Ammonia measurements over multiple fertilisation campaigns using eddy covariance and the integrated horizontal flux method

Abstract. Quantifying field-scale ammonia emissions following nitrogen fertilisation remains challenging, and estimates may differ substantially between measurement methods. We compared quantum cascade laser-based eddy covariance (QCL-EC) with the integrated horizontal flux method using Adapted Low-cost Passive High Absorption (ALPHA) samplers (ALPHA-IHF) during eight urea fertilisation campaigns in winter wheat (2021–2023).

Both approaches identified post-fertilisation emission periods and captured similar dynamics, but paired interval estimates differed. Only 62% of intervals yielded ALPHA-IHF flux estimates, mainly because irregular or weak concentration profiles prevented robust calculation. QCL and ALPHA concentrations were positively correlated, but QCL concentrations were, on average, 31% higher, whereas QCL-EC emission estimates were 23% lower than ALPHA-IHF estimates. Median campaign emission factors were 1.6% for QCL-EC and 3.2% for ALPHA-IHF, respectively. QCL-based estimates were more precise, and differences between the methods varied with short-term ammonia concentration variability, air temperature, friction velocity and ALPHA profile quality.

The contrasting results show that concentration-level patterns do not translate directly into flux estimates and that differences between methods vary with field conditions. QCL-EC resolved short-term net exchange but required turbulence-based quality filtering, whereas ALPHA-IHF yielded time-integrated emission estimates only when the fitted concentration profile met the quality criteria. Without an independent reference, systematic over- or underestimation could not be assigned to either method. Assessments of fertiliser-induced emissions must consider method-specific data coverage and differences in flux representation. The greater precision of QCL-based estimates and high temporal resolution of QCL-EC make it promising for detecting subtle emission differences under low-emission conditions, while improved concentration precision and resolution of weak gradients could broaden ALPHA-IHF applicability.