The detectability of nitrous oxide mitigation efficacy in intensively grazed pastures using a multiple-plot micrometeorological technique

Abstract. Methodologies are required to verify agricultural greenhouse gas mitigation at scales relevant to farm management. Micrometeorological techniques provide a viable approach for comparing fluxes between fields receiving mitigation treatments and control fields. However, they have rarely been applied to spatially verifying treatments aimed at mitigating nitrous oxide emission from intensively grazed pastoral systems. We deployed a micrometeorological system to compare N₂O flux among several ~1.5 ha plots in intensively grazed dairy pasture. The sample collection and measurement system is referred to as the Field-Scale Nitrous Oxide Mitigation Assessment System (FS-NOMAS) and used a tuneable diode laser absorption spectrometer to measure N₂O gradients to high precision at four locations along a 300 m transect. The utility of the FS-NOMAS to assess mitigation efficacy depends largely on its ability to resolve very small vertical N₂O gradients. The performance of the FS-NOMAS was assessed in this respect in laboratory and field-based studies. The FS-NOMAS could reliably resolve gradients of 0.039 ppb between a height of 0.5 and 1.0 m. The gradient resolution achieved corresponded to the ability to detect an inter-plot N₂O flux difference of 26 μg N₂O–N m⁻² h⁻¹ under the most commonly encountered conditions of atmospheric mixing (quantified here by a turbulent transfer coefficient), but this ranged from 11 to 59 μg N₂O–N m⁻² h⁻¹ as the transfer coefficient ranged between its 5th and 95th percentile. Assuming a likely value of 100 μg N₂O–N m⁻² h⁻¹ for post-grazing N₂O fluxes from intensively grazed New Zealand dairy pasture, the system described here would be capable of detecting a mitigation efficacy of 26% for a single (40 min) comparison. We demonstrate that the system has considerably greater sensitivity to treatment effects by measuring cumulative fluxes over extended periods.