Work

Methane and Nitrous Oxide Reshape the Air‐Water Greenhouse Gas Budget of a Tropical Estuarine Delta

Year: 2026

Type: article

Abstract: Estuaries are important sources and sinks of greenhouse gases (GHGs), yet assessments of their radiative balance often focus primarily on carbon dioxide (CO 2 ), with less attention given to methane (CH 4 ) and nitrous oxide (N 2 O). We combine measurements of CH 4 and N 2 O air‐water fluxes with concurrent CO 2 data in a tropical estuarine delta (Paraíba do Sul River, Brazil), based on three surveys under contrasting hydrological conditions. CH 4 concentrations were highest in freshwater‐dominated waters and mangrove creeks and decreased seaward, with greater values during high river discharge. Surface waters were supersaturated in CH 4 , indicating a permanent atmospheric source driven by freshwater inputs and organic matter degradation. In contrast, N 2 O exhibited weaker variability and alternated between source and sink, with concentrations linked to ammonium (NH 4 + ) and apparent oxygen utilization, suggesting nitrification as a dominant control. In CO 2 ‐equivalent terms, CH 4 emissions offset 30%–84% of the CO 2 sink during dry conditions and enhanced CO 2 emissions by 20%–44% during high discharge. N 2 O exerted a smaller but variable influence, either reinforcing or partially counterbalancing CO 2 fluxes. Together, CH 4 and N 2 O offset 10%–85% of the CO 2 sink in dry periods and added 18%–42% to CO 2 emissions during wet conditions. The relative influence of CH 4 and N 2 O was greatest where CO 2 fluxes were near equilibrium, particularly in the mixing and marine domains. Because many tropical deltas exhibit mixing‐driven CO 2 uptake of weak magnitude, CH 4 and N 2 O fluxes can reshape the radiative balance, underscoring the need for combined GHG assessments where CO 2 exchange is low or bidirectional.

Source: Journal of Geophysical Research Biogeosciences

DOI: 10.1029/2026jg009956