Optimizing the formulation of external-soil spray seeding with sludge using the orthogonal test method for slope ecological protection

•We demonstrated that sewage sludge can be an alternative to the turfy soil currently used in external-soil spray seeding.•We explored the effects of spray seeding components (sludge, fiber, soil additive, and bioorganic fertilizer) on soil properties, plant growth, runoff, and soil loss.•The optimized component proportions in the spray seeding substrate are soil-to-sludge ratio of 6:4 (v/v), fiber content of 15% (v/v), and soil additive concentration of 0.6 kg m−3.

External-soil spray seeding has gradually become a widely used technology for slope ecological rehabilitation in China in the 21st century. Meanwhile, with ever-increasing amounts of municipal sewage sludge being produced, sewage sludge has become a new source of solid waste pollution in the environment. Therefore, the goal of this study was to utilize sludge as a new kind of spray seeding component to replace non-renewable turfy soil. Orthogonal experiments were conducted to optimize the substrate formulation regarding the four factors of soil-to-sludge ratio, fiber content, soil additive concentration, and bioorganic fertilizer concentration. The results showed that sludge improved the physical and chemical properties of the substrate, increased plant growth, and within a certain range of dosage, reduced runoff and soil loss. The improvements of soil bulk density, total porosity and soil nutrients were particularly notable (P < 0.01). Plant growth in experimental groups was better than that of control groups (P < 0.05), and Festuca arundinace performed best among the three plants (P < 0.01). In addition, besides the physical and chemical properties, fiber content mainly affected plant growth, and soil additive reduced runoff and soil loss, whereas bioorganic fertilizer showed no obvious effect. After the experiments, a simple model was used to integrate the effects of the four factors on 28 parameters and thus to get the optimum component proportions, which were a soil-to-sludge ratio of 6:4 (v/v), fiber content of 15% (v/v), and soil additive concentration of 0.6 kg m−3.

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