Oxygen isotope content of CO₂ in nocturnal ecosystem respiration: 1. Observations in forests along a precipitation transect in Oregon, USA

Abstract

[1] The oxygen isotope content of nocturnal ecosystem respiration (d 18 O R ) was examined in forests along a precipitation gradient in Oregon, USA, to determine whether site-to-site variation in d18 O R was more strongly related to variation in d 18O 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 (d 18 O) of precipitation, with inland sites receiving isotopically depleted precipitation (more negative d 18O) relative to coastal sites. The d18 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 d 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 d 18 O R .

Oxygen isotope content of CO2 in nocturnal ecosystem respiration: 2. Short‐term dynamics of foliar and soil component fluxes in an old‐growth ponderosa pine forest

“…[3] Factors influencing the oxygen isotope content of ecosystem respiration (d R ) are discussed in a companion paper [Bowling et al, 2003b]. Keeling [1958Keeling [ , 1961 observed that d…”

“…Bowling et al [2003b] presented a set of data quality criteria to determine when Keeling plots can be interpreted with confidence for oxygen isotopes of CO 2 . All Keeling plots in this study met those requirements, which included (1) significant linear regressions (p < 0.01, Student's t-test) and (2) air sampling durations less than 5 hours.…”

“…[8] The companion paper [Bowling et al, 2003b] 18 O of precipitation. Our ecosystem measurements, however, focused on a short-term experiment at a single forest.…”

A field-based method for simultaneous measurements of the δ&lt;sup&gt;18&lt;/sup&gt;O and δ&lt;sup&gt;13&lt;/sup&gt;C of soil CO&lt;sub&gt;2&lt;/sub&gt; efflux

“…Laboratory investigations (Miller et al, 1999) and modeling efforts (Stern et al, 2001) suggest that the effective diffusional fractionation factor will vary depending on the environmental conditions. Error in the estimation of the oxygen isotopic ratio of soil respired CO 2 will impact the results of studies that use the stable isotopes of CO 2 to partition ecosystem gas exchange into its components (Yakir and Wang, 1996;Bowling et al, 2003b).…”

“…Because the global and regional scale carbon budgets include a flux of CO 2 from the soil to the atmosphere, the determination of the isotopic composition of soil CO 2 efflux remains a critical parameter to be measured in these budgets (Ciais and Meijer, 1998;Stern et al, 1999). At ecosystem and regional scales, accurate determination of the isotopic composition of soil respired CO 2 is necessary for partitioning ecosystem gas exchange into its components (Yakir and Wang, 1996;Bowling et al, 2003b).…”