Deciphering the Isotopic Imprint of Nitrate to Reveal Nitrogen Source and Transport Mechanisms in a Tile‐Drained Agroecosystem

Installation of subsurface drainage systems has profoundly altered the nitrogen cycle in agricultural regions across the globe, facilitating substantial loss of nitrate (NO 3 − ) to surface water systems. Lack of understanding of the sources and processes controlling NO 3 − loss from tile‐drained agroecosystems hinders the development of management strategies aimed at reducing this loss. The natural abundance nitrogen and oxygen isotopes of NO 3 − provide a valuable tool for differentiating nitrogen sources and tracking the biogeochemical transformations acting on NO 3 − . This study combined multi‐years of tile drainage measurements with NO 3 − isotopic analysis to examine NO 3 − source and transport mechanisms in a tile‐drained corn‐soybean field. The tile drainage NO 3 − isotope data were supplemented by characterization of the nitrogen isotopic composition of potential NO 3 − sources (fertilizer, soil nitrogen, and crop biomass) in the field and the oxygen isotopic composition of NO 3 − produced by nitrification in soil incubations. The results show that NO 3 − 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 NO 3 − was temporally stable and similar to those of fertilizer and soybean residue during unfertilized periods. This temporal invariance in NO 3 − isotopic signature indicates a nitrogen legacy effect, possibly resulting from N recycling at the soil microsite scale and a large water storage for NO 3 − mixing. Collectively, these results demonstrate how combining field NO 3 − isotope data with knowledge of isotopic fractionations can reveal mechanisms controlling NO 3 − cycling and transport under complex field conditions.