97 Organic Agriculture 25 th Congress NORDIC VIEW TO SUSTAINABLE RURAL DEVELOPMENT June 16-18, 2015 ANAEROBIC DIGESTION OF ANIMAL MANURE – IMPLICATIONS FOR CROP YIELDS AND SOIL BIOTA IN ORGANIC FARMING Anders JOHANSEN 1 , Reidun POMMERESCHE 2 , Hugh RILEY 3 , Anne-Kristin LØES 2 1 Department of Environmental Science – Environmental microbiology & biotechnology, Aarhus University, Denmark 2 Bioforsk Norwegian Institute for Agricultural and Environmental Research, Organic Food and Farming 3 Bioforsk Cereal grains, Potatoes and Vegetables, Norway Emails: ajo@envs.au.dk, anne-kristin.loes@bioforsk.no Abstract. Anaerobic digestion of farmyard manures may help farmers to produce bioenergy instead of using fossil fuels, support cycling of nutrients and reduce greenhouse gas emission. However, compared to pristine slurry, digested slurry has a reduced content of organic carbon which may impact the soil biota negatively due to substrate shortage. Our knowledge on these processes and their infuence on soil quality is scarce. Hence, a feld experiment with two organic cropping systems (grass-clover ley and arable system; at two slurry-application levels) was established in 2011, to study how application of digestates affects crop yields, soil characteristics and soil biota (earthworms, springtails, microbiota). The grass-clover system showed comparable yield levels over 3 years when digested slurry was compared to untreated slurry. Digested slurries had no infuence on soil nutrient concentrations or on soil organic matter levels over the frst 2 years. Application of high levels of manure increased the mortality of both surface-dwelling and soil-living earthworms just after application, but the long-term effect of manure application seemed more positive, especially at low application levels. Springtails and microorganisms seemed only little affected by application of digested slurry. Key words: Collembola, bioenergy, grass-clover, digestate. INTRODUCTION Agriculture is criticized for emitting high amounts of greenhouse gas, both as methane (CH 4 ) and nitrous oxide (N 2 O), through the use of fossil energy and animal husbandry. By anaerobic digestion (AD) of farmyard manures, farmers may produce biogas to replace fossil fuels and reduce the emission of methane produced during storage of slurries. AD may also ease the handling of the manure and reduce viability of animal pathogens and weed seeds [1]. However, the process needs stable temperatures of at least 30°C, so the construction and maintenance of small-scale biogas plants for Nordic conditions requires technically skilled farmers. AD alters the physicochemical composition of the manure, reducing the proportion of easily degradable C in the digested manure. This may negatively affect soil organic matter pools and the soil biota that live on the organic C. Organic nitrogen (N) is mineralised during AD, thus enhancing the proportion of mineral N in the digestate compared with untreated manure [2]. The higher concentration of ammonium, constituting most of the mineral N, may be toxic to soil fauna, such as earthworms [3]. On the other hand, when used as fertilizer, greater availability of N applied in the digestate may increase root and shoot residues due to increased plant growth. This may compensate for the organic C lost during digestion, and support the growth of soil fauna and microorganisms. The effects of AD on the quality of animal manure, and derived effects on crop yields and biota, are studied in the project “SoilEffects” at Tingvoll, NW Norway. Results from 2011-2014 are presented, to evaluate whether AD affects manure, crop yields and quality or soil fauna and microbiota. MATERIALS AND METHODS Anaerobic digestion of manure A biogas plant was established in 2010 at Tingvoll Research Farm, owned by the Norwegian Centre for Ecological Agriculture, to treat the slurry from 25 organically managed dairy cows. In spring 2011, an associated feld experiment was initiated to study long-term effects of AD treated slurry on soil quality characteristics and crop yields [4]. Because a stable digestion process was frst reached in autumn 2011, both the conventional dairy cow slurry and digestate applied in 2011 was obtained from Bioforsk, Soil and Environment Division (Ås), where the digestate was produced in a 6-m 3 batch digester. In later seasons, digested and non-digested slurries were sampled during winter and stored in 1-m 3 plastic containers at Tingvoll. The farm has a loose-housing cow house, from which slurries fow to a collection pit. Until a pump was installed