Nitrogen and Sulfur Availability in Digestates from Anaerobic Co-digestion of Cover Crops, Straw and Cattle Manure

Summary in the original language of the document

Anaerobic digestion (AD) has the potential to enhance the availability of nitrogen (N) from green manures; however, part of the sulfur (S) is lost with the biogas. In this study, the N and S availabilities from untreated and digested cover crop (CC) and straw mixtures were measured after application to soil in the laboratory. In addition, a microplot experiment was carried out to assess the N
availability in a spring barley crop. A CC consisting of red clover (Trifolium pratense L.) and chicory (Cichorium intybus L.) was harvested and ensiled with barley straw (Hordeum vulgare L.) at different ratios (1:0, 10:1 and 3:1). The silages were either digested alone or co-digested with cattle manure (CM) in continuously fed anaerobic reactors. Six months after soil application, the net inorganic N release was − 7%, 15% and 25% of total N for untreated CC:straw ratios of 3:1, 10:1 and 1:0, respectively. AD significantly increased the net inorganic N release of these plant materials to 5%, 26% and 38% (mono-digestion) and 40%, 41% and 43% (co-digestion with CM). Results from the laboratory experiment corroborated the field experiment in which AD significantly increased the nitrogen fertiliser replacement value (NFRV) of the CC:straw from 19 to 41% of total N applied. Gaseous loss of S up to 30% was observed during AD while the total N content remained unchanged. Sulfate from untreated CC:straw and digestates was immobilised soon after application to soil, resulting in low availability of inorganic S. Digestion of CC and straw significantly reduced N immobilisation but clearly did not contribute to inorganic S supply.