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Deciphering the isotopic imprint of nitrate to reveal nitrogen source and transport mechanisms in a tile-drained agroecosystem
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  • Yinchao Hu,
  • Zhongjie Yu,
  • Wendy H Yang,
  • Andrew J Margenot,
  • Lowell E Gentry,
  • Michelle Wander,
  • Richard Mulvaney,
  • Corey A Mitchell,
  • Carlos E Guacho
Yinchao Hu
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Zhongjie Yu
University of Illinois at Urbana Champaign

Corresponding Author:[email protected]

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Wendy H Yang
University of Illinois at Urbana Champaign
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Andrew J Margenot
University of Illinois at Urbana-Champaign
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Lowell E Gentry
University of Illinois at Urbana-Champaign
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Michelle Wander
University of Illinois at Urbana Champaign
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Richard Mulvaney
UNIVERSITY OF ILLINOIS URBANA-CHAMPAIGN
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Corey A Mitchell
University of Illinois at Urbana-Champaign
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Carlos E Guacho
University of Illinois at Urbana-Champaign
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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.