High contributions of anaerobic decomposition to greenhouse gas emissions of agriculturally used peatlands

Globally, peatlands store one third of global soil carbon. Peatlands accumulate carbon under waterlogged anoxic conditions, but current drainage increases oxygen availability enhancing degradation of these carbon reserves. Therefore, drainage is responsible for ∼ 2 % of anthropogenic greenhouse gas (GHG) emissions. GHG emission estimates from drained peatlands are often based on hydrological proxies, but these methods are known to result in consistent inaccuracies. In this research, we propose to improve these estimates by using the redox potential that controls peat degradation more directly as compared to hydrological proxies. We aimed to quantify in-situ (net) soil production rates of CO₂ and CH₄ by combining in-situ redox potential measurements with corresponding laboratory basal respiration rates scaled to in-situ soil temperature. Using this approach, we estimated soil CO₂ and net CH₄ production rates at 12 field sites over multiple years and validated these estimates by comparing them to aboveground Net Ecosystem Carbon Balance (NECB) measurements using continuously operating chambers (for CO₂) and eddy covariance measurements (for CH₄) over the same sites and timeframes. We hypothesized that (1) laboratory incubation measurements can serve as a basis to estimate field-scale CO₂ and CH₄ emissions, (2) compared to water table depth, the redox potential is a more reliable parameter for estimating soil CO₂ production, and (3) anaerobic respiration processes contribute substantially to peat decomposition and soil CO₂ production. Averaged soil production estimates over multipole years for all sites of CO₂ showed strong agreement with measured NECBs (concordance correlation coefficient, CCC = 0.80) and net soil production estimates of CH₄ showed moderately strong agreement (CCC = 0.65) with CH₄ emissions. Using water table depth instead of soil redox condition to calculate soil CO₂ production rates resulted in a very low agreement with measured NECBs (CCC = 0.08) due to overestimation of the prevalence of oxic conditions. Shorter term comparisons generally resulted in lower CCC values, likely due to (bio)chemical legacy effects that balanced out over longer timescales. Anaerobic respiration processes accounted for 68 % of total soil CO₂ production over all sites, with 61 % originating from soil layers that were exposed to oxygen within the past 1.5 years, also likely influenced by biological and chemical legacy effects. By bridging the gap between laboratory and field-scale, our approach provides a valuable tool for assessing GHG emissions from drained peatlands and enhances our understanding of aerobic and anaerobic peat decomposition processes.