A Portable Laser Spectroscopic System for Measuring Nitrous Oxide Emissions on Fertilized Cropland

Nitrous oxide (laughing gas, N₂O) is a relevant greenhouse gas. Agriculture contributes significantly to its emissions. As nitrogen fertilization has been identified as one of the main sources of N₂O, controlled application and reduction of the amount of fertilizer adapted to crop demand is essential to reduce N₂O emissions. This requires detailed studies of the local distribution of the N₂O emission fluxes on different croplands. Consequently, frequent spatially resolved field measurements of N₂O concentrations are needed. A precision in the ppb range close to the ambient N₂O level of 333 ppb is necessary. Tunable laser absorption spectroscopy using quantum-cascade lasers (QCL) as a light source is an established technique for the measurement of N₂O traces. We present the development and validation of a compact portable setup for on-site measurement of N₂O emissions from the soil. The setup differs from previous solutions by using an interband cascade laser (ICL), which has significantly lower power consumption compared to a QCL. The portable measurement setup allows N₂O emission fluxes to be determined with a precision of 3.5% with a measuring duration of 10 min. The developed system enables the detection of increased N₂O emissions because of the fertilization of fields. High N₂O emission fluxes are indicators of the overfertilization of the field. Directly after fertilization, N₂O fluxes between 2.9 and 5.3 µL m^-2 min^-1 depending on the gas acquisition site are measured during the field tests. Over time, the fluxes decrease. The obtained results compare well with data from more precise but also more complex and maintenance-intensive instruments for atmospheric research. With this system, the soil moisture as well as the air humidity and air temperature are recorded. Strong influences on N₂O fluxes by soil moisture were observed. The presented measurement system is a contribution to the establishment of mobile N₂O screening systems that are robust in the field and suitable for comprehensive and routine detection of N₂O emissions from soil.