Impact of Adding Anaerobic Digestate to Soil and Consequences on Crop Performance

Anaerobic digestate is proposed as an alternative to inorganic fertilisers, but a better understanding of how anaerobic digestates impact the soil and how plant growth is influenced is needed for wider acceptance. In this study, a series of pot experiments were conducted growing spring barley (Hordeum vulgare L.) in a range of soils with the application of digestate or synthetic fertiliser. Two application rates corresponding to total nitrogen at 120 and 480 kg ha-1 were used together with unfertilised soil as a control. Growth characteristics were measured as plant height, number of tillers, straw biomass, grain biomass and total biomass. Most growth characteristics (tillering, and straw and grain yield) increased with increasing application rates of nitrogen. An increase of 7-20% in plant height was observed with anaerobic digestate compared to synthetic fertilisers. However, results differed depending on the source of digestate and soil type. The nitrogen balance index (the ratio of the chlorophyll to polyphenolic compounds, which is linked to the nitrogen status of the crop) showed an increase of 40-50% for digestate applied at a nitrogen rate of 480 kg ha-1 compared to the control. By measuring nitrogen as ammonium, nitrate and nitrite in the different soils over 35 days, differential nitrogen release was also demonstrated, with soil water concentrations of ammonium decreasing rapidly after an early peak in all the treatments, and nitrate peaking after days 3-4. Results suggest that digestate may be used to replace synthetic fertilisers when applied in a range of soils.