna) in Nigeria. We quantified the influence of cropping systems and rhizobial inoculation on plant growth, mycorrhizal colonization and diversity of promiscuous soybean and maize grown in rotation. The relationships between these variables and selected soil characteristics in farmers' fields were also examined. Percentage mycorrhizal colonization in promiscuous soybean roots ranged from 7% to 36%, while in maize it varied between 17% and 33%, depending on fields and the previous cropping history. A large variation was also observed for mycorrhizal spores, but these were not correlated with mycorrhizal colonization and did not appear to be influenced by rotation systems. Soybean mycorrhizal colonization was higher (13% increase) in Zonkwa, but not in Zaria, if the preceding crop was maize and not soybean. These differences were related to the soil P concentration, which was positively related to mycorrhizal colonization in Zonkwa but negatively to this parameter in Zaria. The previous crop did not affect mycorrhizal colonization of maize in both locations. Soybean cultivars inoculated with rhizobia had a higher mycorrhizal colonization rate (25%) and more AMF species than maize or uninoculated soybean (19%). Maize grown in plots previously under inoculated soybean also had higher percentage mycorrhizal colonization than when grown after uninoculated soybean and maize. Four AMF genera comprising 29 species were observed at Zaria and Zonkwa. Glomus was the dominant genus (56%) followed by Gigaspora (26%) and Acaulospora (14%). The genus Sclerocystis was the least represented (4%). Received: 28 October 1998     

Nodulation and estimation of symbiotic nitrogen fixation by herbaceous and shrub legumes in Guinea savanna in Nigeria

Twelve herbaceous and shrub legume species were grown in pot and field experiments in five sites representing three agroecological zones in moist savanna in Nigeria. The objectives were to: (1) assess natural nodulation of the legumes and characterize their indigenous rhizobia, (2) determine their need for rhizobia inoculation and (3) estimate the amount of N₂ fixed by each of these legumes. At 4 weeks after planting (WAP), Crotolaria verrucosa was not nodulated at any of the sites while Centrosema pascuorum had the highest number of nodules in all sites. At 8 WAP, all legumes were nodulated, with Mucuna pruriens having the least number of nodules and Stylosanthes hamata the highest. The number of nodules, however, was inversely correlated to the mass of nodules. Significant differences in nodulation of the legume species grown in the field also occurred between and within sites. Mucuna pruriens and Lablab purpureus produced more shoot and nodule biomass than the other legumes in all sites. Growth of most of these legumes responded to fertilizer application, except for C. verrucosa and Aeschynomene histrix. Except for C. verrucosa, average proportion of N₂ fixed was about 80% and this was reduced by about 20% with N fertilizer application. The majority of rhizobia isolates (60%) were slow growing, belonging to the Bradyrhizobia spp. group. Selected rhizobia isolates evaluated on Cajanus cajan, C. pascuorum, M. pruriens and Psophocarpus palustris varied from ineffective to highly effective in Leonard jar conditions. However, only growth of M. pruriens responded to inoculation in potted soils, whereas it was lower than that obtained with N fertilizer application. This indicated the need to screen more rhizobia in order to improve N₂ fixation and growth of legume species such as M. pruriens when it is introduced in soils deficient in N.     

Nitrogen uptake of sorghum (Sorghum bicolor L.) from tree mulch and mineral fertilizer under high leaching conditions estimated by nitrogen-15 enrichment

