Bioavailability of Metals In No Tillage Soybean Crop As Influenced by Sewage Sludge and Industrial Residues.

The recycling of residues in crop soils is one of the most rational forms of using such material as source of nutrients; however, the content of heavy metals can restrict their use in crops. This study has been carried out for seven years in a no tillage area, on a Haplodux soil, in Botucatu, SP, Brazil.  Residues were applied on surface at five times in soil cultivated with soybean in the summer and oat or sorghum in the winter. Treatments include four levels (0, 2, 4 and 8 ton ha^-1) of four residues, two types of sewage sludge, centrifuged (LC; pH 12.0) and  biodigested (LB; pH 7.0), in addition to steel slag (LS) and lime mud (LL) from paper and cellulose industry. Soil sampling was done at depths 0-0.10m and 0.10-0.20m to determine Zn and heavy metals extracted by DTPA. LB added to soil increased content of Zn, Pb, Cr and Cd, compared to LC, which had higher levels of Zn, Pb and Cr. Similarly, LS contributed to the increase of Zn and Cd, while LL increased Cr levels. Metal levels kept within the adequate range considering all used residues and thus without risk of environmental contamination. LB increased Zn levels in leaves and grains, Pb in grains, Cd in leaves and grains, and Ni in grains, compared to LC, LS and LL. Cr and Zn levels in leaves and grains are above the range considered adequate, especially when originated from LB. Ni, Cd, Pb and Zn were more absorbed by grains, while Cr was more concentrated in leaves.  The soybean that received LB had lower growth in height and lower grain yield.