Phillip Nyamugafata

Adsorption of Zn(2+) and Ni(2+) in a binary aqueous solution by biosorbents derived from sawdust and water hyacinth (Eichhornia crassipes).

The Zn(2+) and Ni(2+) adsorption capacities of six biosorbents derived from water hyacinth (Eichhornia crassipes) (WH) and sawdust (SD) were investigated, with activated carbon as the control. The biosorbents were raw biomass (WH, SD), charred WH (BWH) and SD and sulphonated bio-chars of WH and SD. The effect of the initial solution pH and Zn(2+) and Ni(2+) concentrations on adsorption capacity was studied, and adsorption isotherms for Zn(2+) and Ni(2+) evaluated. The initial solution pH significantly influenced adsorption (p < 0.05) but the relationship was generally nonlinear. Zn(2+) suppressed Ni(2+) adsorption on all biosorbents. The adsorption capacities of the biosorbents were statistically (p ≤ 0.05) similar to or higher than that of activated carbon. The effects of pyrolysis and bio-char sulphonation on adsorption were inconsistent and dependent on biomass type; in most cases bio-char was a better biosorbent than the original biomass, while sulphonation resulted in less or comparable adsorption. Adsorption data obeyed at least one of three isotherms (linear, Langmuir and Freundlich) (r(2) = 0.90-0.995, p < 0.05). The study revealed that low-cost biosorbents may be used as alternatives to activated carbon in applications including selective separation of Zn(2+) from multi-metal ion solutions containing Ni(2+), and water and wastewater treatment.

Optimizing dambo (seasonal wetland) cultivation for climate change adaptation and sustainable crop production in the smallholder farming areas of Zimbabwe

Most smallholder farming areas of Zimbabwe have low soil fertility and low rainfall, which has continually decreased over the past years. In recent years, most of the smallholder farming areas have experienced perennial droughts, poor rainfall distribution and crop failures and these have been attributed to climate change and variability. Cultivation of dambos, which are seasonal wetlands, presents a climate change and variability adaptation option for smallholder farmers. This synthesis analyses the role of dambo cultivation in climate change and variability adaptation and discusses future directions for sustainable dambo utilization. The data on current dambo farming practices were collected from literature, surveys and field observations. The results showed that farmers grow crops in dambos as an adaptive strategy to climate change and variability and have largely abandoned upland fields where yields are <1u2005tu2005ha−1 in preference of dambos where yields average 2–3u2005tu2005ha−1. Dambo cultivation offers a buffer against crop failures and has resulted in improved household food security. We conclude that dambo cultivation is a potentially beneficial farmer-driven climate change and variability adaptation strategy. However, if not properly designed and managed, dambo cultivation may result in their degradation hence there is need for further research to evaluate options for sustainable dambo utilization as intensification of dambo agriculture is important for food security.

The effect of hydroxide solutions on the structural stability and saturated hydraulic conductivity of four tropical soils

Industrial and domestic effluents are widely applied to soil as a way of dumping, and/or source of nutrients and water for plant growth. The potential effects of sodium hydroxide (NaOH) and potassium hydroxide (KOH), which are common hydroxides found in industrial effluents/sludge, on aggregate stability, clay dispersion and saturated hydraulic conductivity (Ks) of four tropical soils were studied. For aggregate stability and clay dispersion, the soils were pre-treated with NaOH and KOH solutions (0.003–0.3M, pH = 11.5), and for Ks the NaOH and KOH solutions were used as influent solutions. Aggregate stability and Ks decreased with increased concentration of hydroxide solutions, while clay dispersion increased as concentration of the hydroxide increased. NaOH, which is more commonly used for industrial cleaning, had more negative effects on soil structural stability compared to KOH. Long-term application of a mixture of sewage sludge and effluent to Pension farm soil reduced Ks compared to Churu farm soil (control). Ks decreased to less than 20% for Pension farm soil compared to 40% for Churu farm soil when 0.3M NaOH was used as influent solution, despite having much higher organic matter content in the former. It was concluded that disposal of sewage sludge/effluent high in hydroxides, such as those of sodium (Na) and potassium (K)1 would result in reduced structural stability and Ks. Therefore there is a need to establish critical levels of hydroxides in effluent/sludge that can be disposed onto Zimbabwean soils.

