K. Mulongoy

Properties of worm casts and surface soils under various plant covers in the humid tropics

Abstract Worm casts and the corresponding surface soils were collected in an Ultisol at Onne (south-eastern Nigeria) under a cover of Pueraria phaseoloides and on Alfisols at Ibadan (south-western Nigeria) under Leucaena leucocephala , under Treculia africana and in a secondary forest. The biological and physico-chemical properties of these samples were assessed. Worm casts composed of finer soil particles had higher pH values, higher mineral element concentrations and more water-stable aggregates than the corresponding soils. Their higher humic acid (HA) fulvic acid (FA) contents, and HA: FA ratio, suggested that worm casting activity accelerates the humification process. Numbers of denitrifying microorganisms or cowpea rhizobia were the same in both casts and soils from each site, but worm casts contained more nitrifiers, had a larger biomass-C content and a higher percent of stress-labile N than the corresponding soils. Enzyme activities were also higher in casts than they were in the corresponding topsoils. Granular casts had similar properties as turret-shaped casts collected at the same site. In general, no microbiological and enzymatic differences were found among samples of casts or soils from different sites, but soil and the corresponding casts collected under T. africana were relatively low in organic-C, total N, mineral N, fulvic and humic acids, biomass-C content, nitrifiers and acid phosphatase activity.

Nitrogen contribution by leucaena (Leucaena leucocephala) prunings to maize in an alley cropping system

SummaryThe N uptake of maize was assessed on an Alfisol in a sole crop and in an alley cropping system in southwestern Nigeria. Although the application of prunings increased the maize N content in both sole and alley-cropped maize, the N contributed to the maize by the prunings was low, ranging between 4.4 and 23.8 kg ha−1. This was equivalent to 3.2% and 9.407% of the N released during decomposition of the prunings. Application of the prunings increased the grain yields of the sole maize by 38% and the maize yield in the alley-cropped plots by 104%, compared with yields in the corresponding plots where prunings were not applied. The results indicate that part of the N from the prunings was retained in the soil organic-N pool. Maize N, dry weights and grain yields were lowest in the alley-cropped plots where prunings were removed, probably because of competition between the maize and the hedgerow trees.

Nitrogen fixation of field-inoculatedLeucaena leucocephala (Lam.) de Wit estimated by the15N and the difference methods

The amount of nitrogen fixed byLeucaena leucocephala (Lam.) de Wit was assessed on an Alfisol at the International Institute of Tropical Agriculture located in southwestern Nigeria. Estimated by the difference method, nitrogen fixation of leucaena inoculated with Rhizobium strain IRc 1045 was 133 kg ha−1 in six months. Inoculation with Rhizobium strain IRc 1050 gave a lower nitrogen fixation of 76 kg ha−1. Fertilization with 40 and 80 kg N ha−1 inhibited nitrogen fixation by 43–76% and 49–71%, respectively. Estimates with the15N dilution method gave nitrogen fixation of 134 kg ha−1 in six months when leucaena was inoculated with Rhizobium strain IRc 1045 and 98 kg ha−1 for leucaena inoculated with Rhizobium strain IRc 1050. This nitrogen fixation represented 34–39% of the plant nitrogen. Inoculated leucaena derived 5–6% of its nitrogen from applied fertilizer and 56–54% from soil.

Microbial biomass and maize nitrogen uptake under a Psophocarpus palustris live-mulch grown on a tropical alfisol

Abstract Nitrogen uptake by maize was monitored for five cropping seasons in a live-mulch of Psophocarpus palustris (psopho) established on an Alfisol at the International Institute of Tropical Agriculture, Ibadan, Nigeria. During the first four cropping seasons, psopho and maize competed for soil N in the live-mulch, and maize grown in a psopho live-mulch contained less N than maize grown by itself. In the fifth season, psopho contributed 15 kg N ha −1 to maize in the psopho-maize plots that were cropped continuously for five seasons. Maize grown in a psopho live-mulch that had already been established for four cropping seasons contained 36 kg ha −1 more N than maize grown by itself. This positive N contribution came, at least in part, from increases in microbial biomass and in organic matter derived from the cover crop in the form of litter and earthworm casts. These casts represented a supplementary N reserve of 13–21 kg ha −1 under the cover crop. The casts also had a larger biomass C content, and higher percent stress-labile N (PSN), organic C and total N than the corresponding soils. Biomass C and PSN values were the lowest in soils from bare maize plots, indicating that the live-mulch improved soil nutrient availability and biological properties.

