Nodulation status of native woody legumes and phenotypic characteristics of associated rhizobia in soils of southern Ethiopia

The nodulation of provenances of Acacia seyal, Acacia tortilis and Faidherbia albida, and other indigenous multipurpose tree species were tested in 14 different soil samples collected from diverse agro-ecological zones in southern Ethiopia. Associated rhizobia were isolated from these and from excavated nodules of field standing mature trees, and phenotypically characterized. Indigenous rhizobia capable of eliciting nodules on at least one or more of the woody legume species tested were present in most of the soils. Tree species were markedly different in nodulation in the different site soils. Sesbania sesban and Acacia abyssinica showed higher nodulation ability across the different sites indicating widespread occurrence of compatible rhizobia in the soils. The nodulation patterns of the different provenances of Acacia spp. suggested the existence of intraspecific provenance variations in rhizobial affinity which can be exploited to improve N fixation through tree selection. Altogether, 241 isolates were recovered from the root nodules of trap host species and from excavated nodules. Isolates were differentiated by growth rate and colony morphology and there were very fast-, fast-, slow-, and very slow-growing rhizobia. The bulk of them (68.5%) were fast-growing acid-producing rhizobia while 25.3% were slow-growing alkali-producing types. Fast-growing alkali-producing (2.9%) and slow-growing acid-producing strains (3.3%) were isolated from trap host species and excavated nodules, respectively. All isolates fell into four colony types: watery translucent, white translucent, dull glistering and milky (curdled) type. The diversity of indigenous rhizobia in growth rate and colony morphology suggested that the collection probably includes several rhizobial genera.     

Rhizobia nodulating African Acacia spp. and Sesbania sesban trees in southern Ethiopian soils are metabolically and genomically diverse

The diversity of 110 rhizobial strains isolated from Acacia abyssinica, A. seyal, A. tortilis, Faidherbia albida, Sesbania sesban, Phaseolus vulgaris, and Vigna unguiculata grown in soils across diverse agro-ecological zones in southern Ethiopia was assessed using the Biolog™ system and amplified fragment length polymorphism (AFLP) fingerprinting technique. By cluster analysis of the metabolic and genomic fingerprints, the test strains were grouped into 13 Biolog and 11 AFLP clusters. Twenty-two strains in the Biolog method and 15 strains in the AFLP analysis were linked to eight and four reference species, respectively, out of the 28 included in the study. Most of the test strains (more than 80% of 110) were not related to any of the reference species by both methods. Forty-six test strains (42% of 110) were grouped into seven corresponding Biolog and AFLP clusters, suggesting that these groups represented the same strains, or in some cases clonal descendants of the same organisms. In contrast to the strains from S. sesban, isolates from Acacia spp. were represented in several Biolog and AFLP clusters indicating the promiscuous nature of the latter and widespread occurrence of compatible rhizobia in most of the soil sampling locations. The results showed that indigenous rhizobia nodulating native woody species in Ethiopian soils constituted metabolically and genomically diverse groups that are not linked to reference species.     

Effect of organic residue amendments and soil moisture on N mineralization,maize (Zea maysL.) dry biomass and nutrient concentration

Greenhouse pot experiments using four tropical soils were conducted to measure the effect of crop residues on nitrogen mineralization/immobilization and the growth of maize plants under two soil moisture regimes (pF2.5 and pF3.5). Nitrogen-rich residues of pigeon pea [PP, Cajanus cajan (L.) Millps, C/N, 18.8] enhanced plant growth and increased the assimilation of mineral elements (N, P and K) at both moisture regimes. Less nitrogen-rich residues of haricot beans [HB, Phaseolus vulgaris (L.), C/N, 28.9] had a slightly negative effect on plant growth and the assimilation of mineral elements, and a stronger negative effect was recorded for the most N-poor plant residue, maize [M, Zea mays (L.), C/N, 33.6]. For PP, we estimated the recovery of residue-nutrients in maize plants (net increase in N, P and K assimilation due to PP incorporation) as 19% for N, 88% for P and 86% for K in the high-moisture regime (pF2.5). The equivalent values for the drought-stress regime (pF3.5) were 10, 34 and 38%, respectively. The results demonstrate the immediate enhancement of plant growth by plant residues with a low C/N ratio, even under drought-stress conditions.     

