Keywords: Anaerobic digestate, nutrient management, nitrogen, ammonia, best practice
Anaerobic digestion is the controlled biological decomposition of biodegradable materials (e.g. food waste, livestock manures or purpose-grown crops) in the absence of oxygen, releasing biogas that can be used as a source of heat and/or power. The process also produces a nutrient-rich digestate which is a potentially valuable source of crop available nitrogen (N) together with phosphate and potash. As with any ‘new’ material being recycled to agricultural land it is important to understand the crop N supply characteristics of digestate applications and to develop best management practices that maximise crop N utilisation, whilst minimising emissions to the environment.
Approximately 90% of the total N content of digestate derived from food waste is present in a readily available N form (i.e. ammonium-N), compared to c.65% for digestate derived from livestock manures and purpose grown crops, and 45% for a typical cattle slurry. This, together with a high pH means the potential for N loss via ammonia volatilisation following digestate applications can be considerable. The DC-Agri experimental programme aimed to quantify the crop available N supply from digestate application at 15 sites across the UK. Additional measurements to quantify ammonia emissions to air and nitrate leaching losses to water were carried out at 3 of the sites. Ammonia losses were higher from digestate (40% of total N applied) than from livestock slurry (30% of total N applied) applications reflecting the higher ammonium-N content and pH of the digestate. Bandspreading was effective at reducing ammonia emissions but only where the food-based digestate stayed in a band and rapidly infiltrated into the soil. Nitrate leaching losses following autumn application of food-based digestate to a sandy soil were similar in magnitude to those from livestock slurry at c.15-20% of the total N applied, demonstrating that digestate poses a similar threat as livestock slurries in terms of nutrient leaching to water bodies. Over all 15 sites, spring application timings of food-based digestate resulted in a crop (winter and spring cereals, potatoes and grass) nitrogen use efficiency (NUE) of c.55% compared to just 15% from autumn applications, due to a low crop N uptake and overwinter nitrate leaching losses following autumn applications.
The DC-Agri experimental programme has demonstrated the value of food-based digestate as a source of crop available nitrogen, but also highlighted the potentially high environmental cost of mismanaging these materials due to losses of N via overwinter nitrate leaching and ammonia volatilisation. These findings have directly fed into the development of best practice guidelines which provide a framework for the responsible use of digestates and composts.
Anne Bhogal, Fiona Nicholson, Matt Taylor and Alison Rollet, ADAS Gleadthorpe, Meden Vale, Mansfield, UK
John Williams, ADAS Boxworth, Cambridge, UK.
16 pages, 4 figures, 1 table, 20 references