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

Abstract

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.