Use of δ 15 N and δ 18 O Values for Nitrate Source Identification under Irrigated Crops: A Cautionary Vadose Zone Tale

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 δ¹⁵NNO₃ and δ¹⁸ONO₃ values in leachate collected over 1 yr at 55 cm below raspberry (Rubus idaeus L.) plots receiving either synthetic fertilizer (FT) or poultry manure (MT). The large ranges of δ¹⁵NNO₃ (FT: −2.4 to +8.7‰, MT: +1.6 to +9.6‰) and δ¹⁸ONO₃ (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 δ¹⁵NNO₃ (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 δ¹⁵NNO₃ 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 δ¹⁵NNO₃ values of leachate.