Recycling fly ash from coal or biomass combustion for nutrient recovery and enhanced food production in acidic agroecosystems: A multi-level meta-analysis

Fly ash (FA) is a powdered, alkaline, and potentially hazardous co-product that remains largely underutilized and is predominantly disposed of in landfills, posing environmental risks. In this study, we conducted a meta-analysis of 1,482 studies sourced from leading scientific databases to quantify the impacts of FA application on soil nutrient levels and plant yield across various agro-ecological conditions. The results revealed a significant positive impact of FA on soil nutrient content, increasing Ca by 3.7-fold, P by 3-fold, K by 22 %, Mg by 11.2 %, and C by 0.82 %. However, FA application had a negative impact on soil N content, reducing it by 9.7 %, likely due to: i) elevated soil pH, which accelerates ammonia (NH3) volatilization, stimulates nitrification followed by nitrate (NO3−) leaching, and promotes denitrification under anaerobic conditions, leading to gaseous N losses; and ii) increased salinity and iii) potential heavy metal toxicity at higher FA application rates, both of which further suppress microbial activity and impair N mineralization. Despite this drawback, soil amelioration with FA significantly increased plant yield by 56 %. A multi-moderator approach revealed significant interactions between soil, FA type/dosage, experimental conditions, and plant species, underscoring the complexity of FA's impact. The mechanisms behind these effects are interrelated and include fast hydrolysis of the Ca-/Mg-/K-/P-mineral-rich FA matrix, ion exchange at the soil-FA interface, reduced leaching, improvement in soil structure and water-air relations, all of which contribute to improved nutrient availability, microbial activity, and soil fertility. Given its significant potential for reuse as a soil amendment, FA could contribute to food security by increasing crop production in widely cultivated acidic and nutrient-deficient agroecosystems, while also supporting a circular economy. However, further research is needed to explore the long-term sustainability and environmental safety of FA, including its broader ecological implications.