Plant litter decomposition and microbial characteristics in volcanic soils (Mt Etna,Sicily) at different stages of development

Soils at different developmental stages were sampled from eight sites on the slopes of Mt Etna, Sicily (Italy) and characterized for total C, microbial biomass and microbial respiration. The values of these parameters were greatest for the most developed soils, but differences in recent management and site characteristics limited analysis of trends with soil development across the eight sites. The decomposition kinetics of both intact leaf litter and the water-insoluble fraction of leaf litter from three common species on Etna [Etnean broom (Genista aetnensis), European chestnut (Castanea sativa), and Corsican pine (Pinus nigra)] were determined in four of the soils (the two with the smallest and the two with the largest organic C contents) in a laboratory experiment over 168 days to test two hypotheses. First, that the readily mineralized fraction of added plant C is greater when the plant material decomposes in well-developed soils compared to less developed soils, and second, that the microbial communities in less developed soils are less efficient at mineralizing C from low quality plant residues. The first hypothesis held for Genista and Pinus litter, but not Castanea litter. The second hypothesis was supported for the Castanea and Pinus litter, but not for the Genista litter. Thus, the general applicability of the hypotheses was dependent on the precise source and characteristics of the litter.     

Establishment and effectiveness of added pigeonpea (Cajanus cajan) rhizobia in different soils of narrow abiotic variability

Summary Pot and laboratory experiments were conducted to study the establishment and effectiveness of a streptomycin-sulphate-resistant (1 mg/ml of medium) pigeonpea rhizobia strain (RM₇) in sterile sand and non-sterile soils. Strain RM₇ increased the drymatter yield of pigeonpea plants (Cajanus cajan) by 106% over control plants under sterile conditions. However, when the rhizobia strain was introduced into 14 different non-sterile soils with a narrow abiotic variability, the comparable beneficial effect was observed only in one soil inoculated with log 6.70 cells/pot. At this inoculum rate, the percentage increase in yield over control plants varied from –1 to 140 in different soils. Rhizobium (RM₇), applied at log 3.70 cells/pot (3 kg soil), showed less than 5% establishment in four soils. However, establishment varied from 8% to 72% at a higher level of inoculation (log 6.70 cells/pot). Displacement of native rhizobia and creation of new sites for nodulation by the introduced rhizobia were also affected by soil properties. The increase in shoot dry-matter yield compared with control plants was positively correlated with the percent establishment of RM₇ (r = 0.60^*) in these soils. Experiments showed that some biotic stresses led to poor survival, proliferation and establishment of the added alien in the soil. Therefore, any culture that is efficient in one soil may not produce similar results under all situations.     

Responses of co-occurring populations of Dendrobaena octaedra (Lumbricidae) and Cognettia sphagnetorum (Enchytraeidae) to soil pH, moisture and resource addition

Dendrobaena octaedra (Lumbricidae) and Cognettia sphagnetorum (Enchytraeidae) are the two most dominating soil invertebrates in terms of biomass in boreal coniferous forest soils. A microcosm experiment was set up in order to study the influence of pH, moisture and resource addition on D. octaedra and C. sphagnetorum when both species are simultaneously present. Two kinds of coniferous forest humus were used as substrate, pine stand humus (pH 4.2), and spruce stand humus (pH 4.6); in the third treatment the pine stand humus was adjusted with slaked lime (CaOH₂) to the same initial pH as the spruce stand humus. Each substrate was adjusted to water contents of 25%, 42.5% and 60% of WHC (referred to as ‘dry’, ‘moist’ and ‘wet’). In the second part of the experiment, spruce needle litter and birch leaf litter were separately added into the pine stand humus (‘moist’, unlimed) and compared with a control without litter. The microcosms were plastic jars with 75 g (d.m.) of humus, into which 4 specimens of D. octaedra and 70 specimens of C. sphagnetorum were added. D. octaedra showed the highest biomass and C. sphagnetorum the lowest biomass in the spruce stand humus with higher pH. Moisture did not affect earthworms, while C. sphagnetorum thrived best at the highest moisture. Addition of both kinds of litter increased the numbers and biomass of D. octaedra, while on C. sphagnetorum resource addition had little effect. The results help to explain the abundance of these two species in coniferous forests differing in soil acidity, moisture and fertility.     

