Nitrogen cycling and relationships between ammonia oxidizers and denitrifiers in a clay-loam soil

Abstract

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 (NO3--N) in the first 6 days after compost incorporation, soil NO3--N content remained constant in all the treatments by day 62 suggesting immobilization of N. Then, soil NO3--N content increased, especially in the highest compost dose implying that the immobilization effect has been relieved, at least to some extent. amoA gene copies of ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB) followed strictly the pattern of soil NO3--N content throughout the study providing evidence that both groups were involved in ammonia oxidation and changes in their population can be used as ‘indicator’ for predicting changes in soil nitrification status. Moreover, the strong correlation between AOA 12 and AOB amoA copies (R2: 0.94) and the high slope (13) of the curve suggest that AOA had probably a more important role on ammonia oxidation under conditions of low ammonia availability. Denitrifying genes (nirS, nirK, nosZ) also followed the general pattern of soil NO3–-N and they were strongly correlated with both groups of ammonia oxidizers, and particularly archaea, suggesting strong interrelationships among them. Losses of N through denitrification, as they were estimated from total nitrogen decrease, were inversely related to soil NO3--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 of the MSW compost and hence the competition with aerobic heterotrophs.