, Volume 164, Issue 2, pp 297-310
Date: 09 May 2010

Nocturnal and seasonal patterns of carbon isotope composition of leaf dark-respired carbon dioxide differ among dominant species in a semiarid savanna

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The C isotope composition of leaf dark-respired CO213Cl) integrates short-term metabolic responses to environmental change and is potentially recorded in the isotopic signature of ecosystem-level respiration. Species differences in photosynthetic pathway, resource acquisition and allocation patterns, and associated isotopic fractionations at metabolic branch points can influence δ13Cl, and differences are likely to be modified by seasonal variation in drought intensity. We measured δ13Cl in two deep-rooted C3 trees (Prosopis velutina and Celtis reticulata), and two relatively shallow-rooted perennial herbs (a C3 dicot Viguiera dentata and a C4 grass Sporobolus wrightii) in a floodplain savanna ecosystem in southeastern Arizona, USA during the dry pre-monsoon and wet monsoon seasons. δ13Cl decreased during the nighttime and reached minimum values at pre-dawn in all species. The magnitude of nocturnal shift in δ13Cl differed among species and between pre-monsoon and monsoon seasons. During the pre-monsoon season, the magnitude of the nocturnal shift in δ13Cl in the deep-rooted C3 trees P. velutina (2.8 ± 0.4‰) and C. reticulata (2.9 ± 0.2‰) was greater than in the C3 herb V. dentata (1.8 ± 0.4‰) and C4 grass S. wrightii (2.2 ± 0.4‰). The nocturnal shift in δ13Cl in V. dentata and S. wrightii increased to 3.2 ± 0.1‰ and 4.6 ± 0.6‰, respectively, during the monsoon season, but in C3 trees did not change significantly from pre-monsoon values. Cumulative daytime net CO2 uptake was positively correlated with the magnitude of the nocturnal decline in δ13Cl across all species, suggesting that nocturnal δ13Cl may be controlled by 13C/12C fractionations associated with C substrate availability and C metabolite partitioning. Nocturnal patterns of δ13Cl in dominant plant species in the semiarid savanna apparently have predictable responses to seasonal changes in water availability, which is important for interpreting and modeling the C isotope signature of ecosystem-respired CO2.

Communicated by Marilyn Ball.