Water table depth (WTD) effects on net ecosystem CO2 exchange of boreal
peatlands are largely mediated by hydrological effects on peat
biogeochemistry and the ecophysiology of peatland vegetation. The lack of
representation of these effects in carbon models currently limits our
predictive capacity for changes in boreal peatland carbon deposits under
potential future drier and warmer climates. We examined whether a
process-level coupling of a prognostic WTD with (1) oxygen transport, which
controls energy yields from microbial and root oxidation–reduction
reactions, and (2) vascular and nonvascular plant water relations could
explain mechanisms that control variations in net CO2 exchange of a
boreal fen under contrasting WTD conditions, i.e., shallow vs. deep WTD. Such
coupling of eco-hydrology and biogeochemistry algorithms in a process-based
ecosystem model, ecosys, was tested against net ecosystem CO2 exchange
measurements in a western Canadian boreal fen peatland over a period of
drier-weather-driven gradual WTD drawdown. A May–October WTD drawdown of
∼ 0.25 m from 2004 to 2009 hastened oxygen transport to
microbial and root surfaces, enabling greater microbial and root energy
yields and peat and litter decomposition, which raised modeled 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 modeled
vascular gross primary productivity (GPP) and plant growth. The increase in
modeled vascular GPP exceeded declines in modeled nonvascular (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
modeled 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 caused no
significant WTD effects on modeled seasonal and interannual variations in
net ecosystem productivity (NEP). These modeled trends were corroborated
well by eddy covariance measured hourly net CO2 fluxes (modeled vs. measured: R2 ∼ 0.8, slopes ∼ 1 ± 0.1,
intercepts ∼ 0.05 µmol m−2 s−1), hourly
measured automated chamber net CO2 fluxes (modeled vs. measured:
R2 ∼ 0.7, slopes ∼ 1 ± 0.1,
intercepts ∼ 0.4 µmol m−2 s−1), and other
biometric and laboratory measurements. Modeled drainage as an analog for
WTD drawdown induced by climate-change-driven drying 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 the water table deepen by a further ∼ 0.5 m.
This decline in projected NEP indicated that a further WTD drawdown at this
fen would eventually lead to a decline in GPP due to water limitation.
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