Relationship of root necrosis potential of soil to asparagus (Asparagus officinalis) decline in Switzerland

Summary The possible relationship between asparagus decline and the root necrosis potential (RNP) of soil was investigated for 11 asparagus and 4 nonasparagus soils. Asparagus seedlings were grown in each soil in climate chambers. A root necrosis index was used to determine RNP. RNP was correlated with the decline observed in the field for seven white asparagus crops, but not for three white and one green asparagus crops. Low RNP values were obtained from the soils not planted to asparagus.Fusarium oxysporum andF. oxysporum var. redolens accounted for 7070 of the fungi isolated from necrotic roots in RNP tests. BothFusarium were obtained from all soils. AllF. oxysporum andF. oxysporum var. redolens isolates tested were highly virulent on asparagus seedlings in inoculation experiments.F. solani, Penicillium verrucosum var.corymbiferum, andRhizoctonia violacea were isolated less frequently but were also highly virulent. It is concluded that soil RNP indicated a risk of decline caused by fusaria, but other soil factors were likely to be involved in the aetiology of decline. The study of these factors is necessary to develop a method for the prognosis of decline associated with fusaria.     

施磷对吊兰修复镉污染土壤及土壤酶活性的影响

选用吊兰进行室内盆栽,研究在Cd污染土壤中,施用不同剂量磷肥对吊兰生理指标、土壤酶活性等的影响,为合理施用磷肥,提高Cd污染土壤的植物修复效果提供参考依据。结果表明:吊兰在100mg/kg的Cd污染土壤中能正常生长,施P对吊兰的叶绿素、细胞膜透性、抗氧化酶活性影响显著(p0.01),表现为低浓度促进,高浓度抑制作用。当施磷量为200mg/kg时,叶绿素含量、CAT酶活性最大,外渗电导率和MDA含量最低,POD酶活性开始上升,说明施磷能使吊兰的细胞膜受害程度最低,抗氧化作用增强。同时,施P有利于改善Cd污染土壤的酶系统,对过氧化氢酶、脲酶的活性表现为低浓度促进、高浓度抑制,对蔗糖酶的活性表现相反,对磷酸酶的活性表现抑制作用;且200mg/kg的磷处理时,过氧化氢酶、脲酶的活性最高,蔗糖酶的活性最低。4种土壤酶活性均与P浓度呈高度相关性(p0.05),相关性为脲酶过氧化氢酶蔗糖酶磷酸酶。     

Nitrogen and carbon mineralization of semi-arid shrubland soil exposed to long-term atmospheric nitrogen deposition

Anthropogenic N-deposition represents a significant input of N into semi-arid chaparral and coastal sage scrub (CSS) shrublands of southern California. High levels of atmospheric N deposition have the potential to increase soil C and N mineralization, and we hypothesize that semi-arid shrubland soil exposed to long-term (decades) high N deposition will have significantly higher C and N mineralization potentials. This hypothesis was tested in a laboratory incubation where the inorganic N (NH₄+NO₃) and CO₂ production of soils maintained at a constant temperature of 25°C and a soil moisture of 0.25 g H₂O/g (65% water-filled pore space) were sampled sequentially over a 50-week period. The temporal trend in cumulative C and N mineralization was well described by a first- and zero-order model, respectively. Long-term atmospheric N deposition significantly increased potential N mineralization but not C mineralization, and both the rate and total N mineralization were significantly positively correlated with the surface (0–10 cm) soil δ ¹⁵N natural abundance and negatively correlated with the surface soil C:N ratio. While the incubation techniques used here do not provide realistic estimates of in situ C or N mineralization, these assays indicate that atmospheric N deposition has significantly altered ecosystem N storage and cycling.     

