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Abstract
The effects of increased cobalt additions (0, 50, 100 and 200 v mg v kg m 1 soil) in sewage sludge-amended soil on organic matter, N Kjeldahl, ammonium and nitrate were studied in this experiment. Three different rates of sewage sludge were applied (0, 30 and 60 v tn v ha m 1 ) to soil as main plots, using tomato (Lycopersicum esculentum Mill var. Ramy) such testing cultivation. Plant biomass and nitrogen content in tomato leaf were also monitored. The organic matter in the soil was clearly affected by the fertilization. N Kjeldahl, ammonium and nitrate were favoured by organic treatments. Co seemed to reduce the transformation of ammonium to nitrate on amended soils, with accumulation of ammonium forms, especially at the higher application rates of sewage sludge. This incidence of Co on nitrogen species in soil decreased with the time of experiment, probably due to the reduction of availability of the pollutant. Aerial biomass production and nitrogen content in leaf were increased for the organic fertilization compared to the control. Only very high levels of Co in soil reduced significantly the aerial biomass production of tomato plants in amended soils. Co seemed to induce a decrease of the nitrogen in leaf in the amended soils, but not for the non-fertilized soils.