Exploring oxygen intrusion as explanation for observed emission differences in Dutch peatlands.

Many countries in the world are trying to reduce their carbon emissions to minimize climate change, as described in the Paris Agreement of 2015. Alongside well-known, worldwide sectors contributing to emissions such as transport, industry and mining, drained peatlands can substantially contribute to national emission such as in The Netherlands. The organic matter in peat is susceptible to decomposition; the breakdown of organic tissue and transformation into carbon dioxide, methane and N₂O gas by microbes. Oxic decomposition with oxygen from the air occurs above the groundwater table, while anoxic decomposition occurs in the absence of oxygen above and below the groundwater table. Oxic decomposition is the more optimal method of organic matter breakdown, resulting in faster decomposition rates and higher emissions. To evaluate if yearly emission reduction targets are achieved, models are used to estimate the amount of carbon emissions originating from peat meadow areas. Although many types of models with a wide range of complexities are used, a key element is usually that the groundwater level determines the boundary between the oxic and anoxic decomposition zones. Therefore, a thicker oxic zone will result in more emissions if the other conditions remain stable as more organic matter is exposed to oxygen. However, oxygen intrudes the soil by diffusion and advection. The oxygen intrusion takes time and is limited by pore connectivity and temperature, which vary with depth. Thus, oxygen intrusion on a yearly timescale is likely not linear with the yearly average groundwater level, but depends on the peat type, peat profile and temperature conditions. The model used to estimate the Dutch national carbon emissions applies a continuous linear increase of carbon emissions with decreasing groundwater levels that fits with the Dutch observed emissions. However, countries like Germany, and Denmark favor an S-shaped relationship that describes maximum emissions below a certain groundwater depth.

 

This research uses a dedicated process model to further investigate this relationship between groundwater level and emissions. Specifically, we look at the relationship between oxygen intrusion into the peat soil and carbon emissions. We use a model that is calibrated for a drained peatland under intensive agricultural use. We calibrated on observed CO₂ emissions and soil redox conditions. Subsequently, we vary the oxygen diffusion coefficient in the model, which is likely to depend on decomposition degree, clay fraction and peat compaction. We find that in simulations with limited oxygen intrusion deeper summer groundwater tables do not result in equally more oxygen in the soil. Oxygen intrusion trails the groundwater depth, and this intrusion delay is explained by diffusion limitations and oxygen consumption by microbes near the surface. This limits the potential for decomposition at larger depths during deep groundwater tables. Consequently, we find that the increase of emissions with decreasing yearly average groundwater levels follows the well-known S-curve for peatland with a low oxygen diffusion into the soil or deep groundwater levels. This result is an important step in our understanding of observed emissions for different peat types and groundwater management strategies.