Interactions between drought stress and vesicular-arbuscular mycorrhiza on the growth of Faidherbia albida (syn. Acacia albida) and Acacia nilotica in sterile and non-sterile soils

SummaryFaidherbia albida (syn. Acacia albida) (Del.) A. Chev. and Acacia nilotica (L.) Willd. were grown for 18 weeks in sterile and non-sterile soils inoculated with Glomus clarum (Nicolson and Schenck). During this period, drought stress was imposed for the last 10 (F. albida) or 12 weeks (A. nilotica) at 2-week intervals. A greater number of leaves abscissed in drought-stressed mycorrhizal plants of A. nilotica than drought-stressed non-mycorrhizal and unstressed plants. In F. albida, the number of abscissed leaves was few and similar for all treatments. At the end of the drought stress, inoculation with vesicular-arbuscular mycorrhizal (VAM) fungi in sterile soil increased the plant biomass of the two tree species compared to the control plants. In non-sterile soil, the mycorrhizal growth response of introduced G. clarum equalled the effect of indigenous VAM fungi. There were significant interactions between the mycorrhizal and drought stress treatments and between the mycorrhizal and soil treatments for plant biomass and P uptake in F. albida. The absence of these interactions except for that between the mycorrhizal and soil treatments in A. nilotica indicates that the increased plant biomass and nutrient uptake cannot be attributed directly to a mycorrhizal contribution to drought tolerance. F. albida tolerated the drought stress by producing long tap roots and similar weights of dry matter in shoots and roots. Whereas A. nilotica tolerated the drought stress by developing larger root systems able to explore a greater volume of soil, in addition to leaf abscission, for a favourable internal water status. The introduction of G. clarum increased nodulation by A. nilotica under unstressed conditions, but at the expense of a reduced P uptake in sterile soil.

Time Course of Biological Nitrogen Fixation, Nitrogen Absorption and Biomass Accumulation in Three Woody Legumes

ABSTRACT In order to determine how dependent a woody legume can be upon potential N sources, the time course of N2 fixation and N absorption was studied on Gliricidia sepium cv. ILG50, Leucaena leucocephala cv. K28, and Albizia lebbeck in a screenhouse for 16 months using Senna siamea as a reference tree. Dry matter and N yield in all four species increased steadily with time. The numbers and dry weight of nodules also increased with tree age. N-difference and 15N dilution methods correlated well (r = 0.87–0.93) in defining an increasing pattern of N2 fixation with time in all species, with the highest increment rate between sowing and 8 months. The percent N2 fixed ranged from 17.9 to 74%, 27.7 to 71.9% and 43.6 to 83.6%, equivalent to 191 to 3385 mg, 321 to 2863 mg and 533 to 6419 mg N in Leucaena, Gliricidia and Albizia, respectively, for the period between 4 and 16 months. In contrast to others, G. sepium had its peak of amount of N2 fixed at 12 months after planting. In both the proportion and amount...

Comparisons of the influence of vesicular-arbuscular mycorrhiza on the productivity of hedgerow woody legumes and cassava at the top and the base of a hillslope in alley cropping systems

We investigated the influence of vesicular-arbuscular mycorrhizal (VAM) inoculation on growth and nutrient relationships in two alley-cropping trials, one at the top and the other at the base of a hillslope. Each trial involved three woody hedgerow legumes with cassava (Manihot esculenta Crantz) as the sole intercrop. The hedgerow trees at the base of the slope showed greater survival and higher leaf dry weights than those at the top of the slope, although these parameters were not affected by VAM inoculation, either at the top or the base of the slope. In contrast to survival, the uptake of nutrients, particularly P and N, was higher for inoculated than uninoculated hedgerow trees, both at the top and at the base of slope. Increases in stem and leaf biomass and the uptake of nutrients by the trees were strongly correlated with increases in P uptake, indicating that the improvements were attributable to VAM inoculation. Cassava tuber yields at the base of the slope, from inoculated or uninoculated plants, were significantly greater than the corresponding cassava yields at the top of the slope. These increases at the base of the slope compared to the top of the slope were not attributed to available soil nutrients but to greater VAM spore density. Higher available soil moisture may have been another factor. Increasing the VAM spore density of effective mycorrhiza through proper agronomic practices at the top of a slope may bring about comparable yields on different parts of the slope.

