農場近くの小川は高レベルの温室効果ガスを排出していることが研究で判明(Streams near farms emit high levels of greenhouse gas, studies find)

2024-12-03 イリノイ大学アーバナ・シャンペーン校

＜関連情報＞

• https://aces.illinois.edu/news/streams-near-farms-emit-high-levels-greenhouse-gas-studies-find
• https://pubs.acs.org/doi/full/10.1021/acs.est.4c01285
• https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024GL109623

水文連結性が河川のN2O発生源と動態を制御す Hydrologic Connectivity Regulates Riverine N2O Sources and Dynamics

Minpeng Hu,Zhongjie Yu,Timothy J. Griffis,Wendy H. Yang,Joachim Mohn,Dylan B. Millet,John M. Baker,Dongqi Wang,
Environmental Science & Technology  Published May 23, 2024
DOI:https://doi.org/10.1021/acs.est.4c01285

Abstract

Indirect nitrous oxide (N₂O) emissions from streams and rivers are a poorly constrained term in the global N₂O budget. Current models of riverine N₂O emissions place a strong focus on denitrification in groundwater and riverine environments as a dominant source of riverine N₂O, but do not explicitly consider direct N₂O input from terrestrial ecosystems. Here, we combine N₂O isotope measurements and spatial stream network modeling to show that terrestrial–aquatic interactions, driven by changing hydrologic connectivity, control the sources and dynamics of riverine N₂O in a mesoscale river network within the U.S. Corn Belt. We find that N₂O produced from nitrification constituted a substantial fraction (i.e., >30%) of riverine N₂O across the entire river network. The delivery of soil-produced N₂O to streams was identified as a key mechanism for the high nitrification contribution and potentially accounted for more than 40% of the total riverine emission. This revealed large terrestrial N₂O input implies an important climate–N₂O feedback mechanism that may enhance riverine N₂O emissions under a wetter and warmer climate. Inadequate representation of hydrologic connectivity in observations and modeling of riverine N₂O emissions may result in significant underestimations.

米国コーンベルトからの亜酸化窒素排出に関する同位体制約 Isotopic Constraints on Nitrous Oxide Emissions From the US Corn Belt

T. J. Griffis, Z. Yu, J. M. Baker, D. B. Millet
Geophysical Research Letters  Published: 06 November 2024
DOI:https://doi.org/10.1029/2024GL109623

Abstract

Agriculture is the dominant source of anthropogenic nitrous oxide (N₂O) –a greenhouse gas and a stratospheric ozone depleting substance. The US Corn Belt is a large global N₂O source, but there remain large uncertainties regarding its source attribution and biogeochemical pathways. Here, we interpret high frequency stable N₂O isotope observations from a very tall tower to improve our understanding of regional source attribution. We detected significant seasonal variability in δ¹⁵N_bulk (6.47–7.33‰) and the isotope site preference (δ¹⁵N^SP = δ¹⁵N^α–δ¹⁵N^β, 18.22–25.19‰) indicating a predominance of denitrification during the growing period but of nitrification during the snowmelt period. Isotope mixing models and atmospheric inversions both indicate that indirect emissions contribute substantially (>35%) to total N₂O emissions. Despite the relatively large uncertainties, the upper bound of bottom-up indirect emission estimates are at the lower bound of the isotopic constraint, implying significant discrepancies that require further investigation.

Key Points

• Seasonality of N₂O isotope fingerprint driven by snowmelt and fertilizer input
• Snowmelt and growing season emissions dominated by nitrification versus denitrification pathways
• Isotope fingerprints were used to constrain direct and indirect N₂O emissions

Plain Language Summary

Increasing use of synthetic nitrogen fertilizers for agricultural production is causing higher atmospheric nitrous oxide (N₂O) concentrations. Nitrous oxide is a long-lived greenhouse gas and degrades the protective stratospheric ozone layer. Using tall tower N₂O isotope observations from within the US Corn Belt, we examine how different processes (denitrification vs. nitrification) and sources (corn fields vs. wetlands, rivers, and streams) contribute to variations in atmospheric N₂O. The findings indicate that a substantial amount of nitrogen leakage from agricultural crops contributes to N₂O emissions via indirect sources such as drainage networks. These findings can help inform mitigation strategies targeting nitrogen use and leakage pathways from agricultural systems.