Salt risk is a key limiting factor in high-dose sewage sludge land application

The land application of sewage sludge is a cost-effective disposal method, particularly in ecological projects where high sewage sludge addition can function as soil substitute and improve soil quality. This study aims to evaluate the feasibility of using sewage sludge as a substrate for slope ecological restoration and to identify the key factors that limit its application at higher doses. A 150-day pot experiment using tall fescue (Festuca arundinacea) evaluated substrate characteristics, including water-soluble salts and heavy metals activities, as well as plant growth, physiological responses, and enrichment of the fifteen elements at three addition ratios (10 %, 30 %, and 50 %, w/w). The results revealed that increasing sludge addition led to higher levels of water-soluble salts and heavy metal activities in the substrate. During the seedling stage, heavy metals and salts accumulate in the root system, inhibiting plant growth. However, as the plant matured, heavy metal and salt concentrations decreased, resistance improved, and normal growth resumed. Analysis using Random Forest and Structural Equation Modeling suggested that the primary driver of early growth inhibition was salt stress rather than heavy metal toxicity. From an application and contamination risk assessment perspective, sewage sludge application rates exceeding 30 % (w/w) should be avoided unless pre-treated.