Nitrogen contribution of five leguminous trees and shrubs to alley cropped maize in Ibadan, Nigeria

There are abundant local legume trees and shrubs potentially suitable for alley cropping systems in the sub-Saharan Africa, which are yet to be studied. The nitrogen contribution of two years old Albizia lebbeck and S. corymbosato yield of maize grown in alley cropping was compared to that of Senna siamea, Gliricidia sepium and Leucaena leucocephala in four seasons at Ibadan. Maize shoot biomass and maize grain yield in A. lebbeck alley compared favourably with that in G. sepium and L. leucocephala. Maize biomass and grain yield in S. corymbosa alleys were the lowest. Within A. lebbeck, L. leucocpehala, and G. sepium alleys there were no significant differences in the maize yield in the alleys that received 0, 40 or 80 kg N/ha. Application of more than 40 kg N/ha in S. corymbosa alleys was not necessary as there was no significant increase in maize yield at the higher level of nitrogen. Maize yield and N uptake in A. lebbeck alleys were not significantly different from yield and N uptake in G. sepium, and L. leucocephala at the same fertilizer level. There was a significant correlation between hedgerow tree biomass and maize grain yield. At the end of twelve weeks after pruning application, the organic residues of the pruning applied in the alleys ranged from 5% in G. sepium and 44% in A. lebbeck in the first year compared with the original pruning applied which showed that the slow rate of A. lebbeck decomposition could have a beneficial effect on the soil. The maize N recovery from applied N fertilizer was low (10–22%). Percentage N recovery from the prunings was low in the non-N fixing trees (12–22%), while the recovery was high (49–59%) in A. lebbeck as well as in the other nitrogen fixing tree prunings. Thus A. lebbeck, apart from enhancing maize growth and grain yield like in L. leucocephala and G. sepium, had an added advantage because it remained longer as mulching material on the soil because of its slow rate of decomposition. It was able to survive pruning frequencies with no die-back. This indicates that A. lebbeck is a good potential candidate for alley cropping system in West Africa. S. corymbosa performed poorly compared with the other legume trees. Though it responded to N fertilizer showing a positive interaction between the hedgerow and fertilizer application, it had a high die back rate following pruning periods and termite attack.

Effectivity of indigenous rhizobia for nodulation and early nitrogen fixation with Leucaena leucocephala grown in nigerian soils

Abstract Rhizobium strains isolated from Leucaena leucocephala . Sesbania rostrata , S. grandiflora , S. punctata , Tephrosia vogelii . Acacia albida and Vigna unguiculata growing in Nigerian soils were characterized and tested for their ability to nodulate and fix atmospheric nitrogen with L. leucocephala . Isolates from L. leucocephala , S. rostrata . S. grandiflora and S. punctata were fast-growing and acid producers whereas those from A. albida , T. vogelii and V. unguiculata were slow-growing and alkali producers. Some of these isolates grew in the presence of 500 μg streptomycin ml −1 . The effectivity of the rhizobia isolates on L. leucocephala was tested in Leonard jars. Isolates from all plant species except those from S. grandiflora and V. unguiculata nodulated L. leucocephala but a wide range of effectiveness was observed. Based on this experiment the ten most promising rhizobia were screened for effectiveness in potted soils collected at IITA, Ibadan (transition forest savanna), and at Fashola (savanna, 70 km north of IITA). Establishment of L. leucocephala was poor in soils without previous history of L. leucocephala cultivation due to the presence of only few native leucaena rhizobia ( −1 soil). Rhizobium isolates IRc 1045 and IRc 1050 obtained from L. leucocephala grown at Fashola and IITA were found to be the most effective rhizobia on this host. In addition to their effectiveness and competitiveness they survived well in the field 1 year after their establishment.

Contribution of arbuscular mycorrhizal (AM) fungi and hedgerow trees to the yield and nutrient uptake of cassava in an alley-cropping system

A field trial on alley-cropping was conducted at the University of Ibadan research farm in the 1990/91 cropping season to assess the contributions of arbuscular mycorrhizal (AM) fungi and hedgerow woody legumes to the yield and nutrient uptake of cassava (Manihot esculenta Crantz) as an intercrop in an infertile soil. The trial also investigated the influence of AM fungi on the interplanting of a non-nodulating woody legume Senna siamea (syn. Cassia siamea) with a nodulating woody legume (Leucaena leucocephala). AM contributions to cassava were greater than the hedgerow contributions, which demonstrated that AM associations are an essential component in the nutrition of cassava. In contrast to cassava, AM inoculation only influenced the leaf dry weight and uptake of nutrients of non-interplanted woody legumes but not the above-ground biomass and P uptake of interplanted woody legumes. However, non-inoculated interplanted Leucaena benefited more from indigenous AM fungi than the competing Senna. The negative contributions to the nutrient uptake (K, Ca and Mg) of cassava by hedgerows and the lack of response to AM inoculation in interplanted hedgerow woody legumes could be attributed to root competition among the different plant species growing in close proximity to each other. The present results show that cassava benefits more from AM association than Leucaena which in turn benefits more than Senna in an alley-cropping system.