Reproduction and growth of three deep-burrowing earthworms (Lumbricidae) in laboratory culture in order to assess production for soil restoration

Laboratory experiments were conducted to examine the growth and reproduction of three deep-burrowing lumbricids, Aporrectodea longa, Lumbricus terrestris, and Octolasion cyaneum. The reproductive output was recorded as 18.8, 38.0, and 32.3 cocoons per worm per year for A. longa, L. terrestris, and O. cyaneum, respectively. For the same species, maturity was reached at a mean mass of 3.9, 5.0 and 2.4 g, within 3 months from the hatchling stage by L. terrestris and within 4 months by the other two species. The hatching success of cocoons at 15 and 20°C was within the range of 70–80% for each species, except A. longa at the higher temperature, where a viability of 47% was recorded. Twenty percent of viable O. cyaneum cocoons produced twin hatchlings, compared with only one percent for A. longa and L. terrestris. A combination of these results suggests that a complete life-cycle for each species could be achieved within 6 months (L. terrestris and A. longa) or 7–8 months (O. cyaneum). Each species has particular life-cycle strategies that would aid survival and colonisation, under field conditions, if inoculated into restored soils.     

Effects of inoculation with N2-fixing cyanobacteria on the nitrogenase activity in soil and rhizosphere of wetland rice (Oryza sativa L.)

Summary A greenhouse experiment was conducted with wetland rice (Oryza sativa cv. IR-50) in a clay-loam soil (Fluventic Eutrochrept) to study the effect of cyanobacterial inoculation a mixed culture of Aulosira fertilissima, Nostoc muscorum, Nostoc spp., and Anabaena spp., applied at the rate of 0.15 g (dry weight pot^-1 or 43 kg ha^-1) on acetylene reduction activity in soil and the root system (excised root), and the grain and straw yield. The effects of applying P (40 kg ha^-1), N (60 kg ha^-1), and P+N to the soil were also evaluated. Cyanbacterial inoculation significantly increased (more than 200% on average) photo-dependent acetylene reduction activity in soils, particularly where the indigenous activity was considerably low, i.e. under unfertilized and N-fertilized conditions. The effect of inoculation was prominent at the maximum tillering and grain formation stages of the crop. This inoculation benefit was, however, marginal in P-applied soils (P and P+N), where the indigenous activity was stimulated more than threefold. The inoculation led to a remarkable increase in root-associative acetylene reduction activity after the maximum tillering stage of the crop, particularly with applied N but for other treatments this inoculation effect was not significant. Cyanobacterial inoculation also increased the grain and straw yield of the crop when N was not applied. The grain and straw yield was significantly correlated with the acetylene reduction activity in flooded soils and in the root system during the tillering and maximum tillering stages of rice growth, respectively.     

Nitrogen-supplying capacity of leaves of Dactyladenia barteri (Hook ex olw) and Leucaena leucocephala (Lam.) de Wit in two soils of different acidity from southern nigeria

Summary The evolution of mineral and hydrosoluble organic N released from two soils differing in pH and treated with leaves of Leucaena leucocephala (0, 8.3, 16.7, and 33. g kg^-1 soil), Dactyladenia barteri (syn. Acioa barteri; 0 and 16.7 g), and their mixtures was studied in the laboratory using the aerobic incubation-leaching method. N mineralization in untreated soils and in soils supplemented with 8.3 g leucaena leaves was 41–53% higher in the soil from Onne (pH 4.7) than in the soil from Ibadan (pH 6.2), but the organic N content was similar with these treatments in the leachates of the soils from both locations. The application of 16.7 or 33.3 g of either or both type of leaves reduced the rate of mineral N production during the first 4 weeks, particularly in soils treated with dactyladenia leaves (C:N=36). After this lag period, N mineralization proceeded at a faster rate in the soil from Ibadan treated with 16.7 or 33.3 g of leucaena leaves (C:N=12), even in the presence of dactyladenia leaves. In Ibadan soil, after 12 weeks, mineral N apprently derived from leaves of both dactyladenia and leucaena averaged 6.3% of the N applied, and organic N from leaves averaged 9.5%. The addition of dactyladenia and leucaena leaves did not increases the mineral N content in the acid soil from Onne but leaching of soluble organic N with addition of 16.7 or 33.3 g of leaves contributed an N-mineralizable pool of 5.9% of the N applied.     

