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 unex-plored. 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 (2,700 m2) previously subjected to inten-sive 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 dif-ferent 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), respira-tion rate, microbial biomass C and N, and metabolic activities exhibited an increasing trend after di-gestate application, leading to a recovery of the soil biological fertility (i.e. biological fertility index in-creased 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 miti-gation and support rural communities. In the circular bioeconomy context, afforestation appears as the most promising strategy to take advantage of the restored land.