NEMATODE MANAGEMENT IN RAIN-FED SMALLHOLDER MAIZE PRODUCTION SYSTEMS UNDER CONSERVATION AGRICULTURE IN ZIMBABWE

Nematode infestation in Sub-Saharan Africas (SSA) cropping systems, worsened by poor crop rotations, is a major factor contributing to limited utilisation of applied nutrients and water, leading to low maize ( Zea mays L.) yields particularly on sandy soils. The effects of nematode infestation on maize productivity were evaluated under conservation agriculture (CA) on granitic sandy soils in sub-humid smallholder farms of Goromonzi district of Zimbabwe. Four treatments were tested for three seasons on six smallholder farmers’ fields in a randomised complete block design, each farm being a replicate: fenamiphos 40EC (a commercial synthetic nematicide), lime + fenamiphos 40EC, lime and an untreated control. Results of the study showed that independent application of fenamiphos 40EC and lime significantly reduced plant parasitic nematode infestations in maize roots by more than 10 times those present in the untreated plots while maize yield also increased significantly. Yield increase from fenamiphos and lime applications amounted to 53 and 42% respectively, compared to the untreated controls. Maize yield was negatively correlated with density of Pratylenchus spp. nematodes. Nematode management strategies involving fenamiphos 40EC or lime could significantly reduce maize yield losses in maize-based smallholder farming systems of SSA under CA. It was more economical to use fenamiphos than lime to control nematodes.

Mineralisation of organic fertilisers used by urban farmers in Harare and their effects on maize (Zea mays L.) biomass production and uptake of nutrients and heavy metals

Nutrient release patterns and heavy metal contamination risks associated with animal manure and organic municipal wastes used as basal fertilisers are largely unknown in Zimbabwe. Carbon, nitrogen and phosphorus mineralisation patterns were determined in a 56-day incubation experiment of domestic sewage sludge (SS), pig (PG), poultry (PM) and cattle manures (CM). Maize biomass production, and nutrient and heavy metal (cadmium, copper, nickel and zinc) uptake were determined in a greenhouse experiment. Inorganic fertiliser, PG, PM, SS and CM were applied to the first maize crop and a second maize crop grown without addition of fertilisers. Aboveground biomass was harvested at four weeks after emergence. Fertiliser treatments had significant (p < 0.01) effects on mineralisation. Carbon, nitrogen and phosphorus mineralisation followed the order PG > PM > CM > SS. Maize grown in PM- and PG-amended soils had significantly (p < 0.01) higher biomass, nitrogen and phosphorus uptake than inorganic fertiliser-amended soil in the first crop. The second crop grown in organic fertiliser-amended soils had significantly (p < 0.01) higher biomass and nutrient uptake than inorganic fertiliser-amended soil. Heavy metal uptake was highest in SS-amended soils. Poultry and PG manures mineralised faster than SS and CM. Organic fertilisers had more residual fertility. Nickel toxicity is likely when SS is used as a fertiliser.

Yield, nutrient and heavy metal uptake of leafy vegetables grown in sewage sludge and poultry manure amended soils

Urban farmers in Harare grow vegetables in soils fertilised with poultry manure (PM) and sewage sludge (SS). Feed and storage management influence nutrient supply of these organic amendments. Nitrogen mineralisation of PM and SS were determined in a non-leaching and aerobic incubation experiment. Effects of these amendments on yield, nitrogen (N) and heavy metal uptake by vegetables grown in soils treated with 150 kg N ha−1 from compound mineral fertiliser (7 N:14 P2O5:7 K2O), PM (2.26% N) and SS (3.26% N) were studied. Phosphorus (P) and potassium (K) were added at rates of 50 kg P ha−1 and 60 kg K ha−1, respectively. A second crop was grown without adding amendments. Poultry manure mineralised faster than SS. Yield was significantly higher (p < 0.05) in mineral fertiliser amended soil for the first crop whilst organic amendments resulted in significantly higher yield in the second crop. First-crop nitrogen uptake increased by 53% and 100% (Brassica napus), 92% and 158% (Brassica juncea) over the control for SS and PM, respectively. Zinc, copper, cadmium and nickel uptake was higher with SS than in the other treatments and their concentrations were lower than European Union permissible limits. Poultry manure can be used in place of mineral fertiliser, whilst SS requires early or supplementary application of mineral N for early plant growth.