岩溶区不同恢复阶段檵木根际土壤生态化学计量学特征

[目的]探讨不同植被恢复阶段中檵木(Loropetalum chinense)根际土壤生态化学计量学特征,揭示岩溶石漠化区不同恢复阶段下檵木养分利用规律及限制因子,为岩溶区植被恢复与重建提供科学依据。[方法]利用生态化学计量学方法,分析桂西北岩溶区不同植被恢复阶段(灌木林、原生林)檵木根际土壤生态化学计量学特征。[结果]檵木根际土壤C、N、P含量表现为灌木林阶段高于原生林阶段,而C∶N,C∶P,N∶P比值则刚好相反;同一植被恢复阶段,不同坡位间檵木根际土壤C∶P与N∶P比值存在显著差异;相关性分析表明,土壤C含量与土壤N∶P含量存在显著正相关关系,土壤N,P均与C∶N,C∶P,N∶P存在显著正相关关系,土壤C∶P与C∶N,N∶P存在显著正相关关系。[结论]檵木根际养分供应状况与其生境密切相关,在植被恢复前期(灌木林)檵木生长容易受N素限制,到植被恢复后期檵木生长易受P素限制。     

Effects of plant species on microbial biomass phosphorus and phosphatase activity in a range of grassland soils

Soil P transformations are primarily mediated by plant root and soil microbial activity. A short-term (40 weeks) glasshouse experiment with 15 grassland soils collected from around New Zealand was conducted to examine the impacts of ryegrass (Lolium perenne) and radiata pine (Pinus radiata) on soil microbial properties and microbiological processes involved in P dynamics. Results showed that the effect of plant species on soil microbial parameters varied greatly with soil type. Concentrations of microbial biomass C and soil respiration were significantly greater in six out of 15 soils under radiata pine compared with ryegrass, while there were no significant effects of plant species on these parameters in the remaining soils. However, microbial biomass P (MBP) was significantly lower in six soils under radiata pine, while there were no significant effects of plant species on MBP in the remaining soils. The latter indicated that P was released from the microbial biomass in response to greater P demand by radiata pine. Levels of water soluble organic C were significantly greater in most soils under radiata pine, compared with ryegrass, which suggested that greater root exudation might have occurred under radiata pine. Activities of acid and alkaline phosphatase and phosphodiesterase were generally lower in most soils under radiata pine, compared with ryegrass. The findings of this study indicate that root exudation plays an important role in increased soil microbial activities, solubility of organic P and mineralization of organic P in soils under radiata pine.     

Temperature affects microbial decomposition of cadavers (Rattus rattus) in contrasting soils

The ecology of soils associated with dead mammals (i.e. cadavers) is poorly understood. Although temperature and soil type are well known to influence the decomposition of other organic resource patches, the effect of these variables on the degradation of cadavers in soil has received little experimental investigation. To address this, cadavers of juvenile rats (Rattus rattus) were buried in one of three contrasting soils (Sodosol, Rudosol, and Vertosol) from tropical savanna ecosystems in Queensland, Australia and incubated at 29 °C, 22 °C, or 15 °C in a laboratory setting. Cadavers and soils were destructively sampled at intervals of 7 days over an incubation period of 28 days. Measurements of decomposition included cadaver mass loss, carbon dioxide–carbon (CO₂–C) evolution, microbial biomass carbon (MBC), protease activity, phosphodiesterase activity, and soil pH, which were all significantly positively affected by cadaver burial. A temperature effect was observed where peaks or differences in decomposition that at occurred at higher temperature would occur at later sample periods at lower temperature. Soil type also had an important effect on some measured parameters. These findings have important implications for a largely unexplored area of soil ecology and nutrient cycling, which are significant for forensic science, cemetery planning and livestock carcass disposal.     

