Chemical properties of anaerobic digestates affecting C and N dynamics in amended soils

The optimisation of digestate recycling as fertilisers, based on both environmental and agricultural criteria, requires an evaluation of the effects on C and N dynamics in soil. In the present paper, six digestates from several anaerobic co-digestion experiments, using pig or cattle slurry as the main substrate, were evaluated in short-term incubations in soil.Digestate properties such as dissolved organic-C (DOC), biochemical oxygen demand (BOD) and digestate organic-C mineralised in the soil during the first 7 days represented properly the digestate biodegradability. These, together with their ratios with respect to the total nitrogen (TN) concentration in the digestate, were reliable parameters with respect to defining the C and N dynamics in the soil and hence the N-fertiliser potential of the digested materials. Therefore, highly biodegradable digested materials, represented in the present study by digestates from cattle slurry–glycerine mixtures were not suitable for agricultural use as they caused a high CO2–C production and led to N-immobilisation and/or denitrification after their application to soil. Contrastingly, for less biodegradable digested materials (BOD5 d < 6.0 g O2 L−1 fresh weight, DOC < 5.5 g L−1 fresh weight and DOC/TN < 1.5), less CO2–C was evolved and ammonium was rapidly nitrified in soil—being an available N source for crops.

► N-fertiliser potential and C-mineralisation of digestates depend on their stability.
► Dissolved organic carbon and BOD indicate well-digestate biodegradability.
► Highly biodegradable digested materials require further stabilisation prior to soil use.
► Unstable digested materials lead to strong CO2–C production and N-immobilisation.
► Ammonium in low biodegradable digestates is rapidly nitrified in soil.