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Oxygen isotope content of CO 2 in nocturnal ecosystem respiration: 1. Observations in forests along a precipitation transect in Oregon, USA
Author(s) -
Bowling D. R.,
McDowell N. G.,
Welker J. M.,
Bond B. J.,
Law B. E.,
Ehleringer J. R.
Publication year - 2003
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/2003gb002081
Subject(s) - transect , precipitation , isotopes of oxygen , environmental science , stable isotope ratio , δ18o , ecosystem respiration , isotope analysis , atmospheric sciences , environmental chemistry , ecosystem , hydrology (agriculture) , ecology , chemistry , geology , biology , primary production , physics , geotechnical engineering , quantum mechanics , nuclear chemistry , meteorology
The oxygen isotope content of nocturnal ecosystem respiration (δ 18 O R ) was examined in forests along a precipitation gradient in Oregon, USA, to determine whether site‐to‐site variation in δ 18 O R was more strongly related to variation in δ 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 (δ 18 O) of precipitation, with inland sites receiving isotopically depleted precipitation (more negative δ 18 O) relative to coastal sites. The δ 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 CO 2 , leading to variation in observed δ 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 δ 18 O R . The range of observed δ 18 O R was 21.7 to 35.3‰ (VSMOW), with variation within a single site as large as 10.7‰ (range 24.2 to 34.9‰ at different sites). The results suggested a trend of more positive δ 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 δ 18 O R .