Microbial respiration of biochar- and digestate-based mixtures

Abstract

The addition of biochar or digestate as organic amendments to soils is currently controversially discussed with regard to its positive and negative effects on C mineralization. Organic amendments are generally applied to agricultural fields to improve soil quality and crop yield. In this study, we present results from short-term respiration experiments (90 days), where two different biochars (produced at 400 and 800 °C) as well as digestate from biogas production were added in different combinations to two soils (loamy sand and silt loam). Additionally, both amendments were mixed together into the soil to study interactions between biochar and digestate effects and investigate the interactions of both amendments with clay minerals resulting in a total of 13 mixtures (plus control soils) per soil type. The results indicate that the rate of CO2 evolution was not proportional to the amount of C added to the systems indicating a saturation effect in the C degradation mechanism. More than 40 % of the digestate C was released as CO2 and only 3 % for the biochar soil mixture; the recalcitrant nature of biochar and its suitability for short-term C stabilization in soils (incubation period of 90 days) were shown. Surprisingly, a much lower CO2 release (up to 11-fold) was observed in soil/digestate/biochar compared to soil/digestate mixtures without biochar. This effect was observed even when only 1 % (w/w) biochar was added to the digestate/soil mixtures, indicating that the biochar changed the physicochemical properties of the system. Additional dissolved organic C (DOC) sorption experiments revealed that large quantities of DOC can be sorbed by the biochar reducing the microbial accessible DOC in the liquid phase and as a consequence also the CO2 production.