Application of Different Fractions of Anaerobic Digestate Significantly Influences the Carbon Cycle in Grassland Soils

Applying digestate to soil is of growing interest in agriculture. However, the impacts of digestate on soil biogeochemical cycles often remain unclear, especially after solid-liquid separation of whole digestate (WD). We used a 21 d incubation to examine the effects of WD and solid digestate (SD) on CO₂-C efflux, dissolved organic carbon (DOC), microbial biomass C (C_micro), phospholipid fatty acid (PLFA) and carbon use efficiency (CUE) within two grassland soils of contrasting nutrient status. Application rates for SD and WD were based on recommended N inputs to grassland soils for these organic materials. Compared to un-amended controls, cumulative CO₂-C efflux, C_micro and the fungal:bacterial in soils increased significantly following SD application, regardless of the soil nutrient content (+20% CO₂-C, +29% C_micro, +58% fungal:bacteria for high nutrient soil; +563% CO₂-C, +36% C_micro, +18% fungal:bacteria for low nutrient soil). In contrast, WD produced a significant effect on CO₂-C efflux and fungal:bacterial only in the low nutrient soil. Our results also indicated that both digestate fractions and the initial soil nutrient status affected CUE. Applying both SD and WD to a low nutrient soil potential leads to decreases in soil C stocks, whilst the application of SD to a high nutrient soil can potentially enhance soil C stocks. Digestate application must be carefully planned, accounting for both the nature of the digestate and of the soil, in order to avoid adverse impacts on soil C stocks.