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

International Journal of Recycling of Organic Waste in Agriculture https://doi.org/10.1007/s40093-017-0188-0 ORIGINAL RESEARCH Recycling of biogas digestates in plant production: NPK fertilizer value and risk of leaching Trine A. Sogn1 · Ivan Dragicevic1 · Roar Linjordet3 · Tore Krogstad1 · Vincent G. H. Eijsink2 · Susanne Eich‑Greatorex1 Received: 3 August 2017 / Accepted: 16 December 2017 © The Author(s) 2018. This article is an open access publication 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. Keywords Digestates · Fertilizer · Nitrogen · Phosphorus · Potassium Introduction The core of the bioeconomy is the change from use of fossil raw materials to sustainable production, refinement and utilization of renewable biomass. For instance, production of biogas using organic waste as feedstock represents a plausible activity within the bioeconomy. Organic waste becomes a valuable resource, and compared to fossil fuel utilization, use of biogas yields a reduced carbon footprint * Trine A. Sogn 1 Faculty of Environmental Sciences and Natural Resource Management, Norwegian University of Life Sciences, Ås, Norway 2 Faculty of Chemistry, Biotechnology and Food Science, Norwegian University of Life Sciences, Ås, Norway 3 Norwegian Institute of Bioeconomy Research, Ås, Norway (Rehl and Müller 2013; Uusitalo et al. 2014). Biogas production through anaerobic digestion leaves organic residues, digestates, which are rich in nutrients. If these digestates are utilized in plant production, nutrients already within the nutrient cycle will be recycled and the sustainability of the biogas production process improves (Arthurson 2009; Ladanai and Vinterbäck 2009; Vaneeckhaute et al. 2013). In addition, use of organic fertilizers contributes to maintaining and/or improving soil quality (Hati et al. 2006). Utilization of digestates may replace or at least reduce the use of mineral fertilizer in agronomic plant production. The liquid phase of digestates is usually rich in plant-available nutrients such as ammonium (NH4+), phosphate and potassium (K). Additionally, more nitrogen (N) will be plant available by microbial decomposition and mineralization of the digestates’ solid phase. As anaerobic digestion produces methane (CH4) gas from the organic feedstock, which is subsequently removed for use as biogas, the digestates have a 13 Vol.:(0123456789) International Journal of Recycling of Organic Waste in Agriculture lower carbon to nitrogen ratio (C/N ratio) than the feedstock (Möller and Müller 2012). Although dependent on decomposition time, a C/N ratio lower than about 25 usually entails that the organic matter has a surplus of N relative to the N demand of the soil microbial community, which then can be mineralized to NH4+, i.e. plant-available mineral N (Ferris et al. 1998). Since farmers have to base their N fertilizer strategy on farm level N budgets with relatively high precision, the mineral N fertilizer equivalents or replacement values of different digestates are of importance. Studies with manure have shown low plant availability and then little effect from the organic N on plant growth in the first year of application due to slow mineralization (Möller and Müller 2012; Webb et al. 2013). On the other hand, the N H4+ in the liquid phase is plant-available (Gutser et al. 2005) and may be directly comparable with mineral N fertilizer. Nevertheless, it is common practice for farmers to estimate the fertilization value of manure by considering a small part of organically bound N and a reduced effectiveness of the mineral N (Kirchmann 1985), the latter in order to allow for gaseous and/or leaching losses at application and during the growing season. A major challenge with all organic fertilizers is the predictability of the additional mineral N release by decomposition of the organic matter. Assessments of the fertilizer potential of digestates have been carried out (Alburquerque et al. 2012; Kuszel and Lorencowicz 2015; Cavalli et al. 2016; Riva et al. 2016), but due to the wide variety of feedstock used for biogas production, the efficiency and exact predictability of the fertilizer N value of digestates is difficult. Over time, organic N in the solid phase of digestates will be mineralized and NH4+ will be released. In a production system with a continuous plant cover, the residual effect due to mineralization is positive as it ensures a gradual supply of NH4+ to the plants. However, in cereal production, the timing of the mineralization is crucial. For example, release of mineral N late in the growing season, when plants have less requirement for N, or in seasons without plant cover, will increase the risk of nitrate (NO3−) leaching. In agricultural areas with only one growing season per year and then often months without plant cover, release of mineral N from organic matter decomposition may increase the risk of N leaching. Next to N, K and phosphorus (P) are quantitatively the most important plant nutrients and essential to secure proper growth and development. Phosphorus and K in the feedstock are generally retained during the anaerobic digestion process (Zirkler et al. 2014). In addition to dissolved inorganic phosphate in the liquid phase, the solid phase of the digestates contains both inorganic and organic P (Kataki et al. 2017). As a surplus of phosphate in soils may eventually lead to eutrophication of fr (...truncated)