 The effects of applying either inorganic fertilizer or leaf mulch of Acacia saligna (Labill.) H.L. Wend. on yields of Sorghum bicolor (L.) were compared with an unfertilized control under the high leaching conditions of runoff irrigation in a dry tropical environment. The N use efficiency and transfer from ¹⁵N-labelled (NH₄)₂SO₄ or acacia leaves to the sorghum differed in quantity and quality. Only 6% of the applied mulch N was retrieved in the crop, in contrast to 21% of the fertilizer N. The proportions of N in the crop derived from the fertilizers were small, amounting to 7% and 28%, respectively, in the mineral fertilizer and mulch treatments. However, the application of inorganic fertilizer and mulch significantly increased crop grain yield (P<0.05 and P<0.1, respectively), biomass production and foliar N contents (P<0.05). The inorganic fertilizer improved crop yields to a larger extent than mulching. At the same time, more N was lost by applying (NH₄)₂ SO₄ than leaf mulch: only 37% of the N of applied (NH₄)₂ SO₄ was found in the crop and the soil (0–0.3 m), but 99% of the mulched N. High NO₃ ^– contents in the topsoil of the inorganic fertilized sorghum treatments indicated the risk of N leaching. However, more important may have been gaseous N losses of surface-applied NH₄ ⁺. From a nutrient conservation point of view, mulches should be given preferance to inorganic fertilizers under high soil pH and leaching conditions, but larger improvements of crop yields could be achieved with mineral fertilizers. Received: 29 July 1998     

Time-course of dinitrogen fixation of promiscuous soybean cultivars measured by the isotope dilution method

 Soybean cultivars capable of nodulating with indigenous Bradyrhizobium spp. have been developed by the International Institute of Tropical Agriculture (IITA) and national programs in Africa in order to avoid artificial inoculation by resource-poor farmers in Africa. The current selection procedure for enhanced N₂ fixation is based on an assessment of nodule formation which does not directly quantify the proportions of crop N derived from the atmosphere. We have monitored N accumulation patterns and N₂ fixation in nine promiscuous soybean cultivars with different maturity periods, using the ¹⁵N dilution technique. Nodule development generally peaked at the early podfill stage for all cultivars except Tgx 1519-1D and Tgx 1447-2D in which it continued to increase. The proportion of crop N derived from fixation (%NDFA) ranged between 51% and 67%, 77% and 84%, and 66% and 73% at full bloom, early podfill, and physiological maturity stages, respectively. Total N accumulation increased in all soybean genotypes with increasing plant age. Significant correlations (P<0.001) were established between nodule weight and %NDFA, even though this did not explain the relationship between nodule development and N₂ fixation in cultivars such as Tgx 1519-1D. Promiscuous soybean cultivars retained between 10% and 19% of total N accumulated at the final harvest, in belowground biomass. Our results indicated that these soybean cultivars can derive substantial proportions of plant N from N₂ fixation in soils where compatible indigenous bradyrhizobia populations are adequate and effective. Also, we have substantiated the claims that qualitative nodulation parameters currently used to select varieties with a high N₂ fixation capacity need to be validated with other measurements of N₂ fixation. Received: 5 November 1998     

Urease activity and its relation to soil organic matter, microbial biomass nitrogen and urea-nitrogen assimilation by maize in a Brazilian Oxisol under no-tillage and tillage systems

 We studied the relationship between urease activity (UA) and soil organic matter (SOM), microbial biomass N (N_biom) content, and urea-N fertilizer assimilation by maize in a Dark Red Latosol (Typic Haplustox) cultivated for 9 years under no-tillage (NT), tillage with a disc plough (DP), and tillage with a moldboard plough (MP). Two soil depths were sampled (0–7.5 cm and 7.5–15 cm) at 4 different times during the crop cycle. Urea was applied at four different rates, ranging from 0 to 240 kg N ha^–1. The levels of fertilizer N did not affect the UA, SOM content, and N_biom content. No significant difference between the treatments (NT, DP, and MP) was observed for SOM during the experiment, probably because the major part of the SOM was in recalcitrant pools, since the area was previously cultivated (conventional tillage) for 20 years. The N_biom content explained 97% and 69% of the variation in UA in the upper and deeper soil layer, respectively. UA and biomass N were significantly higher in the NT system compared to the DP and MP systems. The highest maize productivity and urea-N recovery was also observed for the NT system. We observed that the increase in urea-N losses under NT, possibly as a consequence of a higher UA, was compensated for by the increase in N immobilized in the biomass. Received: 2 July 1999     

Responses of nitrous oxide, dinitrogen and carbon dioxide production and oxygen consumption to temperature in forest and agricultural light-textured soils determined by model experiment

