Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: 11/15/2006
Publication Date: 12/29/2006
Citation: Fay, P.A., Hui, D., Proctor, A., Johnson, H.B., Polley, H.W., Jackson, R.B. 2006. Photosynthetic water use efficiency in Sorghastrum nutans (C4) and Solidago canadensis (C3) in three soils along a CO2 concentration gradient. In: Proceedings of the EOS Trans. American Geophysical Union. December 11-15, 2006, San Francisco, CA. Paper No. B41B-0817. Interpretive Summary:
Technical Abstract: The water use efficiency (WUE) of leaf photosynthetic carbon uptake is a key regulator of ecosystem carbon cycles and is strongly sensitive to atmospheric carbon dioxide concentrations [CO2]. However WUE responses to [CO2] typically differ between C3 and C4 species and may differ on varying soil types because of differences in soil moisture retention and plant uptake efficiency. We measured leaf-level photosynthesis (ACO2), stomatal conductance (gs), and transpiration (E) with an infrared gas analyzer to estimate WUE for the C4 grass Sorghastrum nutans and the C3 forb Solidago canadensis in constructed grassland species assemblages growing in three soils arrayed along a 200 – 560 ppm [CO2] gradient in the LYCOG Experiment, in central Texas, USA. LYCOG consists of eighty intact soil monoliths (1 m X 1 m X 1.5 m) representing 3 soil series, Austin (Udorthentic Haplustolls, a mollisol), Bastrop (Udic Paleustalfs, a sandy loam alfisol) and Houston Black (Udic Haplusterts, a vertisol). The monoliths were vegetated by transplanting 8 native perennial prairie species (5 grasses and 3 forbs), including S. nutans and S. canadensis. Both are abundant and widespread; S. nutans is a dominant species throughout much of North American tallgrass prairie, and S. canadensis is one of the most abundant and widespread forbs in North America. ACO2, gs, and E were measured three times during the growing season. Dark-adapted chlorophyll fluorescence (FvFm) was measured concurrently to assess photosynthetic capacity, and leaf water potential (ps leaf) and soil water content were measured to assess plant water status and soil moisture availability. WUE increased strongly (p < 0.0001) at higher [CO2], due to a combination of decreasing E due to decreased gs (p </= 0.0005) and increasing ACO2 (p = 0.0055). This pattern was the same in both species (species x [CO2] ns). There was a corresponding increase in ps leaf (p = 0.01) at higher [CO2], but no [CO2] effect on FvFm. E and gs were lower on Houston than Austin or Bastrop soils (p </= 0.03), however there were no differences in the other leaf parameters between soils. In contrast, volumetric soil water content (0 – 30 cm) was lower on the Bastrop than Austin or Houston soils (p < 0.0001). The disconnect between soil moisture and plant water loss was unexpected, and may indicate limitations on leaf physiology by soil N or other resources, as well as compensatory adjustment in root uptake efficiency, depth profiles, or root length densities among soil types. These results suggest that the WUE of photosynthesis, and thus the relative contribution of these two abundant species to the carbon cycle in grasslands may remain constant over the range of [CO2] in this study. However, the response of leaf transpiration to [CO2] may be constrained differently by soil water content between the soils, possibly because of differences in the distribution plant roots.