ORIGINAL ARTICLE Activity and composition of ammonia oxidizing bacterial communities and emission dynamics of NH 3 and N 2 O in a compost reactor treating organic household waste A ˚ . Jarvis 1 , C. Sundberg 2 , S. Milenkovski 3 , M. Pell 1 , S. Sma ˚ rs 2 , P.-E. Lindgren 4 and S. Hallin 1 1 Department of Microbiology, Swedish University of Agricultural Sciences, Uppsala, Sweden 2 Department of Energy and Technology, Swedish University of Agricultural Sciences, Uppsala, Sweden 3 Department of Ecotoxicology, Ecology Building, Lund University, Lund, Sweden 4 Division of Medical Microbiology, Department of Clinical and Experimental Medicine, Linko ¨ ping University, Linko ¨ ping, Sweden Introduction Composting is an aerobic, self-heated process driven by the activity of different micro-organisms involved in the degradation and stabilization of organic material. In recent decades, many municipalities have established large-scale composts to handle organic waste. There is therefore an urgent need to improve handling and con- trol of the process so that it can be managed to mini- mize environmental hazards and produce a high-quality final product that can be used in agriculture, parks or gardens. Many compost substrates, such as manure and household waste, are rich in nitrogen and the rapid mineralization of proteins in the early stages of compo- sting leads to the production of ammonium (NH 4 + ; Miller 1993; Kowalchuk et al. 1999). However, an alka- line pH during the thermophilic phase of composting may lead to substantial losses of gaseous ammonia (NH 3 ). This represents an environmental hazard as NH 3 , once released in the environment, can cause acidi- fication through transformations leading to the release of protons or eutrophication as NH 3 is also a nitrogen source. In addition, emissions of NH 3 result in a com- post of low nitrogen content, diminishing its value as a Keywords ammonia, ammonia oxidation rate, ammonia oxidizing bacteria, compost, methane, nitrification, nitrous oxide. Correspondence A ˚ sa Jarvis, Ro ¨ rba ¨ cksva ¨ gen 50, SE-757 52 Uppsala, Sweden. E-mail: asa.jarvis@glocalnet.net 2008 ⁄ 0901: received 28 May 2008, revised 8 September 2008 and accepted 9 October 2008 doi:10.1111/j.1365-2672.2008.04111.x Abstract Aims: To monitor emissions of NH 3 and N 2 O during composting and link these to ammonia oxidation rates and the community structure of ammonia oxidizing bacteria (AOB). Methods and Results: A laboratory-scale compost reactor treating organic household waste was run for 2 months. NH 3 emissions peaked when pH started to increase. Small amounts of N 2 O and CH 4 were also produced. In total, 16% and less than 1% of the initial N was lost as NH 3 -N and N 2 O-N respectively. The potential ammonia oxidation rate, determined by a chlorate inhibition assay, increased fourfold during the first 9 days and then remained high. Initially, both Nitrosospira and Nitrosomonas populations were detected using DGGE analysis of AOB specific 16S rRNA fragments. Only Nitrosomonas europaea was detected under thermophilic conditions, but Nitrosospira popula- tions re-established during the cooling phase. Conclusions: Thermophilic conditions favoured high potential ammonia oxida- tion rates, suggesting that ammonia oxidation contributed to reduced NH 3 emissions. Small but significant amounts of N 2 O were emitted during the ther- mophilic phase. The significance of different AOBs detected in the compost for ammonia oxidation is not clear. Significance and Impact of Study: This study shows that ammonia oxidation occurs at high temperature composting and therefore most likely reduces NH 3 emissions. Journal of Applied Microbiology ISSN 1364-5072 1502 Journal compilation ª 2009 The Society for Applied Microbiology, Journal of Applied Microbiology 106 (2009) 1502–1511 ª 2009 The Authors