Soil N2O emissions: how much does soil structure matter?

Nitrous oxide (N₂O) is a greenhouse gas almost 300 times more powerful than CO₂ in terms of global warming potential, and it is also the first ozone-depleting substance emitted in the 21st century. Approximately 43% of N₂O emissions are estimated to be due to anthropogenic activities worldwide, and 52% of this anthropogenic part come from cultivated soils. The main cause of anthropogenic emissions is nitrogen fertilization.

The production, transfer and emission of N₂O from soils are complex multifactorial processes, with a high spatial and temporal variability. Although N₂O production in soils has multiple origins, the main source remains denitrification reactions during microbial respiration under anaerobic conditions. Thus, one of the major soil control factors is the availability of oxygen to soil organisms, which partly depends on the soil structure. The spatiotemporal variability of N₂O emissions is explored by deterministic studies that focus either on the soil microstructure scale, i.e. the scale of N₂O production and microorganism habitat, or on the macrostructure scale, to focus on fluids transfers. However, the influence of soil micro- and macrostructure studied together on N₂O emissions is still poorly known, and represents the objective of this work.

A multi-scale approach was adopted to better understand the determinism of N₂O emissions. The spatial variability of N₂O emissions at the field scale was estimated during a snap-shot campaign on the same soil type with contrasted structural states, induced by different agricultural practices (4 soil modalities crossing strip-till and tillage with compacted or uncompacted areas). 24 soil cylinders were collected in low and high N₂O emission zones and were then scanned by using both X-ray macro- and micro-tomography. Quantitative morphological tools were used to describe soil structure at the macro and micro scales while simultaneously studying other soil properties influencing N₂O emissions (air permeability, gas diffusivity, nitrogen, pH, soil texture, etc.). The 4 soil modalities studied showed contrasted N₂O emissions along with contrasting macrostructural and gas transfer indices. The ongoing work is aimed at clarifying the relationships between multiscale soil structure, gas transfer and other soil factors on N₂O emissions.