Hyporheic zone is the source of nearly 50% of N2O emissions at the global scale
Abstract
Riverine systems contribute an estimated 10% of the entire anthropogenically generated nitrous oxide (N2O) emissions to the atmosphere. The primary biogeochemical transformations that lead to N2O emissions are denitrification and nitrification reactions that are mediated by microbes mainly living in benthic biofilms, subsurface sediments of the hyporheic zone, and within the water column. These three zones are an integral part of the riverine corridor, whose hydro-morphological characteristics depend on land use, land cover, biomes, climate and geological characteristics. Because of the complex interplay that occurs at the local scale among these variables and nutrient transport, N2O riverine emissions have been difficult to estimate. This is epitomized by global estimates that span 2 orders of magnitude from 35 to 6000 GgN2O-N/year. Here, we use a simple dimensionless framework to analyze and predict N2O emissions at the reach scale and then we upscale this framework to global level. The model accounts for hyporheic, benthic and water column processes and predicts reach-scale N2O areal emissions from standard reach-scale hydro-morphological variables: channel depth, width, velocity, average stream slope, median bedload grain size, water temperature, and nitrate and ammonium concentrations. Global emissions are quantified by integrating the local reach scale emissions along the entire river network. The model was developed with data collected by the 2nd Lotic Intersite Nitrogen eXperiment (LINXII) project and validated with measurements taken in different reaches from 10 river networks across the world including headwater streams, medium and large rivers and a tidal system with different morphologies, land covers, biomes and climatic conditions. We used a novel dataset, which has distributed mean annual values of all the required data at 1 km scale resolution, to inform our upscaling model. Results show that 50% of global N2O emissions come from the hyporheic zone of small streams, with widths less than 10 m, and large rivers such as the Amazon, Mississippi and the Yellow Rivers are responsible for less than 30%. This suggests that any policy of water management aiming to reduce N2O emissions from streams would be more effective than targeting large rivers.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B13L2469T
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 0469 Nitrogen cycling;
- BIOGEOSCIENCES;
- 0490 Trace gases;
- BIOGEOSCIENCES