Carbon and nitrogen mineralization dynamics in different soils of the tropics amended with legume residues and contrasting soil moisture contents

Seasonal drought in tropical agroecosystems may affect C and N mineralization of organic residues. To understand this effect, C and N mineralization dynamics in three tropical soils (Af, An₁, and An₂) amended with haricot bean (HB; Phaseolus vulgaris L.) and pigeon pea (PP; Cajanus cajan L.) residues (each at 5 mg g⁻¹ dry soil) at two contrasting soil moisture contents (pF2.5 and pF3.9) were investigated under laboratory incubation for 100–135 days. The legume residues markedly enhanced the net cumulative CO₂–C flux and its rate throughout the incubation period. The cumulative CO₂–C fluxes and their rates were lower at pF3.9 than at pF2.5 with control soils and also relatively lower with HB-treated than PP-treated soil samples. After 100 days of incubation, 32–42% of the amended C of residues was recovered as CO₂–C. In one of the three soils (An₁), the results revealed that the decomposition of the recalcitrant fraction was more inhibited by drought stress than easily degradable fraction, suggesting further studies of moisture stress and litter quality interactions. Significantly (p < 0.05) greater NH₄⁺–N and NO₃⁻–N were produced with PP-treated (C/N ratio, 20.4) than HB-treated (C/N ratio, 40.6) soil samples. Greater net N mineralization or lower immobilization was displayed at pF2.5 than at pF3.9 with all soil samples. Strikingly, N was immobilized equivocally in both NH₄⁺–N and NO₃⁻–N forms, challenging the paradigm that ammonium is the preferred N source for microorganisms. The results strongly exhibited altered C/N stoichiometry due to drought stress substantially affecting the active microbial functional groups, fungi being dominant over bacteria. Interestingly, the results showed that legume residues can be potential fertilizer sources for nutrient-depleted tropical soils. In addition, application of plant residue can help to counter the N loss caused by leaching. It can also synchronize crop N uptake and N release from soil by utilizing microbes as an ephemeral nutrient pool during the early crop growth period.     

The use of visible and near-infrared reflectance measurements for identifying the source of suspended sediment in rivers and comparison with geochemical fingerprinting

Purpose

Visible and near-infrared (Vis-NIR) reflectance measurements may be an alternative technique to identify suspended sediment sources in streams of headwater catchments. In this study, we examined if Vis-NIR reflectance measurements are capable of estimating sediment source contributions to sediment yield and compared this technique with a more conventional (i.e. geochemical) technique.

Materials and methods

Two headwater catchments in Ethiopia, Unta (2,052 ha) and Desera (1,657 ha), were analysed with the same techniques in order to find similarities and differences in the results obtained. The first technique used Vis-NIR spectral analysis as a fingerprint, using a partial least squares regression model. The second technique was a quantitative composite fingerprinting technique using geochemical analysis of source materials and suspended sediment samples. As a comparison, the partial least squares model was also used on the geochemical data. In August and September 2009, 30 soil samples of three different land uses (landslides, croplands, and grazing lands) and 21 suspended sediment samples at the catchment outlet were collected. Source samples were sieved to <63 μm. Geochemical analyses consisted of total element concentrations, percentage carbon, percentage nitrogen, and atom percentage ¹⁵N and δ¹³C. Reflectance measurements were taken on dried source samples with a spectrometer.

Results and discussion

Neither technique was able to predict the contributions of the three land use types; they could only distinguish between landslide and topsoil material. The agreement between the results of both techniques was significant for the Unta catchment (R ²?=?0.80) but not for the Desera catchment (R ²?=?0.39). The uncertainty of the technique using Vis-NIR reflectance measurements was slightly higher than with the geochemical approach. Both techniques revealed that topsoil erosion played an important role during storm runoff discharges. Using the partial least squares model for the geochemical data revealed that uncertainty can differ greatly when using other statistical techniques.

Conclusions

The quantitative composite fingerprinting technique using spectral signatures from both source and suspended sediment samples was able to quantify the contribution of two source materials (landslides and topsoil). It provided a faster and more cost effective alternative to the conventional geochemical procedure.     

Biofiltration of Chloroform in a Trickle Bed Air Biofilter Under Acidic Conditions

Water, Air, & Soil Pollution - Concern for the incorrect disposal of potentially toxic substances in aquatic environments is growing due to adverse effects caused to the organisms exposed to...