Oxygen isotope content of CO2 in nocturnal ecosystem respiration:: 1.: Observations in forests along a precipitation transect in Oregon, USA -: art. no. 1120

The oxygen isotope content of nocturnal ecosystem respiration (delta(18)O(R)) was examined in forests along a precipitation gradient in Oregon, USA, to determine whether site-to-site variation in delta(18)O(R) was more strongly related to variation in delta(18)O of precipitation or to evaporative processes that isotopically modify water pools within ecosystems. Measurements were made over 4 years at sites ranging in mean annual precipitation from 227 to 2760 mm. There was a gradient in the isotopic content (delta(18)O) of precipitation, with inland sites receiving isotopically depleted precipitation (more negative d18O) relative to coastal sites. The delta(18)O of water in plant xylem generally followed the isotopic pattern of precipitation. Inland forests were drier than coastal forests, leading to a gradient in the vapor pressure deficit of air that caused isotopic enrichment of soil and leaf water. The enriched soil and leaf water pools influenced the isotopic composition of respired CO2, leading to variation in observed delta(18)O(R) (Keeling-plot intercepts). Keeling plots with non-significant (p > 0.01) regression slopes and those sampled over a time period (t) greater than 5 hours yielded unacceptably high uncertainty in delta(18)O(R). The range of observed delta(18)O(R) was 21.7 to 35.3parts per thousand (VSMOW), with variation within a single site as large as 10.7parts per thousand (range 24.2 to 34.9parts per thousand at different sites). The results suggested a trend of more positive delta(18)O(R) at inland sites relative to those nearer the coast, indicating that fractionation due to evaporative enrichment overshadowed the original isotopic composition of precipitation as a first order control on delta(18)O(R).