Short-term usage of sewage sludge as organic fertilizer to sugarcane in a tropical soil bears little threat of heavy metal contamination

A field experiment was carried out to study the effect of application rates of sewage sludge and mineral nitrogen and phosphate fertilizers on As, Ba, Cd, Cr, Cu, Ni, Pb, Se, and Zn concentration in soil, cane plant, and first ratoon (residual effect) in a Typic Hapludult soil. To allow an analysis by means of response surface modeling, four rates of sewage sludge (0, 3.6, 7.2 and 10.8 t ha−1, dry base), of N (0, 30, 60 and 90 kg ha−1) and of P2O5 (0, 60, 120 and 180 kg ha−1) were applied in randomized block design, in a 4 × 4 × 4 factorial scheme, with confounded degrees of freedom for triple interaction, with two replications. To evaluate the residual effect of the sludge applied to cane plant on the cane ratoon growth, mineral NK fertilizers were applied at the rates of 120 kg ha−1 N and 140 kg ha−1 of K2O, on all treatments. The application rates of mineral nitrogen and phosphate fertilizers did not affect statistically the heavy metal concentration in the soil and in the sugarcane plants. Sewage sludge application increased As, Cd, Cu, Ni, Pb, and Zn concentrations in soil, but values did not exceed the quality standard established by legislation for agricultural soils. Although the concentrations of metals in the plants were very low, the uptake of heavy metal by sugarcane plants was generally increased by sewage sludge doses. The use of sewage sludge based on N criteria introduces a small amount of heavy metal into the agricultural system, however it poses no hazard to the environment.

► Sewage sludge application provides nutrients for sugarcane production in the tropics.
► Metal concentrations in the soil generally increased but not above critical limits.
► Metal concentrations in sugarcane generally increased but concentrations were small.
► The small amount of heavy metals from sludge poses little risk to the environment.