Assessing organic amendments for improving SOM dynamics and microbial functioning in degraded quarry soils

This study assesses the effects of various organic amendments, included vegetable compost from garden waste, compost from vegetable and fruit residues, and sewage sludge stabilized, applied individually and in mixtures, on restoring degraded quarry soils in arid Southeast Spain. Changes in soil physical and chemical properties, enzyme activities, fatty acid methyl-esters and organic matter were monitored to assess restoration relative to unamended and natural soils. Amendments led to significant organic matter improvements, microbial community structure and activity compared to unamended ones. However, results varied depending on amendment type and weather conditions, which influenced soil moisture and temperature. Organic matter increased by up to 2.4% above control (0.53%), approaching natural soil levels. Pyrolysis-GC/MS revealed molecular changes, emphasizing persistence and transformation of specific compounds over time. Soils with vegetable compost had 15–20% higher lignin derivatives, aromatic compounds, and polysaccharides than control, while sewage sludge increased nitrogenous compounds. Amended soils showed significantly higher enzyme activity than controls, indicating enhanced microbial function. Sewage sludge and mixed treatments showed microbial lipid levels 20–30 times higher than controls, while vegetable compost treatments led to 8–10 times higher values, suggesting enhanced microbial biomass and diversity. These findings support the use of organic amendments to accelerate soil recovery, particularly through enhanced biological activity and improved organic matter quality. Amendment type shapes soil recovery, offering guidance for optimizing restoration strategies. Vegetable compost and sewage sludge improve soil quality and support waste valorization in a circular economy, but their use requires careful management and long-term sustainability evaluation. • Quarry soils restored with sludge and plant compost were investigated over time • The amendments increased and maintained SOM content compared to the control over time. • Organic amendments stimulated microbial activity and SOM dynamics • As the amended soils stabilized, their properties became similar to natural soils • Climatic conditions affect the effectiveness of amendments in soil restoration