Soil greenhouse gas emissions from drained and rewetted agricultural bare peat mesocosms are linked to geochemistry.

In view of climate considerations regarding the management of peatlands, there is a need to assess whether rewetting can mitigate greenhouse gas (GHG) emissions, and notably how site-specific soil-geochemistry will influence differences in emission magnitudes. However, there are inconsistent results regarding the correlation of soil properties with heterotrophic respiration (R) of carbon dioxide (CO), methane (CH), and nitrous oxide (NO) from bare peat. In this study, we determined 1) soil-, and site-specific geochemical components as drivers for emissions from R on five Danish fens and bogs, and 2) emission magnitudes under drained and rewetted conditions. For this, a mesocosm experiment was performed under equal exposure to climatic conditions and water table depths controlled to either -40 cm, or -5 cm. For the drained soils, we found that annual cumulative emissions, accounting for all three gases, were dominated by CO, contributing with, on average, 99 % to a varying global warming potential (GWP) of 12.2-16.9 t COeq ha yr. Rewetting lowered annual cumulative emissions from R by 3.2-5.1 t COeq ha yr for fens and bogs, respectively, despite a high variability of site-specific CH emissions, contributing with 0.3-3.4 t CO ha yr to the GWP. Overall, analyses using generalized additive models (GAM) showed that emission magnitudes were well explained by geochemical variables. Under drained conditions, significant soil-specific predictor variables for CO flux magnitudes were pH, phosphorus (P), and the soil substrate's relative water holding capacity (WHC). When rewetted, CO and CH emissions from R were affected by pH, WHC, as well as contents of P, total carbon and nitrogen. In conclusion, our results found the highest GHG reduction on fen peatlands, further highlighting that peat nutrient status and acidity, and the potential availability of alternative electron acceptors, might be used as proxies for prioritising peatland areas for GHG mitigation efforts by rewetting.