The major features of an infestation by the invasive weed legume gorse (Ulex europaeus) on volcanic soils in Hawaii

Gorse (Ulex europaeus) infestation occupies over 4,000 ha of agriculture and conservation lands on the southeastern slope of Mauna Kea on the Island of Hawaii. The aim of this investigation is to identify ecological features associated with this weed invasion by comparing the gorse-infested areas to the surrounding uninfested areas of this landscape. The soils within the gorse infestation are more acidic, resulting in higher levels of KCl-extractable Al and lower levels of Mehlich III-extractable Ca, Mg, Mn, and Zn. Yet, gorse accumulates higher concentrations of Ca, Zn and, Cu than the kikuyu grass (Pennesitum clandestinum), which is ubiquitous throughout the site. The Ca:Al and Mg:Al molar charge ratios of the soils are lowest within the epicenter of the gorse infestation, while the molar ratios are highest in the gorse apical stem tissues. All gorse plants are nodulated and have higher nitrogen contents than the surrounding kikuyu grass. Furthermore, the δ¹⁵N of the gorse stem tissues approaches 0‰, suggesting that nitrogen is being symbiotically fixed from the atmosphere. Characterization of the Bradyrhizobium isolated from gorse nodules shows similarities and distinctions to Bradyrhizobium isolated from the endemic legume koa (Acacia koa) within the same location. Population densities of the indigenous Bradyrhizobium are higher within the gorse rhizosphere than the kikuyu grass. Soil acidification, nutrient depletion, and symbiotic nitrogen fixation distinguish gorse-infested areas from the surrounding uninfested areas. These observations suggest that gorse has a competitive advantage over kikuyu grass under conditions of soil nutrient deficiency.     

Dynamics and availability of phosphorus in the rhizosphere of a temperate silvopastoral system

A field rhizosphere study was carried out over a period of 12 months on a 6-year-old silvopastoral trial in New Zealand. The trial comprised radiata pine (Pinus radiata) with lucerne (Medicago sativa) and perennial ryegrass (Lolium perenne) understoreys. The study was initiated because of the unique interrelationships between roots in silvopastoral systems and a paucity of understanding about the processes involved in phosphorus (P) dynamics in temperate silvopastoral systems. Improving our understanding in this area has important implications for nutrient management in silvopastoral systems. Rhizosphere soils were analysed to determine inorganic (Pi) and organic (Po) P fractions, macroporous resin Pi and Po, phosphatase enzyme activity, microbial biomass carbon and pH. Concentrations of labile Pi were consistently greater and Po lower in tree rhizosphere soil compared to the companion understorey, indicating that radiata pine when grown with a productive understorey mineralised Po to a greater extent than either understorey species. Tree rhizosphere soil from under lucerne and lucerne rhizosphere soil contained the lowest concentrations of total Pi and Po compared with tree under ryegrass and ryegrass rhizosphere soils. This was partly attributed to higher levels of phosphatase enzyme activity in the lucerne rhizosphere soils. The results suggest the combination of lucerne with radiata pine may enhance greater utilisation of soil P, although this requires further investigation. Lower levels of labile Po, and higher levels of labile Pi and phosphatase enzyme activity, were determined in tree and understorey lucerne and ryegrass rhizosphere soils in spring compared with autumn. This data confirmed that overall rates of soil organic P mineralisation are greatest in spring.     

Influence of ectomycorrhizal mat soils on lignin and cellulose degradation

Summary The ectomycorrhizal fungus Hysterangium setchellii (Fisher) forms extensive hyphal mats at the soillitter interface with the roots of the host tree Douglas fir Pseudotsuga menziesii [(Mirb.) Franco]. Microbial biomass, and lignin and cellulose decomposition rates were measured seasonally for 1 year, using ¹⁴C techniques in ectomycorrhizal mat soils and adjacent non-mat soils in a second-growth Douglas fir forest. The microbial biomass and cellulose degradation rates were 3–6 times higher in ectomycorrhizal mat soils than in adjacent nonmat soils. Lignin degradation rates were higher in ectomycorrhizal mat soils than adjacent non-mat soils. Our data suggest that the ectomycorrhizal fungus H. setchellii provides a microenvironment with increased microbiological activity which results in faster lignin and cellulose decomposition.     

