Probing the biological sources of soil N2O emissions by quantum cascade laser-based 15N isotopocule analysis

Isotopocule analysis by quantum cascade laser spectrometry (QCL) is a promising approach for in situ, real-time tracking of the biological sources of soil N2O emissions. However, background atmospheric N2O is an important source of variability in the measurement of isotopocule ratios (i.e., 14N15N16O/14N14N16O and 15N14N16O/14N14N16O) of gas samples. Here, a method based on Keeling plot for determining the intramolecular 15N distribution in N2O is introduced. The sensitivity and reliability of this method are examined against N2O of known 15N site preference (SP), and N2O produced from fungal and bacterial isolates, soils with different moisture contents and organic amendments, and a soil chamber under field conditions. The isotopocules of N2O determined by standard gases supported that the Keeling plot method was more reliable than the averaging method. Using this method, SP of N2O was greater in fungal than bacterial denitrifiers, as well as in soil at 60% water filled pore space (WFPS) than 80-100% WFPS. This method also determined that the SP of N2O was distinct between soils of different substrate complexity. Further, we observed a N2O SP between −18.9‰ and 2.2% in a soil flux chamber deployed in a corn field after 2 d of rainfalls that is consistent with the SP of N2O produced from bacterial denitrification and nitrifier denitrification. Our data demonstrate that this Keeling plot method provides accurate discrimination of biological sources when N2O is analyzed by the QCL system.