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Journal Article 
Nitrogen cycling and relationships between ammonia oxidizers and denitrifiers in a clay-loam soil 
Paranychianakis, NV; Tsiknia, M; Giannakis, G; Nikolaidis, NP; Kalogerakis, N 
In Press 
Applied Microbiology and Biotechnology
ISSN: 0175-7598
EISSN: 1432-0614 
This study investigated the effect of municipal solid waste (MSW) compost (0, 50, and 100 t/ha) on N cycling and the microorganisms involved in it, in a clay-loam soil. After a release of nitrates (NO(3) (-)-N) in the first 6 days after compost incorporation, soil NO(3) (-)-N content remained constant in all the treatments until day 62, suggesting N immobilization induced by the soil used in this study. Then, soil NO(3) (-)-N content increased in all treatments and especially in the highest compost dose, providing evidence that immobilization effect has been at least partially relieved. amoA gene copies of ammonia-oxidizing archaea (AOA) and bacteria (AOB) followed the overall pattern of soil NO(3) (-)-N content; however, no differences were found in amoA gene copies among treatments, except in the last sampling, an effect attributed to the slight differences in the potential nitrification rate among them. Ammonia oxidizer pattern provided evidence that both groups were involved in ammonia oxidation and changes in their abundance can be used as 'indicator' to predict changes in soil nitrification status. Moreover, the strong correlation between AOA and AOB amoA copies (R (2) = 0.94) and the high slope (13) of the curve suggest that AOA had probably an important role on ammonia oxidation. Denitrifying genes (nirS, nirK, nosZ) also followed the general pattern of soil NO(3) (-)-N, and they were strongly correlated with both groups of ammonia oxidizers, and particularly AOA, suggesting strong interrelationships among them. Losses of N through denitrification, as they were estimated by total nitrogen, were inversely related to soil NO(3) (-)-N content. Similar to ammonia oxidizers, denitrifying gene copies did not differ among compost treatments an effect that could be probably explained by the low availability of organic-C in the MSW compost and hence the competition with aerobic heterotrophs. 
Ammonia-oxidizing archaea; Ammonia-oxidizing bacteria; Nitrification; Immobilization