Compositional Shifts in Microbial Diversity under Traditional Banana Cropping Systems of Sub-Saharan Africa.

Improvements in the crop productivity, soil health, and sustainable intensification should be premised on the better understanding of interactions between the cropping systems and soil microbial diversity. In this study, we assessed variations in the microbial communities across the traditional banana-based cropping systems of contrasting monocrop vigor (vigorous or V vs. non-vigorous or NV) and the cropping system (monocrop or MC vs. intercropped or IC) using 16S rDNA (V3-V4) and ITS2 amplicon deep sequencing via Illumina platform. Sequencing results of the bacterial and fungal communities showed high variability among MC and V cropping systems. The abundances of Proteobacteria, Firmicutes, and Actinobacteria were significantly higher in NV (non-vigorous) and V (vigorous) cropping systems; and the abundances of Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes in the MC (monocropping) than IC (intercropping). There were high relative abundances of Pseudomonas (6.1-37.43%), Bacillus (4.5-20.4%), Rhizobium (1.4-6.5%), and Devosia (1.5-6.7%) in the cropping systems. The dominant family of fungal class Incertae_sedis was Mortierellales, which accounted for 8.79-41.12% of total taxa. This result indicated that the cropping systems are vital for supporting the dynamic microbial diversity specifically beneficial for bacterial communities that helps in promoting synergistic plant-soil interactions and total productivity under resource poor conditions of smallholder farmers in sub-Saharan Africa (SSA).

Effects of agrochemicals on the beneficial plant rhizobacteria in agricultural systems.

Conventional agriculture relies heavily on chemical pesticides and fertilizers to control plant pests and diseases and improve production. Nevertheless, the intensive and prolonged use of agrochemicals may have undesirable consequences on the structure, diversity, and activities of soil microbiomes, including the beneficial plant rhizobacteria in agricultural systems. Although literature continues to mount regarding the effects of these chemicals on the beneficial plant rhizobacteria in agricultural systems, our understanding of them is still limited, and a proper account is required. With the renewed efforts and focus on agricultural and environmental sustainability, understanding the effects of different agrochemicals on the beneficial plant rhizobacteria in agricultural systems is both urgent and important to deduce practical solutions towards agricultural sustainability. This review critically evaluates the effects of various agrochemicals on the structure, diversity, and functions of the beneficial plant rhizobacteria in agricultural systems and propounds on the prospects and general solutions that can be considered to realize sustainable agricultural systems. This can be useful in understanding the anthropogenic effects of common and constantly applied agrochemicals on symbiotic systems in agricultural soils and shed light on the need for more environmentally friendly and sustainable agricultural practices.

Characteristics of municipal fresh solid wastes from the selected large urban centres in Uganda: Implication for re-use and soil amendment strategies.

In Uganda, the municipal solid wastes are generally a menace to the environment, ranging from indiscriminate dumping, open burning, and landfills, which would be utilized to augment agricultural fields through organic manure. The National Environment Management Authority (NEMA) of Uganda, however from 2009 to 2012 initiated and implemented a Clean Development Mechanism project. This project was established and conducted in nine urban centers with the key objective of reducing methane and other environmental nuisances while generating compost manure. The in-coming fresh municipal wastes at composting facilities were sorted into six categories; i) wood and wood products, ii) food and food wastes, iii) textiles, iv) garden, yard and park wastes, v) paper and pulp, and vi) glass, plastics, and metals. These were laboratory analyzed based on standard procedures, characterized and investigated for the pH, total organic carbon (TOC), total nitrogen (TN), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) (g kg-1). Statistical analyses were performed based on One-Way ANOVA, implemented in the SPSS program. The results indicate that the municipal fresh solid wastes were mainly dominated by biodegradable organic matter; garden, yard, and park wastes (49%), food and food wastes (43.2%), and the other wastes falling below 5.4%. Overall, the pH was 7.7 ± 0.02, TOC 318.2 ± 2.90, TN 12.1 ± 0.10, C/N 26.7 ± 0.20, P 4.4 ± 0.04, K 35.0 ± 0.49, Ca 38.9 ± 0.51, and Mg 5.8 ± 0.09. The concentration of the fresh wastes and macro-nutrients varied per municipality and were congruent with the economic activities and population lifestyles. We detected the effect of season/month on the concentration of wastes which corresponded with the various agronomical activities. The results from this study suggest that the notion of composting is potentially a viable organic waste management strategy in the country which can ultimately generate sufficient organic manure for agricultural input and thus enhanced carbon sequestration.Implications: In this study, we characterized the in-coming fresh municipal solid wastes and investigated the pH, total organic carbon (TOC), total nitrogen (TN), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) (g kg-1). We found that the municipal fresh solid wastes were mainly dominated by biodegradable organic matter (>90%). The results from this study suggest the notion of composting to be potentially a viable organic waste management strategy in Uganda which can ultimately generate sufficient organic manure for agricultural input. This is typically vital for enhancing carbon sequestration towards minimizing greenhouse gas emissions.

Biophysical potential of crop residues for biochar carbon sequestration, and co-benefits, in Uganda.

