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Water table depth (WTD) effects on net ecosystem CO2 exchange of boreal\udpeatlands are largely mediated by hydrological effects on peat\udbiogeochemistry and the ecophysiology of peatland vegetation. The lack of\udrepresentation of these effects in carbon models currently limits our\udpredictive capacity for changes in boreal peatland carbon deposits under\udpotential future drier and warmer climates. We examined whether a\udprocess-level coupling of a prognostic WTD with (1) oxygen transport, which\udcontrols energy yields from microbial and root oxidation–reduction\udreactions, and (2) vascular and nonvascular plant water relations could\udexplain mechanisms that control variations in net CO2 exchange of a\udboreal fen under contrasting WTD conditions, i.e., shallow vs. deep WTD. Such\udcoupling of eco-hydrology and biogeochemistry algorithms in a process-based\udecosystem model, ecosys, was tested against net ecosystem CO2 exchange\udmeasurements in a western Canadian boreal fen peatland over a period of\uddrier-weather-driven gradual WTD drawdown. A May–October WTD drawdown of\ud ∼  0.25 m from 2004 to 2009 hastened oxygen transport to\udmicrobial and root surfaces, enabling greater microbial and root energy\udyields and peat and litter decomposition, which raised modeled ecosystem\udrespiration (Re) by 0.26 µmol CO2 m−2 s−1 per\ud0.1 m of WTD drawdown. It also augmented nutrient mineralization, and hence root\udnutrient availability and uptake, which resulted in improved leaf nutrient\ud(nitrogen) status that facilitated carboxylation and raised modeled\udvascular gross primary productivity (GPP) and plant growth. The increase in\udmodeled vascular GPP exceeded declines in modeled nonvascular (moss) GPP\uddue to greater shading from increased vascular plant growth and moss drying\udfrom near-surface peat desiccation, thereby causing a net increase in\udmodeled growing season GPP by 0.39 µmol CO2 m−2 s−1\udper 0.1 m of WTD drawdown. Similar increases in GPP and Re caused no\udsignificant WTD effects on modeled seasonal and interannual variations in\udnet ecosystem productivity (NEP). These modeled trends were corroborated\udwell by eddy covariance measured hourly net CO2 fluxes (modeled vs. measured: R2  ∼  0.8, slopes  ∼ 1 ± 0.1,\udintercepts  ∼ 0.05 µmol m−2 s−1), hourly\udmeasured automated chamber net CO2 fluxes (modeled vs. measured:\udR2  ∼ 0.7, slopes  ∼ 1 ± 0.1,\udintercepts  ∼ 0.4 µmol m−2 s−1), and other\udbiometric and laboratory measurements. Modeled drainage as an analog for\udWTD drawdown induced by climate-change-driven drying showed that this boreal\udpeatland 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)\udshould the water table deepen by a further  ∼ 0.5 m.\udThis decline in projected NEP indicated that a further WTD drawdown at this\udfen would eventually lead to a decline in GPP due to water limitation.\udTherefore, representing the effects of interactions among hydrology,\udbiogeochemistry and plant physiological ecology on ecosystem carbon, water,\udand nutrient cycling in global carbon models would improve our predictive\udcapacity for changes in boreal peatland carbon sequestration under changing\udclimates

biogeosciences

Coupled eco-hydrology and biogeochemistry algorithms enable the simulation of water table depth effects on boreal peatland net CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; exchange