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.