Behzad Mortazavi

Temporal variability in 13C of respired CO2 in a pine and a hardwood forest subject to similar climatic conditions

Temporal variability in the 13C of foliage (δ13CF), soil (δ13CS) and ecosystem (δ13CR) respired CO2 was contrasted between a 17.2-m tall evenly aged loblolly pine forest and a 35-m tall unevenly aged mature second growth mixed broadleaf deciduous forest in North Carolina, USA, over a 2-year period. The two forests are located at the Duke Forest within a kilometer of each other and are subject to identical climate and have similar soil types. The δ13CF, collected just prior to dawn, was primarily controlled by the time-lagged vapor pressure deficit (VPD) in both stands; it was used for calculating the ratio of intercellular to ambient CO2 (Ci/Ca). A remarkable similarity was observed in the relationship between Ci/Ca and time-lagged VPD in these two forests despite large differences in hydraulic characteristics. This similarity emerged as a result of physiological adjustments that compensated for differences in plant hydraulic characteristics, as predicted by a recently proposed equilibrium hypothesis, and has implications to ecophysiological models. We found that in the broadleaf forest, the δ13C of forest floor CO2 efflux dominated the δ13CR, while in the younger pine forest, the δ13C of foliage respired CO2 dominated δ13CR. This dependence resulted in a more variable δ13CR in the pine forest when compared to the broadleaf forest due to the larger photosynthetic contribution. Given the sensitivity of the atmospheric inversion models to δ13CR, the results demonstrate that these models could be improved by accounting for stand characteristics, in addition to previously recognized effects of moisture availability, when estimating δ13CR.

Carbon isotopic discrimination and control of nighttime canopy δ18O‐CO2 in a pine forest in the southeastern United States

[i] Canopy and soil CO 2 concentration and isotopic measurements were conducted in a slash pine forest during a progressive drought in the southeastern United States (1) to determine and compare variations in δ 13 C of foliage (δCf), soil (δCs), and ecosystem-respired CO 2 (δCr) and (2) to evaluate the usefulness of a two end-member oxygen isotope ratio (δ 18 O of CO 2 ) approach to partition nighttime ecosystem respiration into soil and plant components at different heights within the canopy. The δCf was enriched by 2.2 ± 0.3 (standard error) %o during the extreme drought in May relative to September when precipitation was above normal. The enrichment in δCf exceeded changes in δCr and δCs for the same time period (1.6 ± 0.5 and 1.0 ± 0.4‰, respectively). Lower variations in the 13 C of soil-respired CO 2 relative to the variations of the autotrophic component can buffer changes in δCr, which integrates the 13 C signature of canopy and soil respired CO 2 . Application of a two end-member model to canopy δ 18 O-CO 2 indicated that soil CO 2 contribution to nighttime CO 2 buildup within the canopy decreased from 100% at the soil surface to 0% within the canopy in September. In May, during the extreme drought period, soil respiration rate was 2.7 times lower than the rate in September despite similar soil temperatures (23° in May versus 19°C in September). Soil and aboveground respiration equally contributed to the nocturnal CO 2 buildup within the canopy in May. Our model was limited in that it produced an upper limit value for soil respiration and neglected respiration by woody tissue.

Does the 13C of foliage‐respired CO2 and biochemical pools reflect the 13C of recently assimilated carbon?

Isotopic labelling experiments were conducted to assess relationships among (13)C of recently assimilated carbon (deltaC(A)), foliage respiration (deltaC(F)), soluble carbohydrate (deltaC(SC)), leaf waxes (deltaC(LW)) and bulk organic matter (deltaC(OM)). Slash pine, sweetgum and maize were grown under (13)C depleted CO(2) to label biomass and then placed under ambient conditions to monitor the loss of label. In pine and sweetgum, deltaC(F) of labelled plants (approximately -44 and -35 per thousand, respectively) rapidly approached control values but remained depleted by approximately 4-6 per thousand after 3-4 months. For these tree species, no or minimal label was lost from deltaC(SC), deltaC(LW) and deltaC(OM) during the observation periods. deltaC(F) and deltaC(SC) of labelled maize plants rapidly changed and were indistinguishable from controls after 1 month, while deltaC(LW) and deltaC(OM) more slowly approached control values and remained depleted by 2-6 per thousand. Changes in deltaC(F) in slash pine and sweetgum fit a two-pool exponential model, with the fast turnover metabolic pool (approximately 3-4 d half-life) constituting only 1-2% of the total. In maize, change in deltaC(F) fits a single pool model with a half-life of 6.4 d. The (13)C of foliage respiration and biochemical pools reflect temporally integrated values of deltaC(A), with change in isotopic composition dampened by the size of metabolic carbon reserves and turnover rates.