Underestimation of Anaerobic Decomposition Rates in Sphagnum Litterbag Experiments by the Holocene Peatland Model Depends on Initial Leaching Losses

Abstract. The Holocene Peatland Model (HPM) is a widely applied model to understand and predict long-term peat accumulation. Here, we test whether the HPM can predict decomposition of available Sphagnum litterbag data along a gradient from oxic to anoxic conditionsand estimate parameter values from the litterbag data.

Large uncertainties in available litterbag data allow predictions of the HPM to fit decomposition rates estimated from litterbags by adjusting initial leaching losses and decomposition rates estimated from the litterbag data within the range of their uncertainties. Specifically, with standard parameter values, the HPM assumes larger initial leaching losses and smaller decomposition rates than estimated from the litterbag data alone. Therefore, improved tests of the HPM rely on future litterbag experiments that allow a more accurate estimation of initial leaching losses and decomposition rates.

When estimating HPM parameters from the litterbag data and assuming smaller initial leaching losses, our analysis indicates that the HPM with standard parameter values underestimates anaerobic decomposition rates for several species and assumes a too steep decrease of decomposition rates from oxic to anoxic conditions. This may be caused by not considering effects of water table fluctuations on aerobic and anaerobic decomposition rates.

Whether the discrepancies are reproducible and the estimated parameter values may be an easy fix to account for effects of water table fluctuations in long-term predictions needs further investigation. Based on previous sensitivity analyses of the HPM, the updated parameter estimates can cause differences in predicted 5000 year C accumulation up to 100 kg m⁻².