Use of δ15N and δ18O 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 delta N-15(NO3) and delta O-18(NO3) 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 delta N-15(NO3) (FT: -2.4 to + 8.7%, MT: + 1.6 to + 9.6%) and delta O-18(NO3) (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 delta N-15(NO3) (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 delta N-15(NO3) 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 delta N-15(NO3) values of leachate.