A comparison of chamber-based methods for measuring N2O emissions from arable soils

Abstract

Static chamber-based flux measurements with gas chromatography are commonly used to estimate nitrous oxide (N₂O) emissions from arable soils. The LI-COR 7820 N₂O/H₂O (LI-7820) enables higher-frequency in situ measurements, but side-by-side comparisons with traditional methods are limited. To address this gap, we compared non-steady-state chamber methods including non-flow-through (NFT) and flow-through (FT) chamber methods under field and laboratory conditions with plant cover or bare soil. The LI-7820 was used with the LI-8200S smart chamber (FT-1: ⌀ 20 cm) and a self-built chamber (FT-2: 60 × 60 cm), and compared to differently sized NFTs (1–4: 75 × 75, 27 × 37, 60 × 60, and ⌀ 20 cm) with manual sampling with gas chromatography. Field experiments showed high RMSE for daily N₂O fluxes within 20 days after fertilizer application between FT-1 and NFTs, particularly for maize and spring barley (183 and 214 μg N₂O-N m⁻² h⁻¹), which dropped sharply after 20 days (47 and 54 μg N₂O-N m⁻² h⁻¹, respectively). FT-2 and NFT-3 for pastures had lower RMSE and MAE, both below 40 μg N₂O-N m⁻² h⁻¹. In the incubation experiment, bare soil showed smaller error values, remaining below 26 μg N₂O-N m⁻² h⁻¹. Significant differences were observed between the cumulative N₂O emissions measured with NFTs and FT-1, while differences were not significant between NFT-3 and FT-2. Several factors may explain these differences. The smaller chamber dimensions of FT-1 may influence water and nitrogen distribution and constrain the capture of spatial heterogeneity, while NFTs could be affected by prolonged deployment times and in-chamber pressure changes. Furthermore, the lack of water-vapor correction in NFTs, unlike the LI-7820, contributed to discrepancies between methods. Understanding these nuances including the impact of the chamber design, is essential for enhancing the comparability of N₂O emissions and getting closer to achieving unbiased measurements of the true flux.