Coupled eco-hydrology and biogeochemistry algorithms enable simulation of water table depth effects on boreal peatland net CO ₂ exchange

Abstract. Water table depth (WTD) effects on net ecosystem CO2 exchange of boreal peatlands are largely mediated by hydrological effects on peat biogeochemistry, and eco-physiology of peatland vegetation. Lack of representation of these effects in carbon models currently limits our predictive capacity for changes in boreal peatland carbon deposits under future drier and warmer climates. We therefore tested whether the effects of WTD variation on net ecosystem CO2 exchange of a Western Canadian boreal fen peatland could be modelled through a process-level coupling of a prognostic WTD dynamic, which arises from equilibrium between vertical and lateral water fluxes, with oxygen transport, which controls energy yields from microbial and root oxidation–reduction reactions, and vascular and non-vascular plant water relations in an ecosystem model ecosys. Ecosys successfully simulated a May–October WTD drawdown by ~ 0.25 m measured in the fen from 2004 to 2008, which was attributed to reduced precipitation relative to evapotranspiration, and reduced lateral recharge relative to discharge. This WTD drawdown hastened oxygen transport to microbial and root surfaces, enabling greater microbial and root energy yields, and peat and litter decomposition, which raised modelled ecosystem respiration (Re) by ~ 0.26 μmol CO2 m−2 s−1 per 0.1 m of WTD drawdown. It also augmented nutrient mineralization, and hence root nutrient availability and uptake, which resulted in improved leaf nutrient (nitrogen) status that facilitated carboxylation, and raised modelled vascular gross primary productivity (GPP) and plant growth. The increase in modelled vascular GPP exceeded declines in modelled non-vascular (moss) GPP due to greater shading from increased vascular plant growth, and moss drying from near surface peat desiccation, thereby causing a net increase in modelled growing season GPP by ~ 0.39 μmol CO2 m−2 s−1 per 0.1 m of WTD drawdown. Similar increases in GPP and Re left no significant WTD effects on modelled seasonal and interannual variations in net ecosystem productivity (NEP). These modelled trends were corroborated against eddy covariance hourly net CO2 fluxes (modelled vs. measured: R2 ~ 0.8, slopes ~ 1 ± 0.1, intercepts ~ 0.05 μmol m−2 s−1), and against other automated chamber, biometric, and laboratory measurements. Modelled drainage as an analog for climate change showed that this boreal peatland would switch from a large carbon sink (NEP ~ 160 g C m−2 yr−1) to carbon neutrality (NEP ~ 10 g C m−2 yr−1) should water table deepened by a further ~ 0.5 m. Therefore, representing the effects of interactions among hydrology, biogeochemistry and plant physiological ecology on ecosystem carbon, water, and nutrient cycling in global carbon models would improve our predictive capacity for changes in boreal peatland carbon sequestration under changing climates.

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