The influence of tropical grass species on soil nitrification and denitrification across savanna ecosystems

Although limited by nutrient-poor soils primary productivity in certain humid savannas can be as high as in tropical forests. This high productivity can be partly attributed to the ability of certain perennial grasses (Poaceae) to inhibit nitrification, a process known as biological nitrification inhibition (BNI). By decreasing nitrate production and thereby reducing nitrogen (N) losses through nitrate leaching and N2O emissions from denitrification, BNI can lead to a more nitrogen-conserving ecosystem. However, the extent to which this capacity is widespread among tropical grasses remains largely unknown. This study explores the potential of tropical savanna grasses to modulate nitrification and influence soil (de)nitrifiers across savannas worldwide. We investigated the activities and abundances of nitrifiers and denitrifiers in the rhizosphere soils of 41 tropical Poaceae species from six sites in Africa and South America. Among the species studied, six significantly reduced nitrification compared to bare inter-tussock soil, and this reduction was associated with a lower abundance of ammonia-oxidizing bacteria (AOB), but not with ammonia-oxidizing archaea (AOA). In contrast, 10 other species significantly enhanced soil nitrification, which was positively correlated with an increase in AOA abundance. Moreover, lower nitrate concentrations in the rhizosphere of nitrification-inhibiting savanna species were associated with reduced denitrification rates, suggesting an indirect limitation of denitrification mediated by nitrate availability. However, this pattern was not observed in the stimulating species. Our study reveals that tropical Poaceae species presumably use a diverse range of strategies to modulate nitrification, suggesting that these grasses play distinct roles in regulating nitrogen cycling processes across different savannas worldwide.