Shifting consensus in moisture modifier of decomposition towards the optimum in well-drained mineral soils instead of mid- or high- moisture levels of organic soils in boreal forest

The lack of consensus of functional dependency of soil respiration on moisture among the Earth system models (ESMs) contributes significantly to uncertainties in their projections.

Based on data of soil organic C stocks and CO₂ emissions from the boreal ecotone between mineral soil forests and adjacent peatlands with organic soils in Finland, we derived the field-based moisture response of respiration in a maximum range of moisture conditions (extending from xeric and mesic forests to water saturated mires). Using Bayesian data assimilation technique, we coupled Yasso07 soil carbon model with the heuristic bell shape moisture function, approximating the enzyme and oxygen limitations. As expected, the Yasso07 model fitted with the revised moisture modifier on data from catena of organic-mineral soils outperformed the previous model version in peatlands.

Unlike the most found optimum of decomposition in ESM in mid- or high- moisture levels, our optimum or the highest rate of decomposition correspond to well-drained conditions of mineral soils.

We speculated that the reason for the shift in the moisture optimum of the functional form was its accounting for long-term processes leading to a larger C mineralization in mineral soils related to extreme events, such as prolonged elevated moisture or rewetting after droughts, which enhance microbial access to previously protected or labile C pools and may not be detected in short-term incubation studies.

Although, the moisture modifier derived here improved the match between the modelled and measured SOCs of peatlands, a shift in consensus from current decomposition rate modifiers used in ESMs requires further evaluation before it can be largely applied for the landscape level semiempirical processed-based modelling of the mineral and organic soil C stocks and CO₂ emissions.