CO2 emissions and mineral nitrogen dynamics following application to soil of undigested liquid cattle manure and digestates

Anaerobic digestion of animal manures, and separation of digestates into a liquid (LF) and a solid fraction (SF) affect manure N availability to crops. Digestates and LF potentially supply more readily available N to crops compared to undigested manures and SF, due to their higher ammonium content, lower C to organic N ratio and lower dry matter content. Studies on digestates decomposition in soil are still scarce compared to those on undigested manures. We therefore carried out an incubation experiment to measure CO2 emissions and soil mineral N dynamics following addition to soil of: ammonium sulphate (AS); digested cattle slurry-maize mix (DSMM); LF and SF of DSMM; undigested cattle slurry (US). The incubation lasted 181 days and was conducted at 25 °C and constant soil water content. Mineralization of C from US (51% of manure C after 181 days) was higher than that of digested manures (32-34%). Availability of applied ammonium after 181 days was similar for AS, DSMM and LF (70-78% of added ammonium), and was higher than that of SF and US (8 and 46% of added ammonium, respectively). Physical-chemical differences among manures explained main differences in decomposition dynamics. Indeed, US and SF induced net N immobilization (9-16% of manure organic N at day 181) due to high C to organic N ratio, high cellulose and volatile fatty acids content. Conversely, DSMM and LF induced net N mineralization (30% of manure organic N) due to a low C to organic N ratio and cellulose content. The findings of this laboratory experiment confirm the availability of manure N previously measured in a field experiment, where the same manures were applied to the same soil for the fertilization of silage-maize.