 The experiment, carried out on a forest and arable light-textured soil, was designed to study the temperature response of autotrophic and heterotrophic N₂O production and investigate how the N₂O flux relates to soil respiration and O₂ consumption. Although N₂O production seemed to be stimulated by a temperature increase in both soils, the relationship between production rate and temperature was different in the two soils. This seemed to depend on the different contribution of nitrification and denitrification to the overall N₂O flux. In the forest soil, almost all N₂O was derived from nitrification, and its production rate rose linearly from 2  °C to 40  °C. A stronger effect of temperature on N₂O production was observed in the arable soil, apparently as a result of an incremental contribution of denitrification to the overall N₂O flux with rising temperature. The soil respiration rate increased exponentially with temperature and was significantly correlated with N₂O production. O₂ consumption stimulated denitrification in both soils. In the arable soil, N₂O and N₂ production increased exponentially with decreasing O₂ concentration, though N₂O was the main gas produced at any temperature. In the forest soil, only the N₂ flux was related exponentially to O₂ consumption and it outweighed the rate of N₂O production only at >34  °C. Thus, it appears that in the forest soil, where nitrification was the main source of N₂O, temperature affected the N₂O flux less dramatically than in the arable soil, where a temperature increase strongly stimulated N₂O production by enhancing favourable conditions for denitrification. Received: 26 August 1998     

Genetic variability in dinitrogen fixation between cowpea [Vigna unguiculata (L.) Walp] cultivars determined using the nitrogen-15 isotope dilution technique

 N fixed in 16 cultivars of cowpea [Vigna unguiculata (L.) Walp] inoculated with effective Bradyrhizobium strains collected from the West African MIRCEN culture collection was measured by ¹⁵N isotope dilution technique. In all plant parts, significant differences in the percentage of N derived from the atmosphere (%Ndfa) and the amount of Ndfa occurred between the cultivars. Ndoute variety exhibited the highest %Ndfa (74.33% in shoots; 60.90% in roots) and accumulated more fixed N (960 mg N plant^–1 and 38 mg N plant^–1 in shoots and roots, respectively). Therefore this cultivar should be selected as the highest N-fixing cowpea cultivar. It also should be used in a breeding programme to contribute to the development of cultivars that could stimulate an intensive use of cowpea in many different cropping systems in Africa with a view to maintaining soil fertility. Received: 14 June 1999     

Effects of trefoil cover crop and earthworm inoculation on maize crop and soil organisms in Reunion Island

 Traditional tree fallows have been abandoned on the western coast of the Reunion Island because of the increasing need for cultivated land. Soil fertility is no longer restored and crop yields have decreased drastically. The leguminous plant, Lotus uliginosus (trefoil), used as a cover crop, has made possible the control of erosion, the restoration of soil macrofauna, especially earthworms, and the increase in crop yields. When trefoil was associated with earthworms (Amynthas corticis), the densities of maize, the yields of maize stalk and dry matter, the yield of trefoil fodder dry matter, and the biomass and respiratory activity of soil microflora were considerably increased. The combined effects of their association led to a significant decrease in populations of the plant-parasitic nematode, Pratylenchus vulnus, in maize roots, and in the population of borers. Some soil chemical features were modified. Received: 10 September 1997     

Evolution of burrow systems after the accidental introduction of a new earthworm species into a Swiss pre-alpine meadow

 The unintentional introduction of a new earthworm species (Aporrectodea nocturna) into a Swiss pre-alpine meadow resulted in a great increase in earthworm density in the newly colonized area (386 m^–2) compared with the density observed in the natural area (273 m^–2) where an earthworm community was already present. To investigate the impact of this introduction on the burrow systems, eight soil cores (length 25 cm, diameter 16 cm) were taken (four in the colonized area and four in the natural area) and analysed with computer-assisted X-ray tomography. The resulting images were processed to obtain the 3D-skeleton reconstructions of the earthworm burrow systems. Due to high variability in these burrow systems, only slight differences were observed between the two areas. The total burrow length and the mean burrow lengths tended to be greater in the colonized area. Moreover, the distribution of pore numbers with depth showed different patterns with a maximum for depths between 10 cm and 15 cm in the colonized area and a maximum for depths between 20 cm and 25 cm in the natural area. These differences may have been related to: (1) the particular behaviour of A. nocturna, which was observed to cast at the surface in this site, and (2) the predominance of juvenile earthworms around the colonization front. These differences were sufficient to create significant effects on the continuity of the burrow systems (assessed by the number of different pathways between virtual horizontal planes) for the two areas. The colonized area was characterized by a greater pore continuity, which could have resulted in enhanced transfer properties. Received: 2 July 1999     

