Application of digestate from low-tech digesters for degraded soil restoration: Effects on soil fertility and carbon sequestration

The application of digestate to soil represents a common practice for its recycling, but its application to degraded lands to achieve their restoration and sequester organic C into the soil is still almost unexplored. In this context, this study describes a first attempt to use digestate from a low-tech digester for degraded soil restoration in Colombia. An experimental site (2700 m²) previously subjected to intensive mono-cultivation was treated with digestate application for 4 months (40 Mg ha^-1 dry weight). Soil samples were collected (0, 4, 8, and 12 months after digestate application) to evaluate chemical and biochemical parameters, as well as total soil organic C stocks and their fractionation among different pools. Results showed that soil pH (from 5.3 to 6), total organic C (from 1.9 to 3 %), total N (from 0.17 to 0.27 %), available P (from 10 to 68 mg kg^-1), exchangeable nutrients content (K, Mg, Ca, Fe), respiration rate, microbial biomass C and N, and metabolic activities exhibited an increasing trend after digestate application, leading to a recovery of the soil biological fertility (i.e. biological fertility index increased from 8 to 19 in a range from 1 to 20). Digestate promoted C sequestration in the more stable and recalcitrant pools. Soil application of digestate from low-tech digesters may thus represent a win-win resource recovery strategy to enhance degraded land recovery, contribute to climate change mitigation and support rural communities. In the circular bioeconomy context, afforestation appears as the most promising strategy to take advantage of the restored land.