Sewage Sludge Stabilized with Clay Minerals: Its Impact on Soil Properties and Ryegrass Growth

In the perspective of finding environmental-friendly and cheap methods for the stabilization of sewage sludge to be used in agriculture, sludge treated with bentonite, mixed saponite-attapulgite, or zeolite was evaluated as soil amendment for ryegrass ( Lolium perenne L.) growth. The clay minerals or Ca(OH) 2 were added to dewatered sewage sludge at rates 30% or 10%, respectively. Control sample without additives was also prepared. All materials were added to two soils (alkaline and acid) at rates 10 and 30 g kg −1 and used in a pot experiment with ryegrass. Additional treatments of inorganic N-P-K fertilization and no fertilization (control) were included. For both soils, the amendments affected differentially pH and increased their electrical conductivity (below 4 dS m −1 ). Moreover, they increased available NO 3 -N, P, and K compared to control, with their respective concentrations being 23–107, 29–79, and 132–675 mg kg −1 for the alkaline and 10–29, 33–87, and 70–570 mg kg −1 for the acid soil. Total concentrations of heavy metals were below the permissible levels. Basal respiration and microbial biomass C responded to application of all amendments at the highest rate. Both rates increased ryegrass’s aboveground-biomass yield, which was 37–67% and 54–79% higher than control for the lower and higher rate, respectively, as nutrients’ uptake. Sewage sludge stabilized with certain clay minerals could be used in agriculture with application rates less than 30 g kg −1 (≈120 Mg ha −1 ) to be considered both beneficial for crops’ growth and safe, regarding soil salinization, NO 3 -N leaching, P, and heavy metals build-up risks.