Nitrous oxide emissions from an irrigated sandy-clay loam cropped to maize and wheat

 Nitrous oxide (N₂O) emissions were measured from an irrigated sandy-clay loam cropped to maize and wheat, each receiving urea at 100 kg N ha^–1. During the maize season (24 August–26 October), N₂O emissions ranged between –0.94 and 1.53 g N ha^–1 h^–1 with peaks during different irrigation cycles (four) ranging between 0.08 and 1.53 g N ha^–1 h^–1. N₂O sink activity during the maize season was recorded on 10 of the 29 sampling occasions and ranged between 0.18 and 0.94 g N ha^–1 h^–1. N₂O emissions during the wheat season (22 November–20 April) varied between –0.85 and 3.27 g N ha^–1 h^–1, whereas peaks during different irrigation cycles (six) were in the range of 0.05–3.27 g N ha^–1 h^–1. N₂O sink activity was recorded on 14 of the 41 samplings during the wheat season and ranged between 0.01 and 0.87 g N ha^–1 h^–1. Total N₂O emissions were 0.16 and 0.49 kg N ha^–1, whereas the total N₂O sink activity was 0.04 and 0.06 kg N ha^–1 during the maize and wheat seasons, respectively. N₂O emissions under maize were significantly correlated with denitrification rate and soil NO₃ ^–-N but not with soil NH₄ ⁺-N or soil temperature. Under wheat, however, N₂O emissions showed a strong correlation with soil NH₄ ⁺-N, soil NO₃ ^–-N and soil temperature but not with the denitrification rate. Under either crop, N₂O emissions did not show a significant relationship with water-filled pore space or soil respiration. Received: 11 June 1997     

Management of a previously eroded tropical Alfisol with herbaceous legumes: Soil loss and physical properties under mound tillage

A study was carried out on a previously eroded Oxic Paleustalf in Ibadan, southwestern Nigeria to determine the extent of soil degradation under mound tillage with some herbaceous legumes and residue management methods. A series of factorial experiments was carried out on 12 existing runoff plots. The study commenced in 1996 after a 5-year natural fallow. Mound tillage was introduced in 1997 till 1999. The legumes – Vigna unguiculata (cowpea), Mucuna pruriens and Pueraria phaseoloides – were intercropped with maize in 1996 and 1998 while yam was planted alone in 1997 and 1999. This paper covers 1997–1999. At the end of each year, residues were either burned or mulched on respective plots. Soil loss, runoff, variations in mound height, bulk density, soil water retention and sorptivity were measured. Cumulative runoff was similar among interactions of legume and residue management in 1997 (57–151 mm) and 1999 (206–397 mm). However, in 1998, cumulative runoff of 95 mm observed for Mucuna-burned residue was significantly greater than the 46 mm observed for cowpea-burned residue and the 39–51 mm observed for mulched residues of cowpea, Mucuna and Pueraria. Cumulative soil loss of 7.6 Mg ha⁻¹ observed for Mucuna-burned residue in 1997 was significantly greater than those for Pueraria-mulched (0.9 Mg ha⁻¹) and Mucuna-mulched (1.4 Mg ha⁻¹) residues whereas in 1999 it was similar to soil loss from cowpea treatments and Pueraria-burned residue (2.3–5.3 Mg ha⁻¹). There were no significant differences in soil loss in 1998 (1–3.2 Mg ha⁻¹) whereas Mucuna-burned residue had a greater soil loss (28.6 Mg ha⁻¹) than mulched cowpea (6.9 Mg ha⁻¹) and Pueraria (5.4 Mg ha⁻¹). Mound heights (23 cm average) decreased non-linearly with cumulative rainfall. A cumulative rainfall of 500 mm removed 0.3–2.3 cm of soil from mounds in 1997, 3.5–6.9 cm in 1998 and 2.3–4.6 cm in 1999, indicating that (detached but less transported) soil from mounds was far higher than observed soil loss in each year. Soil water retention was improved at potentials ranging from −1 to −1500 kPa by Mucuna-mulched residue compared to the various burned-residue treatments. Also, mound sorptivity at −1 cm water head (14.3 cm h^−1/2) was higher than furrow sorptivity (8.5 cm h^−1/2), indicating differences in hydraulic characteristics between mound and furrow. Pueraria-mulched residues for mounds had the highest sorptivity of 17.24 cm h^−1/2, whereas the least value of 6.96 cm h^−1/2 was observed in furrow of Mucuna-burned residue. Pueraria phas eoloides was considered the best option for soil conservation on the previously eroded soil, cultivated with mound tillage.     

