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_h) of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) from bare peat. In this study, we determined 1) soil-, and site-specific geochemical components as drivers for emissions from R_h 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 CO₂, contributing with, on average, 99 % to a varying global warming potential (GWP) of 12.2–16.9 t CO₂eq ha⁻¹ yr⁻¹. Rewetting lowered annual cumulative emissions from R_h by 3.2–5.1 t CO₂eq ha⁻¹ yr⁻¹ for fens and bogs, respectively, despite a high variability of site-specific CH₄ emissions, contributing with 0.3–3.4 t CO₂ ha⁻¹ yr⁻¹ 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 CO₂ flux magnitudes were pH, phosphorus (P), and the soil substrate’s relative water holding capacity (WHC). When rewetted, CO₂ and CH₄ emissions from R_h 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.

This publication is supported by the WET HORIZONS project.