Effects of vegetation cover and season on soil nitrifiers in an African savanna: Evidence of archaeal nitrifier inhibition by grasses

Abstract

For the first time in a savanna, we analysed the joint influence of climate seasonality and plant cover type on the abundances and transcription levels of bacterial and archaeal nitrifiers, and nitrification enzyme activity (NEA). The dominant tree and grass species stimulated and inhibited nitrification, respectively: NEA was 4-fold higher and 5-fold lower under trees and grasses than in bare soil, respectively. The abundances of bacterial nitrifiers (AOB-amoA) followed the same trend as the abundances of total bacteria did, with higher abundances under grasses and trees than in bare soil and higher abundances during the wet season, but AOB-amoA transcripts were always below detection limit. In contrast, the abundances of archaeal nitrifiers (AOA-amoA) were 13-fold lower under grasses and 2-fold higher under trees than in bare soil, and the AOA-amoA transcript level decreased under grasses during the wet season. Our results show that nitrifier abundances and transcriptional activities are highly seasonal and that nitrification is likely to be dominated by AOA-amoA in these savanna soils. We show for the first time that a dominant grass species (Hyparrhenia diplandra) of humid African savannas inhibits nitrification through marked decreases in AOA-amoA expression levels.