Increasing organic matter/carbon contents of soils is one option proposed to offset climate change inducing greenhouse gas (GHG) emissions, under the auspices of the UNFCC Paris Agreement. One of the complementary practices to sequester carbon in soils on decadal time scales is amending it with biochar, a carbon rich byproduct of biomass gasification. In sub-Saharan Africa (SSA), there is a widespread and close interplay of agrarian-based economies and the use of biomass for fuel, which makes the co-benefits of biochar production for agriculture and energy supply explicitly different from the rest of the world. To date, the quantities of residues available from staple crops for biochar production, and their potential for carbon sequestration in farming systems of SSA have not been comprehensively investigated. We assessed the productivity and usage of biomass waste from maize, sorghum, rice, millet, and groundnut crops; specifically quantifying straw, shanks, chaff, and shells, based on measurements from multiple farmer fields and household surveys in eastern Uganda. Moreover, allometric models were tested, using grain productivity, plant height, and density as predictors. These models enable rapid and low-cost assessment of the potential availability of feedstocks at various spatial scales: individual cropland, farm enterprise, region, and country. Ultimately, we modeled the carbon balance in soils of major cropping systems when amended with biochar from biomass residues, and up-scaled this for basic scenario analysis. This interdisciplinary approach showcases that there is significant biophysical potential for soil carbon sequestration in farming systems of Uganda through amendment of biochar derived from unused residues of cereals and legume crops. Furthermore, information about these biomass waste flows is used for estimating the rates of biochar input that could be made to farmlands, as well as the amounts of energy that could be produced with gasifier appliances.

Transgenic banana plants expressing Xanthomonas wilt resistance genes revealed a stable non-target bacterial colonization structure.

Africa is among the continents where the battle over genetically modified crops is currently being played out. The impact of GM in Africa could potentially be very positive. In Uganda, researchers have developed transgenic banana lines resistant to banana Xanthomonas wilt. The transgenic lines expressing hrap and pflp can provide a timely solution to the pandemic. However, the impact of the transgenes expression on non-target microorganisms has not yet been investigated. To study this effect, transgenic and control lines were grown under field conditions and their associated microbiome was investigated by 16S rRNA gene profiling combining amplicon sequencing and molecular fingerprinting. Three years after sucker planting, no statistically significant differences between transgenic lines and their non-modified predecessors were detected for their associated bacterial communities. The overall gammaproteobacterial rhizosphere microbiome was highly dominated by Xanthomonadales, while Pseudomonadales and Enterobacteriales were accumulated in the pseudostem. Shannon indices revealed much higher diversity in the rhizosphere than in the pseudostem endosphere. However, the expression of the transgenes did not result in changes in the diversity of Gammaproteobacteria, the closest relatives of the target pathogen. In this field experiment, the expression of the resistance genes appears to have no consequences for non-target rhizobacteria and endophytes.

Banana-associated microbial communities in Uganda are highly diverse but dominated by Enterobacteriaceae.

Bananas are among the most widely consumed foods in the world. In Uganda, the country with the second largest banana production in the world, bananas are the most important staple food. The objective of this study was to analyze banana-associated microorganisms and to select efficient antagonists against fungal pathogens which are responsible for substantial yield losses. We studied the structure and function of microbial communities (endosphere, rhizosphere, and soil) obtained from three different traditional farms in Uganda by cultivation-independent (PCR-SSCP fingerprints of 16S rRNA/ITS genes, pyrosequencing of enterobacterial 16S rRNA gene fragments, quantitative PCR, fluorescence in situ hybridization coupled with confocal laser scanning microscopy, and PCR-based detection of broad-host-range plasmids and sulfonamide resistance genes) and cultivation-dependent methods. The results showed microhabitat-specific microbial communities that were significant across sites and treatments. Furthermore, all microhabitats contained a high number and broad spectrum of indigenous antagonists toward identified fungal pathogens. While bacterial antagonists were found to be enriched in banana plants, fungal antagonists were less abundant and mainly found in soil. The banana stem endosphere was the habitat with the highest bacterial counts (up to 10(9) gene copy numbers g(-1)). Here, enterics were found to be enhanced in abundance and diversity; they provided one-third of the bacteria and were identified by pyrosequencing with 14 genera, including not only potential human (Escherichia, Klebsiella, Salmonella, and Yersinia spp.) and plant (Pectobacterium spp.) pathogens but also disease-suppressive bacteria (Serratia spp.). The dominant role of enterics can be explained by the permanent nature and vegetative propagation of banana and the amendments of human, as well as animal, manure in these traditional cultivations.

Comparison of four low-technology composting methods for market crop wastes.

Four methods for composting, pit-cover (PC), pit-open (PO), above ground-open (AO) and above ground-covered (AC), were compared for their effect in accelerating compost maturity using market crop waste (MCW). The composting process was monitored through determining changes in temperature and pH, with compost maturity indices measured in terms of variations in water-soluble carbon (WSC), ammonium-N (NH(4)(+)-N) and nitrate-N (NO(3)(-)-N), and C/N, NH(4)(+)-N/NO(3)(-)-N, WSC/organic N and WSC/total N (TN). Total organic carbon (TOC), nitrogen (TN), potassium (TK) and phosphorus (TP) were also determined. Temperature rapidly increased from mesophilic to thermophilic and gradually reduced through maturation phases. A similar pattern was recorded for pH, which reached a slightly alkaline level at maturity. Composting significantly (p<0.05) decreased the concentrations of TOC, TN, TP, TK, NH(4)(+)-N and WSC and increased that of NO(3)(-)-N. All four low-technology composting methods used in this study produced mature composts within 63 days. Suitable maturity indicators for MCW compost were C/N ratio<12, WSC<1%, NH(4)(+)-N<400 mg kg(-1), NH(4)(+)-N/NO(3)(-)-N<0.2, WSC/TN, WSC/organic-N<1. On the basis of these indicators, the AC method generally enhanced maturity faster than the AO, PC and PO methods. Pit methods require less investment and are recommended for the smallhold farmers.