Use of anaerobic digestate to substitute inorganic fertilisers for more sustainable nitrogen cycling

Fertiliser applications significantly contribute to agricultural greenhouse gas emissions. While crop yields and quality depend on the type and amount of chemical fertiliser supplied, there are limitations to fertiliser use, as continuous application of inorganic fertilisers can negatively affect soil fertility and result in mineral leaching. Organic fertiliser produced by Anaerobic Digestion (AD) of sewage waste (AD digestate) has been proposed as a substitute for chemical fertilisers. To investigate the potential for the use of AD digestate in pasture systems, grassland field trials were conducted on three independent farm enterprises in Ireland. AD digestate and chemical fertilisers were applied separately or in various combinations and the effects on soil mineral content, grass yield and quality compared. The analyses revealed significant improvement of soil nitrification activity along with improved fodder yield and quality. In particular, digestate application increased crude protein content of fodder by up to 22.8 % compared to fodder from plots without digestate application, that had up to 14.8% crude protein. The increase could be attributed to the white clover percentage increase from 2% (no AD digestate) to up to 31% on plots where AD digestate was applied. Strikingly, grass yields were 1.5-2-fold higher than the five-year national average (of 59.7 ± 4 kg DM/ha/day) on the plots where AD digestate was applied, either alone or in combination with chemical fertilisers. The results of this study highlight the potential of AD digestate application as a more sustainable alternative to inorganic fertilisers in grass-based pasture systems.