Genotypic diversity and symbiotic effectiveness of rhizobia isolated from root nodules of Phaseolus vulgaris L. grown in Tunisian soils

 One hundred and sixty isolates of rhizobia were sampled from the root nodules of common bean (Phaseolus vulgaris L.) cultivated in Tunisian soil samples originating from three geographically distinct fields. Plasmid profiling was used as a primary method to rapidly screen the isolates, and then 38 plasmid types were recorded among the 160 isolates. A sample representing the majority of plasmid types was chosen for further characterization by restriction fragment length polymorphism (RFLP) analysis of genomic DNA using chromosomal and symbiotic gene probes, and by their ability to nodulate a potential alternative host, Leucaena leucocephala. One third of the isolates showed a high similarity to Rhizobium gallicum isolated from common bean in France, another third showed the same characteristics as the R. etli-R. leguminosarum group, while the remaining isolates could not be related to any of the five species nodulating bean. When reexamined for nodulation, some of these isolates, showing similarities to R. tropici and Agrobacterium with respect to colony morphology and growth in different media, failed to nodulate their original host. The R. gallicum-like isolates, R. etli-like isolates, and R. leguminosarum-like isolates were recovered from regions where bean is frequently grown, while in fields which had not been cultivated with beans for at least the 10 previous years, solely unrecognized taxa of ineffective isolates were recovered. We detected variations in the symbiotic regions, but certain pSym RFLP patterns for nifH were conserved between Tunisian, French, and Austrian populations of bean rhizobia. Evaluation of symbiotic effectiveness showed that R. gallicum-like isolates and R. etli-like isolates were effective, whereas some R. leguminosarum-like isolates were ineffective. Furthermore, effective isolates were also found among the unrecognized taxa. Received: 10 March 1998     

Legumes as dry season fallow in upland rice-based systems of West Africa

Declining fallow length in traditional upland rice-based cropping systems in West Africa results in a significant yield reduction due mainly to increased weed pressure and declining soil fertility. Promising cropping system alternatives include the use of weed-suppressing legumes as short duration fallows. N accumulation, N derived from the atmosphere (Ndfa), weed suppression, and the effects on rice yield were evaluated in 50 legumes, grown at four sites in Côte d'Ivoire with contrasting climate, soils, and rice production systems. The sites were located in the derived and the Guinea savanna and in the bimodal and the monomodal rainfall forest zones. Legume and weed biomass during the fallow were determined at bimonthly intervals. Percent Ndfa by biological N fixation was determined by ¹⁵N natural abundance. Fallow vegetation was cleared and rice seeded according to the practice of local farmers and the cropping calendar. Weed biomass and species composition were monitored at monthly intervals. Legume fallows appear to offer the potential to sustain rice yields under intensified cropping. Biomass was in most instances significantly greater in the legume fallow than in the "weedy" fallow control, and several legume species suppressed weed growth. N accumulation by legumes varied between 1–270?kg N ha^–1 with 30–90%?Ndfa. Across sites, Mucuna spp., Canavalia spp., and Stylosanthes guianensis showed consistently high N accumulation. Grain yields of rice which had been preceded by a legume fallow were on average 0.2?Mg ha^–1 or about 30% greater than that preceded by a natural weedy fallow control. At the savanna sites where fallow vegetation was incorporated, Mucuna spp. and Canavalia ensiformis significantly increased rice yield. In the bimodal forest zone, the highest rice yield and lowest weed biomass were obtained with Crotalaria anagyroides. In general, the effects of legume fallows on rice yield were most significant in environments with favourable soil and hydrological conditions.     

