In a sustainable society, nutrients from waste products in urban areas should be recycled as much as possible. Many organic waste products can be used as soil amendments after some treatment, and there is evidence that additions of organic matter can have positive effects in some situations. Treatment processes will usually be some sort of decomposition process, arerobic or anerobic, to stabilize the material. There are several treatment options, but anarobic digestion is often one of the best, because it gives biogas for energy in addition to a soil amendment. Anaerobic digestion also conserve nitrogen better than other treatment options (e.g. composting), because it is a closed process. Nutrients also become more plant available during the digestion process, so that digestate is usually one of the best fertilizers of organic origien. Problems with use of digestate as fertilizer are that high ammonium content and high pH means that nitrogen is easily lost as ammonia during handeling and spreading, and that high water content means that transport to areas that need the fertilizer can be expesive and negate any environmental benefits. Water content can be reduced by de-watering or drying/evaporation. After de-watering, most of the plant available nutrients will be in the liquid phase, whilst nitrogen will be lost as ammonia during drying. We are assessing sorption as a way to increase the fraction of nutrients in the solid phase as well as reducing losses during drying. As part of this we are investigating if sorption has any effect on plant availability and vunerability to leaching losses. The solid fraction of digestate is a good source of organic matter as well as some nutrients. Composting can be an alternative to anarobic digestion as treatment of organic waste, but it can also be post-treatment of digestate. We are investigating effects of composting the solid fraction of digestate, including vermicomposting. Composting, particularly vermicomposting appears to have some positive effects on plant growth and make the material more suitable as a substate for plant growth, e.g. potting soil. However, emissions of greenhouse gases during composting of digestate can be an issue.

Bente Foereid has experience in research work on carbon and nutrient cycling in managed and semi-natural systems. She has experience in studies of decomposition, plant-soil interactions and greenhouse gas emission. She has done experimental and modelling work.

At NIBIO her main area of research is on the use of organic residues and waste products (e.g. digastate, compost and biochar) as fertilizers and soil improvers.