Physical and isotopic characterization of evaporation from Sphagnum moss

Evaporation from and water transfer within living and dead (but undecomposed) Sphagnum mosses is important biologically and hydrologically, but understanding of the internal mass-transfer mechanisms remains incomplete. A column experiment was conducted to characterize liquid and vapour fluxes and the profiles of relative humidity, temperature and the hydrogen- and oxygen-isotope composition of Sphagnum pore waters evaporating under controlled conditions. A constant water table at 20 cm depth was established in six identical columns fed by a common water reservoir (δ18O = −13.0‰; δ2H = −85.8‰). Evaporation from the columns averaged 4.5 mm d−1 at the average chamber temperature and relative humidity of 20.7 °C and 27.1%, respectively. The columns developed upward-convex profiles of relative humidity and isotopic composition within the first day that persisted throughout the experiment. Isotopic data from columns sampled after 1, 2, 4, 7 and 15 days were strongly constrained by an evaporation line with the linear relation δ2H = 3.8δ18O −36.1 (R2 = 0.99; n = 25), consistent with the expected evaporative-enrichment trajectory under chamber conditions. Calculated vapour flux accounted for only ∼1% of the total mass flux within the columns, reflecting the dominance of liquid-phase capillary flow. While this calculated vapour flux was small, it decreased markedly near the surface, where evaporative cooling may have resulted in condensation of vapour, simultaneously increasing the liquid water content of the surface mosses. The presence of a vapour pressure deficit down to about 15 cm below the surface indicate that both evaporation and upward vapour diffusion were occurring at depth within the Sphagnum columns, but modelling shows that in situ fractionation alone within the columns cannot explain the extent of the observed enrichment. Rather, the enrichment of the heavy isotopes wherever evaporation is occurring and their consequent downward diffusion are needed to explain the observed profiles. Coupled advection–diffusion modelling of these profiles yielded estimates of the effective liquid-phase diffusivities in Sphagnum pore waters of 2.380 (±0.020) × 10−5 cm2 s−1 for 1H1H18O and 2.415 (±0.015) × 10−5 cm2 s−1 for 1H2H16O, in good agreement with accepted values.