Deciphering the isotopic imprint of nitrate to reveal nitrogen source
and transport mechanisms in a tile-drained agroecosystem
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.