Leucaena leucocephala seedling response to vesicular-arbuscular mycorrhizal inoculation in soils with varying levels of inherent mycorrhizal effectiveness

Summary The symbiotic effectiveness of vesicular-arbuscular mycorrhizal (VAM) fungi present in widely differring tropical soils was evaluated in a greenhouse experiment. Small volumes of field soil, a standard inoculum (Glomus aggregatum) or both were introduced into a fumigated sand-soil medium amended with nutrients for optimum VAM activity. Leucaena leucocephala (Lam.) de Wit var. K8 was grown in the medium as an indicator plant. VAM effectiveness was monitored as a function of time by determining the P status of pinnules. The soils differed from each other with respect to the time their endophytes required for the expression of initial and maximum effectiveness and in the level of maximum effectiveness they exhibited. The effect of mycorrhizal inoculation, calculated as the ratio of the areas enclosed by the effectiveness curve of G. aggregatum to that enclosed by the effectiveness curves of test soils, was found to be a good indicator of the response of L. leucocephala to inoculation of soils with G. aggregatum Hawaii Institute of Tropical Agriculture and Human Resources Journal series No. 3285     

Effects of glucose and rhizodeposits (with or without cysteine-S) on immobilized-35S, microbial biomass-35S and arylsulphatase activity in a calcareous and an acid brown soil

Our aim was to study the effects of C (as glucose and artificial rhizodeposits) on S immobilization, in relation to microbial biomass‐S and soil arylsulphatase (ARS) activity, in contrasting soils (a calcareous and an acid brown soil). The glucose‐C and artificial rhizodeposit‐C with or without cysteine were added at six rates (0, 100, 200, 400, 600 and 800 mg kg^?1 soil) to the two soils and then incubated with Na₂³⁵SO₄ for 1 week prior to analysis. The percentages of ³⁵S immobilized increased when C as glucose and rhizodeposit (without cysteine) were added to both soils. With cysteine‐containing rhizodeposit, the percentages of ³⁵S immobilized remained relatively stable (23.5% to 29.9%) in the calcareous soil, but decreased in the acid brown soil (52.7% to 31.5%). For both soils, cysteine‐containing rhizodeposit additions showed no significant correlation between immobilized‐³⁵S and microbial biomass‐³⁵S, suggesting that microorganisms immobilized cysteine‐S preferentially instead of ³⁵S from the tracer (Na₂³⁵SO₄). In the calcareous soil, a positive and significant correlation was found between ARS activity and microbial biomass‐³⁵S (r = 0.85, P < 0.05) when glucose was added. We also saw this correlation in the acid brown soil when rhizodeposit‐C without cysteine was added (r = 0.90, P < 0.05). Accordingly, the results showed the presence of extracellular arylsulphatase activity of 48.7 mg p‐nitrophenol kg^?1 soil hour^?1 in the calcareous soil and of 27.0 mg p‐nitrophenol kg^?1 soil hour^?1 in the acid brown soil.     

The abundance of heterocystous blue-green algae in rice soils and inocula used for application in rice fields

Summary Algal populations were quantified (as colony-forming units [CFU] per square centimetre) in 102 samples of rice soils from the Philippines, India, Malaysia and Portugal, and in 22 samples of soil-based inocula from four countries. Heterocystous blue-green algae (BGA) were present in all samples. Nostoc was the dominant genus in most samples, followed by Anabaena and Calothrix. In soils, heterocystous BGA occurred at densities ranging from 1.0 × 10² to 8.0 × 10⁶ CFU/cm² (median 6.4 × 10⁴) and comprised, on average, 9% of the total CFU of algae. Their abundance was positively correlated with the pH and the available P content of the soils. In soil-based inocula, heterocystous BGA occurred at densities ranging from 4.6 × 10⁴ to 2.8 × 10⁷ CFU/g dw (dry weight), comprising only a moderate fraction (average 13%) of the total algae. In most soils, the density of indigenous N₂-fixing BGA was usually higher than that attained by applying recommended rates of soil-based inoculum. Whereas research on the practical utilization of BGA has been mostly directed towards inoculation with foreign strains, our results suggest that attention should also be given to agricultural practices that enhance the growth of indigenous strains already adapted to local environmental conditions.Maître de Recherches ORSTOM (France), Visiting Scientist at IRRI     

