Recycling of biogas digestates in plant production: NPK fertilizer valueand risk of leaching

Abstract

Purpose The main purposes of the study were to assess the NPK fertilizer value of biogas digestates in different soils and
to evaluate the risk of unwanted nutrient leaching.
Methods The fertilizer value of digestates from anaerobic digesters was investigated in a greenhouse pot experiment with
wheat in three different soils; silt, loam and sand. The digestates were based on different feedstock and had a low, dry matter
content. The fertilizing effect of digestates was compared to mineral fertilizer and manure. To investigate the fate of excess
nutrients in soil after the growing season, the pots were leached after harvest. A complementary soil column leaching experiment
without plants was carried out in the laboratory.
Results The concentration of ammonium in digestates provided a good indicator of the nitrogen fertilizer value of the digestates.
In the silt and loam, the ammonium N fraction in digestates had a fertilizer replacement value equal to that of mineral
fertilizer N, whereas the replacement value was higher in the nutrient poor sandy soil. Digestates often have a ratio between
nitrogen, phosphorus and potassium which is not favourable for plant growth. However, the suboptimal balance did not result
in reduced plant growth or unwanted leaching from soil.
Conclusions The results show that digestates from biogas production based on fundamentally different feedstock are promising
as NPK fertilizers. The N fertilization can simply be based on the digestate NH4
+ concentration and, at least for wheat
production, considerable variation in the concentrations of K and P can be tolerated.