Anaerobic Digestate as a Soil Amendment: Impacts on Crop Production, Soil Ecology, and Environmental Quality: A Review

The global push for a circular bioeconomy and renewable energy has led to a surge in anaerobic digestion (AD), generating vast quantities of digestate. This byproduct is increasingly positioned as a biofertilizer, yet its agronomic performance and environmental impacts are highly variable and not fully understood, particularly in comparison to traditional compost. While numerous studies have assessed the short-term fertilizing effect of digestate, a comprehensive synthesis that bridges its immediate agronomic performance with its long-term impacts on soil carbon sequestration, microbial ecology, and greenhouse gas fluxes remains elusive. Here, we review over two decades of scientific literature (2000–2025) to provide a comprehensive analysis of digestate as a soil amendment. The major points are the following: 1) Digestate's high concentration of readily available nitrogen makes it a potent, fast-acting fertilizer, often producing crop yields equivalent or superior to those of mineral fertilizers. 2) This rapid nutrient release, however, creates a significant risk of environmental loss through ammonia volatilization and nitrate leaching if not managed with precision and can, under certain conditions, lead to higher nitrous oxide emissions than synthetic fertilizers. 3) The impact of digestate on long-term soil health—particularly physical properties, carbon sequestration, and the full soil food web—reveals a complex dilemma of short-term risks versus long-term benefits, with new evidence highlighting its potential for restoring degraded lands. 4) The feedstock is the primary determinant of digestate quality, influencing everything from nutrient ratios and carbon quality to contaminant loads. 5) Integrated approaches, such as co-composting, advanced digestate conditioning, and novel formulations with amendments like biochar, offer promising pathways to combine the energy benefits of AD with the soil-building properties of traditional amendments. This review provides a critical synthesis to guide the sustainable integration of digestate into modern agroecosystems.