2024-09-12 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/legacy-corn-nitrogen-fertilizer-study-shows-lengthy-impact-tile-drained-systems
- https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024JG008027
硝酸塩の同位体インプリントを解読し、排水システムによる農業生態系における窒素源と輸送メカニズムを明らかにする Deciphering the Isotopic Imprint of Nitrate to Reveal Nitrogen Source and Transport Mechanisms in a Tile-Drained Agroecosystem
Yinchao Hu, Zhongjie Yu, Wendy H. Yang, Andrew J. Margenot, Lowell E. Gentry, Michelle M. Wander, Richard L. Mulvaney, Corey A. Mitchell, Carlos E. Guacho
Journal of Geophysical Research Published: 31 July 2024
DOI:https://doi.org/10.1029/2024JG008027
Abstract
Installation of subsurface drainage systems has profoundly altered the nitrogen cycle in agricultural regions across the globe, facilitating substantial loss of nitrate (NO3−) to surface water systems. Lack of understanding of the sources and processes controlling NO3− loss from tile-drained agroecosystems hinders the development of management strategies aimed at reducing this loss. The natural abundance nitrogen and oxygen isotopes of NO3− provide a valuable tool for differentiating nitrogen sources and tracking the biogeochemical transformations acting on NO3−. This study combined multi-years of tile drainage measurements with NO3− isotopic analysis to examine NO3− source and transport mechanisms in a tile-drained corn-soybean field. The tile drainage NO3− isotope data were supplemented by characterization of the nitrogen isotopic composition of potential NO3− sources (fertilizer, soil nitrogen, and crop biomass) in the field and the oxygen isotopic composition of NO3− produced by nitrification in soil incubations. The results show that NO3− isotopes in tile drainage were highly responsive to tile discharge variation and fertilizer input. After accounting for isotopic fractionations during nitrification and denitrification, the isotopic signature of tile drainage NO3− was temporally stable and similar to those of fertilizer and soybean residue during unfertilized periods. This temporal invariance in NO3− isotopic signature indicates a nitrogen legacy effect, possibly resulting from N recycling at the soil microsite scale and a large water storage for NO3− mixing. Collectively, these results demonstrate how combining field NO3− isotope data with knowledge of isotopic fractionations can reveal mechanisms controlling NO3− cycling and transport under complex field conditions.
Plain Language Summary
Installation of subsurface tile pipes in many poorly drained agricultural lands has facilitated a substantial loss of nitrate (NO3−) to surface water systems. However, the nitrogen sources and related processes controlling NO3− export from tile-drained agricultural systems remain unclear. This study employed stable isotope techniques to investigate how NO3− is biologically produced and hydrologically transported in a tile-drained field. Stable isotopes are chemical variants of the same element and have long been used as a tracer of nitrogen cycling in environmental systems. By combining field measurements of NO3− isotopes in tile drainage with a detailed understanding of how these isotopes are altered by microbial reactions, we estimated the original isotope ratios of NO3− and compared these ratios to those of potential nitrogen sources in the field. The results show that the original isotope ratios of NO3− were similar to those of ammonia fertilizer and soybean biomass nitrogen and did not vary over time when there was no fertilizer input to the system. These findings indicate the presence of a large NO3− pool in the soil and a time lag between the moments when the source nitrogen was introduced into the system and when the NO3− was exported via tile drainage.
Key Points
- The oxygen isotopic composition of nitrate produced by soil nitrification varied with the degree of soil nitrite accumulation
- The dual isotopes of nitrate in tile drainage exhibited coupled variations and were highly responsive to variations in tile discharge
- Combining field nitrate isotope data with the isotopic systematics of nitrification reveals a legacy effect controlling nitrate dynamics