Inorganic fertilizer enrichment of soil: effect on decomposition of plant litter under subhumid tropical conditions

 Litter decomposition is controlled by many factors, including the quality of the litter and conditions within the soil environment. The decomposition and N and P release from prunings of three agroforestry plant species (Sesbania sesban, Croton megalocarpus, and Calliandra calothyrsus) were evaluated on field plots which were amended with: (1) no fertilizer (control), (2) 120 kg N ha^–1 applied as urea (urea), and (3) 150 kg P ha^–1 applied as triple superphosphate (TSP) over a period of 77 days. For all litters, the percentage of initial mass, N and P remaining (dry weight basis) over time followed the single exponential model and was strongly influenced by the interaction of plant species and fertilizer. Decay rate constants (k) of loss of litter mass (k _B), and release of N (k _N), and P (k _P) varied among litters and fertility regimes. The k _B, k _N and k _P of Sesbania and Croton were enhanced more by urea than by TSP and control treatments. For Calliandra, k _B with TSP was higher (0.016 day^–1) than with urea (0.012 day^–1) and control (0.012 day^–1). Sesbania and Croton showed no differences in k _B, k _N and k _P between control and TSP. For Sesbania and Croton, leaching may have accelerated the release of P to meet the needs of decomposer organisms, resulting in no significant effects of TSP on their decomposition processes. The low rate of decomposition of Calliandra may be partly due to its high lignin content. In conclusion, the higher the total N or P of litter, the less likely would be the significant effects of additions of the inorganic form of fertilizer on the decomposition processes. Received: 1 July 1998     

Organic residues affect phosphorus availability and maize yields in a Nitisol of western Kenya

 The effects of organic residues and inorganic fertilizers on P availability and maize yield were compared in a Nitisol of western Kenya. Leaf biomass of Calliandra calothyrsus, Senna spectabilis, Croton megalocarpus, Lantana camara, Sesbania sesban, and Tithonia diversifolia were incorporated into the soil at 5 Mg ha^–1 for six consecutive seasons in 3 years and responses compared with those following the application of 120 kg N ha^–1, 0 kg P ha^–1 (0P); 120 kg N ha^–1, 10 kg P ha^–1; and 120 kg N ha^–1 25 kg P ha^–1 as urea and triple superphosphate (TSP); K was supplied in all treatments. Addition of Tithonia, Lantana and Croton increased soil resin-extractable P over that of fertilizer-amended soil throughout the first crop, but the amounts in the former treatments became similar to those for soils amended with inorganic fertilizers for subsequent crops. Addition of Sesbania, Calliandra and Senna had a similar effect on resin P as inorganic fertilizers. Total maize yields after six seasons were tripled by the application of Tithonia compared to 0P, and were higher than those of the Calliandra, Senna, Sesbania and Lantana treatments, and similar only to that of the Croton treatment. P recovered in the above-ground biomass and resin P, immediately after the implementation of the treatments, was higher in the Senna, Sesbania, Croton, Lantana and Tithonia (35–77%) treatments than in the inorganic fertilizer treatments (21–27%). The P content of organic residues, and the soluble C:total P ratio, were the main residue parameters predicting soil P availability and maize yield. All organic residues used in this study can replace inorganic fertilizers for the enhancement of P availability and maize production, while an additional benefit could be obtained from the use of Croton, Lantana and Tithonia. Received: 19 January 2000