Assessing the mitigation of peat oxidation in Frisian managed pastures using mobile eddy covariance systems

Most low-level peatlands in The Netherlands have been converted to pastures for dairy-production as early as 500 years ago, leading to drainage, peat oxidation, soil subsidence and CO₂ emissions. Climate policies prescribe drastic mitigation of these emissions, while cultural and economic interests of dairy production cannot be ignored either. This leads to proposals for often highly technical measures elevating the groundwater table or otherwise limiting oxidation while maintaining the productivity of the land. In the northern Fryslân Province drainage has traditionally been deeper than in the Western Netherlands. The province and its peat meadow programme is actively assessing the effectiveness of a range of measures, including sub-surface (drain) and surface (furrow) irrigation, dynamic ditch levels and flooding as well as soil manipulation techniques.

Wetterskip Fryslân and Wageningen University since 2021 have been jointly monitoring greenhouse gas emissions (CO₂ and CH₄) from a selection of up to 16 pastures implementing these measures (treatment and control), using a set of four roving (mobile) eddy covariance (EC)systems but maintaining fixed environment monitoring in each site. This yields discontinuous data sets spread of the years, which we completed to annual series and annual Net Ecosystem Carbon Budgets (NECB, or NBP) using advanced machine learning techniques completed with harvest and manure data. 

The analysis yields consistent time series with quantifiable uncertainty. However, in most cases the effectiveness of the mitigation methods could not be demonstrated. Apart from methodological considerations, this indicates important secondary factors affecting the emissions, including cattle management, soil clay content, etc.

The relationship with ground water table is also not significant among these sites. If alternative ground water metrics are considered, however, explanatory power improves. Air filled pore space was calculated from soil moisture profiles and is a better predictor than groundwater table, while the depth of ground water below a top clay layer also has explanatory power. Finally, we explored an interesting delayed effect of ground water on emissions.  

One measure that does seem consistently effective is surface (trench) irrigation. Dynamic ditch level management seems to lead to higher, not lower emissions, while in some cases we find consistent carbon uptake in a pasture. Cropping on peat soils is clearly unfavourable, no matter the mitigation measure. Comparison with national-scale emission reporting models shows that our measurements are showing similar uncertainties. All in all, the mobile EC approach proves to be a powerful tool to assess real-world effectiveness of mitigation measures while it also confronts policy makers with the often tough reality of scientifically underpinning mitigation measures.