Post-digestate composting benefits and the role of enzyme activity to predict trace element immobilization and compost maturity

Authors: GURMESSA, BIYENSA - Polytechnic University Of Marche; COCCO, STEFANIA - Polytechnic University Of Marche; Ashworth, Amanda; PEDRETTI, ESTER - Polytechnic University Of Marche; ILLARI, ALESSIO - Polytechnic University Of Marche; CARDELLI, VALERIA - Polytechnic University Of Marche; FORNASIER, FLAVIO - The Research Center For The Study Of The Relationship Between Plant And Soil(CREA-RPS); RUELLO, MARIA - Consiglio Per La Ricerca In Agricoltura E L'Analisi Dell'economia Agraria, Unita Di Ricerca Per I S; CORT, GIUSEPPE - Polytechnic University Of Marche

Submitted to: Bioresource Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/9/2021
Publication Date: 7/16/2021
Citation: Gurmessa, B., Cocco, S., Ashworth, A.J., Pedretti, E.F., Illari, A., Cardelli, V., Fornasier, F., Ruello, M.L., Cort, G. 2021. Post-digestate composting benefits and the role of enzyme activity to predict trace element immobilization and compost maturity. Bioresource Technology. 338. Article 125550. https://doi.org/10.1016/j.biortech.2021.125550.
DOI: https://doi.org/10.1016/j.biortech.2021.125550

Interpretive Summary: Solid digestate, a byproduct of biogas production, is often considered a waste although legal status differs among countries. Some have policies that encourage direct use as fertilizer, while others do not consider it as a fertilizer because of possible associated environmental risks. Low nutrient supply are other disadvantages associated with direct usage of digestate. Researchers in the U.S. and Italy set out to evaluate co-compositing potential of maize silage, cereal mill byproduct, poultry litter, and a combination of poultry litter and maize silage to improve this bioenergy waste stream. This study found that positive digestate quality (e.g. nutrient content and enzyme activities) and anti-quality traits (e.g. heavy metals) improved following composting, regardless of co-composting material. Therefore, the overall findings give an overview of the outcomes of post-digestate composting, not only to reduce possible environmental risks, but to improve its use as fertilizer for the U.S and Europe following bioenergy production.

Technical Abstract: Biogas digestate is widely used as a direct source of crop nutrients, despite its low nutrient concentration and high salinity. The current study set out to 1) improve the quality of digestate with composting or co-composting with maize silage, cereal mill byproduct, poultry litter, and a combination of poultry litter and maize silage, and 2) understand the dynamics of extracellular enzyme activities during composting and their relationship with compost quality indicators. Results showed that maize silage reduced time to maturity of the compost by 20 days. After 90 days of composting, total N and plant available P and K increased up to 93, 62, and 51%, respectively, and the greatest (P<0.05) values for total N and P were obtained with maize silage, likely owing to greater C/N ratio, thereby moderating biodegradation. In contrast, major trace elements were immobilized, and more than 40% immobilization was obtained for Al, Ba, Fe, Zn, and Mn, irrespective of treatments. All the enzyme activities, except arylsulfatase and a-glucosidase, were increased and enriched at the maturity phase. These activities were negatively correlated with organic matter content and soluble trace elements (p<0.05), thus magnifying the benefit of enzyme activities not only as index of compost maturity, but also as an indicator of trace element immobilization. Similarly, the redundancy analysis revealed about 70% of the variations in enzyme activities were explained by trace elements. Overall, post-digestate composting improved digestate quality and enzyme activities may be useful for predicting compost maturity and immobilization of trace elements for enhanced compost utilization.