Geological modulation of N2O emissions in rivers globally

Fertilizer application has been recognized as the major driver of the N ₂ O emissions in rivers. However, behind anthropogenic nitrogen (N) loading, the intrinsic ability of rivers to modulate N ₂ O emissions may vary across regions but has long been overlooked and remains unclear. Here we uncover the hidden but substantial control of riverine N ₂ O emissions by bedrock geology via regional field observations and global data analyses. We discovered the lower denitrification but higher N ₂ O production rates in rivers with silicate-dominated bedrocks compared to those with carbonate-dominated bedrocks. On one hand, the larger sediment grain size found in the silicate-dominated rivers shortens porewater residence time for reaction, which diminishes the likelihood of complete denitrification (NO ₃ ^- →N ₂ ) but simultaneously increases the potential for N ₂ O production through incomplete denitrification (NO ₃ ^- →N ₂ O). Concurrently, lower total organic carbon (TOC) in benthic sediments and lower water pH further abate N ₂ O reductase and enhance N ₂ O production in silicate-dominated rivers. More importantly, the dissimilarities in N ₂ O production due to bedrock geology are reflected in riverine N ₂ O emissions, where N ₂ O emissions are 45% to 185% higher in silicate-dominated rivers than in carbonate-dominated rivers per square kilometer worldwide. Globally, by incorporating a geological factor, we further demonstrate that N from fertilizer is more likely to be converted into N ₂ O in silicate regions, while in carbonate regions, it is more likely to be transformed into N ₂ , which further emphasizes the geological modulation of riverine N ₂ O emission as a global overlooked phenomenon. Hence, we highlight the geographically uneven urgency for improvement of fertilizer management to mitigate global warming. Regions dominated by silicate bedrock should have more strict fertilizer management due to high risks of N ₂ O production and emission.