Greenhouse gas balance of new organic fertilizers derived from anaerobic digestion

Summary

Anaerobic digestion is an environmental technology that not only produces biogas to substitute fossil fuel, but also digestate that can serve as organic fertilizer. In this project, post-digestion treatment processes produced new fertilizer materials with contrasting characteristics. The nitrogen use efficiency and nutrient value to crops, and environmental impacts such as GHG emissions, of both traditional and post-digestion products were studied to find solutions towards sustainable agriculture. The study quantifies 1) emissions of N2O, CH4, CO2 and NH3 from untreated manure and digestate products during storage and N2O emissions after field application, 2) yield performance and N use efficiency; and 3) overall greenhouse gas emissions intensity of manure and digestate products at field and farm level. Digestate products showed 70-90% less CH4 emissions during 11 weeks of storage compared to untreated manure, though higher potential for NH3 losses. Since CH4 emission during storage is the largest component of the GHG balance for both untreated manure and digestates (Baral et al., 2018), the processing of digestates looks promising as strategy for GHG mitigation. The field study will demonstrate if the new treatment technologies also show improvements in N use efficiency. Given that no synthetic chemicals are involved in digestate procesing, the strategy is potentially available to organic farms, where N is typically limiting productivity. Biogas technologies depend on subsidies to compete with fossil fuel, but compared with other GHG mitigation options for agriculture, strategies based on biogas treatment may be cost-effective.