The response of ammonia oxidizing archaea and bacteria in relation to heterotrophs under different carbon and nitrogen amendments in two agricultural soils

Abstract

Ammonia-oxidizing archaea (AOA) and bacteria (AOB) drive nitrification and therefore critically modulate plant nitrogen (N) utilization, ecosystem N retention and environmental effects of reactive N. Previous studies have shown that abiotic factors (e.g., soil ammonium concentration, pH) can largely control the abundance and composition of ammonia oxidizers. However, whether the biotic factors, such as heterotrophic microbes play a role in impacting AOA and AOB remain unknown. Here, we conducted two experiments to assess the impacts of heterotrophs on AOA and AOB. First, a microcosm experiment was designed to create environments with different competition intensities between heterotrophic microbes and ammonia oxidizers through adding different amounts and ratios of organic C (cellulose) and mineral N [(NH4)2SO4] into two agriculture soils with long-term distinct fertilization histories. Along with the carbon to nitrogen (C/N) ratio gradient, rapid decreases in AOA and AOB abundances occurred accompanied with increased total microbial biomass and activities (respiration), suggesting intense competition between heterotrophic microbes and ammonia oxidizers. Pyrosequencing data revealed that different C/N ratios of substrate had significant impacts on the composition of the AOB but not on AOA communities. Second, to test whether there were inhibitive interactions through metabolic compounds, we examined the effect of water extracts of soils amended with high ratios of cellulose and ammonium sulfate on AOA /AOB abundances. The results showed that the extracts from substrates with C/N ratio of 50 and 100 reduced AOA and AOB abundance significantly, although this negative effect abated over time. Together, our findings indicate that both direct competition and inhibition by microbial metabolites critically affect AOA and AOB communities, providing new insights into the mechanisms that underlie ammonia oxidizer dynamics in agricultural ecosystems.