BIOSOLID FROM SEWAGE TREATMENT PLANT INCREASES THE GROWTH OF JACK BEAN (Canavalia ensiformis) IN A COPPER CONTAMINATED SOIL

Copper (Cu) mining have been carried out for more than a century in the Minas do Camaquã region, southern Brazil. Currently, this site has an extensive area impacted by the disposal of tailings and the soil has high levels of Cu. As a result of this contamination, the site has little or no vegetation cover, increasing the risk of other environmental compartments being affected. Phytoremediation can be used to mitigate this problem, and its efficiency can be enhanced by the use of organic amendments. Therefore, this study sought to evaluate the effect of sewage biosolids on the development of jack bean (Canavalia ensiformis) in a copper-contaminated soil after the cultivation of black oats (Avena strigosa). A biosolid obtained from the sludge of an aerobic domestic sewage treatment system was used in this study. The pots with soil from the area impacted by copper mining tailings (739 mg kg^-1 of Cu Mehlih) received increasing doses of biosolid (0, 90, 180, 360, 720 and 1440 kg ha^-1 of N) and black oat was grown in a greenhouse. After 75 days black oats were harvested, and then jack bean were cultivated in the same soil, using the same treatments of the black oat, but without new addition of biosolid. The jack bean was harvested at flowering to determine dry mass and copper content in tissues. The dry mass of the shoot increased with the addition of biosolid, but no significant difference was observed between treatments for dry mass of the roots. The highest dose (1440 kg ha^-1 of N) promoted the highest dry mass production of the shoots (20 g pot^-1), but did not differ (p < 0.001) from doses of 180, 360 and 720 kg ha^-1 of N. In the control treatment (0 kg ha^-1 of N) the plant presented less than 8 g pot^-1of aerial biomass. The highest levels of Cu in shoot biomass were observed in the intermediate doses (180 and 360 kg ha^-1 of N) and in the control treatment (average of 22.11 mg kg^-1), differing (p < 0.05) only from the dose of 1440 kg ha^-1 of N (12.42 mg kg^-1). In the roots, no significant difference (p = 0.525) in Cu content was observed. Thus, our study indicates that intermediate doses (180 and 360 kg ha^-1 of N) of biosolids promoted plant growth similar to the higher doses and increase the Cu contents in the shoot biomass. More studies must be carried out to evaluate the effects of biosolids on other soil and plant parameters, but our results indicate that the use of biosolids can be an alternative for increasing vegetation cover in phytoremediation strategies for soils contaminated by Cu mining tailings.