Optimized design of flux chambers for online measurement of NH3 emission after field application of slurry with full-scale farm machinery

Liquid animal manure (slurry) can be utilized as a valuable nutrient source for crop production but is also a significant source of ammonia (NH₃) emissions, which negatively affect the environment and human health.

Emission depends on several factors, including application technique, climatic conditions, and slurry and soil properties. Despite increased knowledge there are still important effects and interactions that are not quantitatively understood. Reliable emission measurements and assessment of new low-emission application technologies are needed for emission inventories and research aimed at reducing emission.

Different methods can be used to measure NH₃ emission after field application of slurry and can roughly be sorted into two categories: micrometeorological and enclosure methods. Micrometeorological methods yield accurate flux measurements and slurry can be applied by full-scale farm machinery, but replication is impractical and usually omitted, making statistical comparisons difficult. In contrast enclosure methods, such as dynamic chambers, only require a small plot area, making replication possible. However, most dynamic chamber designs can only be used with manual application of the slurry, which is not always representative of application of full-scale machinery, especially when there is an interaction between the slurry application aggregate and the soil.  

A new design of dynamic chambers enabling application of slurry both manually and by full-scale farm machinery has been developed. The dynamic chambers are connected to a cavity ring-down spectrometer for online measurements of NH₃, allowing for high time-resolution measurements with a low detection limit, high recovery, and relatively low variation among chambers.

Several configurations of the design were investigated in silico with computational fluid dynamic (CFD) in order to optimize airflow through the chamber and the configuration to ensure turbulence homogeneity of the emitting surface and sufficient mixing of the air within the chamber.

The new dynamic chambers were tested in three field trials where a different method of applying the slurry was used in each trial: manual application, application with a field trial system with 3-m slurry boom, and application by a 30-m farm-scale slurry boom. Statistical power analysis was performed to estimate the number of replicates required for detecting several effect sizes for each application method. Furthermore, the flux measurements from the new chambers will be compared with wind tunnel measurements (manual application) and the backward Lagrangian Stochastic (bLS) dispersion technique (3-m and 30-m boom application).

This new design allows for representative measurements of NH₃ flux after application of slurry in the field, making it possible to statistically assess the effect of individual variables affecting flux dynamics and thereby further increase knowledge on NH₃ emissions mitigation options.