image: Anthropogenic sources constitute the primary component of N2O emissions in global rivers and are mainly manifested as two patterns: baseline emissions and localized hotspots. The k values of the EF lines for natural, agricultural, and urban rivers were determined at 0.02, 0.09, and 0.05, respectively, indicating increasing nutrient levels and basic N2O emissions in the different types of rivers. The corresponding median ΔN2O concentrations were 0.02, 23.9 and 21.7 nM, respectively. The median NH4+, NO3- and DOC concentrations for the basic emissions (blue) and localized hotspots (red) are shown for the different rivers. Additionally, the median ΔN2O concentration at localized hotspots reached 204.0 nM (11%) and 231.4 nM (14%) in agricultural and urban rivers, respectively. The priority control of organic and NH4+ pollution could eliminate hotspots and reduce the emissions of agricultural and urban rivers by 51.6% and 63.7%, respectively. However, further restoration of baseline emissions on nitrate removal is a long-term challenge.
Credit: ©Science China Press
This study is led by Dr. Guodong Ji (Department of Environmental Engineering, Peking University). As a long-lived (116 ± 9 years) greenhouse gas, nitrous oxide (N2O) is 265-298 times the global warming potential of CO2 on a 100-year timescale and it is also involved in stratospheric ozone depletion. Global rivers are a significant anthropogenic source for N2O emissions. In the estimation and management of indirect N2O emissions (IPCC), riverine N2O fluxes are commonly assumed to linearly increase with nitrate loading with a fixed emission factor EF5r (0.26%). However, global rivers are highly heterogeneous. Uncertainties will inevitably widen when extrapolating the simplistic factor for the global estimation. Additionally, our understanding of riverine N2O emissions remains fragmented with a bias towards N-rich rivers, yet no clear large-scale pattern has been identified for global rivers. Based on these simplistic and fragmented understandings, it is difficult for us to comprehend the underlying mechanism of riverine N2O emissions and quantify human impacts. This, in turn, impedes the development of effective mitigation strategies.
Guodong, together with his student Shuo, seek to solve this complex problem through data mining. They are trying to explain how N2O is emitted and how to reduce it in global rivers using the simplest mathematical curve. Fortunately, they found a universal EF-line in global rivers! It can well guide the sustainable management of riverine N2O emissions.
It was revealed that EF5r decreased with increasing NO3- concentration, precisely conforming to the inversely proportional function EF5r = k/[NO3-] (R2 = 0.90) for natural rivers. A k value of 0.02 represents situations without human disturbance, which could serve as a reference for the overall nutrient level and basic N2O emissions in rivers. Regarding agricultural and urban rivers, anthropogenic impacts caused an overall increase in their baselines and the emergence of hotspots. The k values of agricultural and urban rivers increased to 0.09 and 0.05, respectively, with 11% and 14% of points becoming N2O hotspots.
The team calculated that the priority control of organic and NH4+ pollution could eliminate hotspots and reduce the emissions of agricultural and urban rivers by 51.6% and 63.7%, respectively. This finding is of great significance to practical engineering. In the future, the well-managed urbanization, including the land intensification, the improvement of sewage collection and the expansion of wastewater treatment capabilities, are favorable for rapid eliminating hotspots in urbans. Meanwhile, agricultural centralization and the identification of critical source areas are important for managing N2O hotspots in agricultural watersheds. However, considering population growth and the massive demand for fertilizer, nitrate will inevitably be leached from intensive arable systems. Further restoration of baseline emissions on nitrate removal is a long-term challenge.
See the article:
Sustainable management of riverine N2O emission baselines
https://doi.org/10.1093/nsr/nwae45
Journal
National Science Review