Karyotypic and genome size variation in Cajanus cajan (L.) Millsp. (pigeonpea) and some wild relatives

Detailed karyotypes and 4C DNA amounts have been studied in five cultivars of Cajanus cajan and 20 species belonging to Cajanus, Rhynchosia, Dunbaria, Flemingia and Paracalyx. C. cajan shows intraspecific variability and its karyotype is most similar to that of C. cajanifolius (sect. Cajanus) in the morphology and number of satellite chromosomes and the lack of any correlation between chromosome size within the complement and asymmetry. Karyotypes of C. lineatus and C. sericeus belonging to sect. Atylia are similar with respect to maximum r-index and the ratio of longest and shortest chromosomes in their respective complements. C. acutifolius (sect. Frutocosa) is distinct in having a very low ratio (1.44) between the longest and the shortest chromosomes, while in C. albicans, C. goensis, C. scarabaeoides (sect. Cantharospermum) there is no chromosome pair with r-index >2.0. C. mollis and C. volubilis (sect. Volubilis) show similarity with regard to ratio between the longest and the shortest chromosomes and a distinct chromosome pair with an arm ratio of 2.7 and C. platycarpus is distinct (sect. Rhynchosoides) in having the smallest ratio (1.36) between the longest and the shortest chromosomes. 4C DNA amounts in Cajanus and Rhynchosia vary between 3.28 pg to 11.69 pg and 4.92 pg to 11.13 pg respectively, while in Dunbaria, Flemingia and Paracalyx these vary between 4.70 and 7:06 pg. In the genus Cajanus karyotypic features and 4C DNA amounts agree with the sectional classification and bring out a close relationship between C. cajan and C. cajanifolius. This is supported by studies based on seed protein patterns, isozyme analysis, trypsin and chymotrypsin inhibitor patterns, RFLP and RAPD data and crossability relationships. A clear relation between DNA amount and abnormalities effecting the extent of pairing and recombination in interspecific hybrids shows the importance of this study in developing future breeding programme involving C. cajan and its wild relatives that are of potential value.     

Effect of agroforestry woody species on earthworm activity and physicochemical properties of worm casts

The effects of five agroforestry woody species (Dactyladenia barteri, Gliricidia sepium, Leucaena leucocephala, Senna siamea andTreculia africana) on the surface aasting activity ofHyperiodrilus africanus were studied in an Alfisol (Oxic Paleustalf) in southwestern Nigeria. Casting activity under the woody species decreased in the following order:Dactyladenia sp. (26.4 Mg ha^-1 year^-1)>Gliricidia sp. (24.4 Mg ha^-1 year^-1)>Treculia sp. (22.9 Mg ha^-1 year^-1)>Leucaena sp. (18.6 Mg ha^-1 year^-1)>Senna sp. (18.3 Mg ha^-1 year^-1). These differences in casting activity were partly explained by microclimatic effects. Irrespective of the woody species, the worm casts were higher in clay and silt contents, bulk density, water-stable aggregates, pH, organic C, exchangeable cations, effective cation exchange capacity, and extractable P levels than the corresponding surface soils. The woody species did affect the physicochemical properties and P sorption of the worm casts. The content of water-stable aggregates of worm casts decreased in the following order:Dactyladenia sp.>Treculia sp.>Senna sp.>Leucaena sp.>Gliricidia sp. Large differences in extractable P levels were observed.Senna sp. was associated with the highest extractable P level (11.5 mg kg^-1) andTreculia sp. the lowest (4.9 mg kg^-1). P sorption was highest on worm casts underDactyladenia sp. and lowest on those underTreculia sp. Without fertilizer application, there were no significant differences in the dry weight of maize grown in the different worm casts. With NPK applications, the dry weight of maize grown in worm casts associated withTreculia sp. was significantly lower than that of maize grown in the other worm casts, mainly due to the low extractable P level. Despite a high organic C and exchangeable K status, maize grown in the worm casts still responded significantly to N and K applications. The N uptake by maize grown in worm casts associated withTreculia sp. was lower than that in the other treatments.