The role of agricultural fertilization intensity on fluvial GHG fluxes from tropical and temperate catchments.

Catchment land use intensity can influence C and N cycling within streams, with implications for fluvial greenhouse gas (GHG) fluxes (CO2, CH4, N2O). This study analyzed annual datasets of hydrological, water physico-chemical parameters, GHGs saturations (%), and their fluxes from 78 stream sites in tropical and temperate catchments with contrasting agricultural fertilization intensities. In both climatic regions, instream GHG saturations correlated negatively with dissolved oxygen and mainly positively with dissolved inorganic nitrogen (DIN) (p < 0.05, r2 = 0.30-0.49), suggesting nitrogen-driven biogenic GHG production. In contrast to these similarities, agricultural intensity shaped how fluvial GHG fluxes scaled with distal drivers such as stream size, land use, and discharge. In tropical catchments with mainly extensive agriculture, CO2, and N2O fluxes decreased with stream size (p < 0.05, R2 = 0.77-0.82). However, in temperate catchments with intensive agriculture, land use was more important than stream size, with DIN concentrations and GHG fluxes increasing with agricultural land use (p < 0.05, R2 = 0.46-0.53). Agricultural fertilization intensity also seemed to influence CO2 sources during high-discharge events across the two study regions. Based on inferences from the respiratory quotient (range; 0.5-2.8), peak CO2 fluxes in the tropical catchments were linked to external CO2 inputs, while those in the temperate catchments linked to instream respiration under elevated nitrogen conditions. The findings of this study show that it is essential to consider gradients of agricultural fertilizer intensity to scale fluvial GHG emissions with their distal drivers, information critical for upscaling reach-scale GHG emissions.