Biochemically driven isotope effects differ for n-alkane hydrogen and for cellulose hydrogen and oxygen among eudicot plant species and between years

Plant organic compounds such as cellulose or n-alkanes are often utilized as recorders of oxygen (δ¹⁸O) or hydrogen (δ²H) isotopic signals to inform on past climate or environmental conditions, or on plant physiological changes. This is because these compounds can persist in the geologic record for decades to millennia or longer in select cases. Yet, large differences have often been observed among plant organic compound δ²H or δ¹⁸O values for species growing in a single location due to the balance between variable leaf water isotopic enrichment and variable biochemical isotopic effects among species. Distinguishing between these sources of variability and making use of these signals is an ongoing challenge, in part because of the limited number of studies that have explored the extent to which the different drivers influence the isotopic composition of each compound class and element within a single location.

We present a detailed assessment of isotopic variation in relevant plant water pools and cellulose δ²H and δ¹⁸O values, in combination with n-alkane δ²H values in 192 eudicot species grown in a botanical garden in a single growing season, as well as year-to-year comparisons for consecutive years (2019-2020). Our results show that variation in leaf water δ²H values were not a strong driver for the observed variation in organic compound δ²H values across eudicot species. Additionally, while correlation between δ²H and δ¹⁸O values found in plant source water and leaf water was transferred to cellulose, the explanatory power of this correlation was strongly diminished. This indicates that additional biochemical isotope fractionation caused substantial variation in organic compound δ²H and/or δ¹⁸O values across species. Moreover, variation in cellulose δ²H values were poorly correlated with δ²H values from n-alkanes, suggesting that the biochemical pathways associated with different compounds were accompanied by varying isotope effects. Lastly, cellulose δ²H and δ¹⁸O values changed more than n-alkane δ²H values from one year to the next. This implies that, cellulose δ²H and δ¹⁸O values are more sensitive to environmental differences between growing seasons compared to δ²H values from n-alkanes, and thus that the environmental forcing effects on isotope values are not equal between compounds. Overall, we found that variation in organic compound δ²H, and possibly also δ¹⁸O, values across species and between growing seasons was substantially more strongly driven by biochemical isotope fractionation than by isotope values of plant water. Therefore, to the extent that it is possible, biochemical responses to environmental changes should be considered in interpretations of organic compound δ²H and δ¹⁸O values to reconstruct the past. Furthermore, there is potential to recover plant responses to environmental changes from plant organic compound δ²H and/or δ¹⁸O values when the measurements are incorporated into multi-compound or multi-proxy paleoenvironmental and paleophysiological inquiries.