Use of δ¹⁵N and δ¹⁸O Values for Nitrate Source Identification under Irrigated Crops: A Cautionary Vadose Zone Tale

Abstract

Source nitrogen (N) identification of leachate or groundwater nitrate is complicated by N source mixing and N and oxygen (O) isotope fractionation caused by microbial N transformations. This experiment examined the δN and δO values in leachate collected over 1 yr at 55 cm below raspberry ( L.) plots receiving either synthetic fertilizer (FT) or poultry manure (MT). The large ranges of δN (FT: -2.4 to +8.7‰, MT: +1.6 to +9.6‰) and δO (FT: -9.9 to -0.3‰, MT: -10.9 to +1.7‰) values in leachate collected under crop rows prohibited the reliable identification of the applied N sources on individual sampling dates. However, the mass-weighted average δN (FT: +3.2‰, MT: +7.3‰) values in leachate were significantly different and can be explained by accounting for the estimated contributions of nitrate and δN values of the various N sources, including applied fertilizer (-0.7‰) or manure (+7.9‰), nitrate-rich irrigation water (+9.0‰), and nitrate from soil N mineralization and nitrification (FT: +3.7‰, MT: +4.6‰; the seasonal timing of which is unknown). This study illustrates the importance of characterizing all major N sources and considering the seasonal variation of these sources and of N cycling processes, as they contribute to the δN values of leachate.

Global patterns of nitrate isotope composition in rivers and adjacent aquifers reveal reactive nitrogen cascading

“…Shallow phreatic aquifers showed strongly dampened seasonal nitrate and isotopic variability compared to river waters. Given NO 3 − concentrations are generally buffered in aquifers over longer timeframes (e.g., years), the range of δ 15 N values in groundwater reflect longer-term climatic and land-use changes or established processes taking place in the soil and the unsaturated zone before NO 3 − leaches into the groundwater system 30,61 . The lack of a clear seasonal variation of δ 15 N and δ 18 O of NO 3 − in groundwater is attributed to nitrifying and mixing of nitrate sources introduced at different points or periods in time due to variable residence times (from months to years) in the unsaturated zone 38,62 .…”

Nitrogen, irrigation, and alley management effects on nitrate leaching from raspberry

“…This was achieved by measuring leachate under the raspberry row and alley using PCAPS over a 4-yr period including the establishment year (April 2009 to March 2010) and three cropping years (April 2010 to March 2013). Leachate NO 3 concentration and NO 3 isotope composition for two treatments on selected sampling dates over a 1-yr period from this study have been previously reported (Loo et al, 2017).…”

Nitrate pollution source apportionment, uncertainty and sensitivity analysis across a rural-urban river network based on δ15N/δ18O-NO3− isotopes and SIAR modeling

“…In this complex watershed including soil, farmland, wetlands and riverine transformations of N, the biological processes driving N cycling were deemed sufficient to initiate significant isotopic fractionation (Denk et al, 2017;Yu et al, 2020). Thus, we incorporated 15 N-fractionation factors for atmospheric deposition nitrate (0‰), nitrogen fertilizer (3.9 ± 2.3‰), soil nitrogen (− 4.1 ± 1.4‰) and municipal sewage (− 0.6 ± 1.2‰) during the SIAR modeling process (Ostrom et al, 1998;Loo et al, 2017;Zhang et al, 2018;Ren et al, 2022).…”