Heavy Metal Concentrations and Basal Respiration in Contaminated Substrates used in the Cuban Urban Agriculture

Abstract

The input of heavy metals in agricultural systems depresses the soil quality and compromises the food safety owing to crop contamination. Metals also affect the structure, morphology, function, and activity of microorganisms in soils and substrates. Heavy metals can inhibit basic processes of microbial metabolism such as respiration. The present study aimed (i) to assess the relationship between cadmium (Cd), lead (Pb), zinc (Zn), and mercury (Hg) in contaminated organic substrates used in the Cuban urban farms (the so-called organoponicos) and the basal respiration; (ii) to study the changes in the basal respiration of substrates added with increasing metals concentrations; and (iii) to evaluate how increasing Hg concentrations in the substrates affect the basal respiration during a 135-day period. In general, the Cd, Pb, and Zn concentrations in the substrates of the organoponicos from Havana province are within the permissible values for organic substrates, while substrates prepared from municipal solid wastes from Havana and all substrates from Guantanamo province had metal concentrations exceeding the maximum permissible levels. The basal respiration was correlated negatively with the pollution load index. The toxic effect on microorganisms was also dependent on the metal concentration, the substrates, and the metal considered, with Cd and Hg having the most significant effect on diminishing microbial activity. Although Hg contamination reduced the basal respiration in the first 24 h of substrate-metal contact, at the 135th day of incubation, the basal respiration was higher than non-polluted substrates. This finding suggests the high metabolic activity of metal-resistant microorganisms in the substrates.