Effect of Sewage Sludge and Barnyard Manure on Growth, Nutrient Uptake, and Heavy Metal Accumulation in Lettuce

This study investigated the effects of barnyard manure and different doses of sewage sludge on yield performance, micronutrient uptake, and heavy metal accumulation in lettuce (Lactuca sativa L.) under pot conditions. The treatments included: (1) Control, (2) Barnyard manure (BYM, 5 ton ha⁻¹), (3) Sewage sludge at 1.25 ton ha⁻¹ (SS1), (4) 2.5 ton ha⁻¹ (SS2), (5) 5.0 ton ha⁻¹ (SS3), and (6) 7.5 ton ha⁻¹ (SS4). According to the Soil Water Protection Regulation, the heavy metal concentrations in the sewage sludge remain below the permissible limit for applying sewage sludge to the soil. Results demonstrated that the highest sludge dose (7.5 ton ha⁻¹) significantly enhanced fresh biomass of both shoots and roots compared to BYM. Moreover, sewage sludge treatments improved the concentrations of Fe, Zn, Cu, and Mn in both roots and leaves. However, they also led to elevated accumulation of Cd, Pb, and Cr in root tissues. The observed increase in plant growth is likely attributed to the high nitrogen content and low C/N ratio of the sewage sludge, which accelerated organic matter mineralization and enhanced nutrient availability through increased microbial activity. Despite the rise in heavy metal uptake, their concentration per unit of biomass was reduced due to the dilution effect. Overall, the application of sewage sludge to soils with low organic matter significantly improved plant development and nutrient acquisition, albeit with a potential risk of heavy metal accumulation.