Application of sewage sludge as a soil biofertilizer enhances crop productivity of alfalfa plants (Medicago sativa L.) and improves soil quality without heavy metal toxicity

The rising economic and environmental burdens associated with intensive use of chemical fertilizers, required to compensate for continuous nutrient depletion from agricultural soils, limit sustainable crop production. In parallel, the increasing generation of sewage sludge (SS) poses a major disposal challenge, necessitating the development of safe, cost-effective, and sustainable management strategies. Here, alfalfa plants were grown in soil amended with different sewage sludge (SS) rates (0, 10, 20, 30, and 40 g/kg soil) to evaluate its potential as a biofertilizer for improving soil quality, crop productivity, and nutrient recirculation to soil–plant systems. Our results showed that, among the tested SS doses, the 10 g/kg dose significantly improved the growth parameters of the alfalfa plants. The levels of the tested nine heavy metals (HMs) (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn) in the alfalfa plants grown on the SS treated soil did not exceed the permissible limits in plant shoots, with an indication that alfalfa plants had the ability to recirculate micronutrients to food chains. Our results showed that a 10 g/kg SS dose was adequate to improve the post-harvest soil properties including organic matter (OM), the contents of the macronutrients; nitrogen (N), phosphorous (P) and potassium (K), and soil water holding capacity (SWHC), with no toxic levels of the tested HMs in the post-harvest SS-amended soil. Overall, the results presented here suggest that soil amendment with SS dose of 10 g/kg is reasonable to enhance plant growth and improve soil quality without considerable hazard effects on agricultural soil or plant health in terms of HMs toxicity.