Release of nutrients and heavy metals from biochar-amended soil under environmentally relevant conditions

Biochar is a potential amendment for improving soil fertility due to its richness of nutrients, P, K, Ca, and Mg. However, soil amended with metal-rich biochars may pose a risk of heavy metal release to the environment. Biochars derived from pig manure and sewage sludge (PM-biochar and SS-biochar) were investigated for their nutrient and heavy metal release in two soils (acidic and alkaline soil) under simulated landfill and acid rain conditions. Results showed that under both environmental conditions, adding PM-biochar into the soil increased K, P, and Mg release significantly by about 40-50 times, while only 2-4 times increase of the nutrients was observed in the SS-biochar-amended soil. The Ca release was higher in the SS-biochar-amended soil than in the PM-biochar-amended soil. Higher P, Ca, and Mg nutrient release was observed in alkaline soil than in acidic soil under the two environmental conditions though K release was not significant in both soils. A kinetic study in solution illustrated that the release of nutrients from biochar was initially via desorption and diffusion under environmental conditions and then through slow dissolution of insoluble species. More release of nutrients and heavy metals was observed in the biochar-amended soil under the landfill condition than under the acid rain condition. Although this release was limited under the acid rain condition, leaching of Fe and Mn exceeded the limitations of the groundwater standard value of China. Overall, biochar could be utilized as a prospective soil fertilizer by supplying nutrients such as P, K, Ca, and Mg, while the release of Fe and Mn should be paid more attention due to the risk of these metals impacting groundwater.