Photosynthetic water use efficiency and biomass of Sorghastrum nutans (C4) and Solidago canadensis (C3) in three soils along a CO2 concentration gradient

The water use efficiency (WUE) of leaf photosynthetic carbon uptake is a key regulator of plant production in grasslands. However WUE may differ with soil type because of differences in soil moisture retention and plant uptake efficiency. We measured soil water content (SWC, 0-30 cm, %), leaf-level WUE (ratio of photosynthesis, A_CO2, to transpiration, E) and end-of-season biomass in the C₄ grass Sorghastrum nutans and the C₃ forb Solidago canadensis growing in well-established multi-species communities on three soil types (Austin, Bastrop, and Houston series) along an experimental [CO₂] gradient. SWC was lower on the Bastrop than Austin or Houston soils (p < 0.0001).  WUE did not differ between soil types for either species, but the biomass of both S. nutans and S. canadensis was 2 to 4- fold greater on Bastrop and Houston than on Austin soils (p ≤ 0.05).  However, photosynthetic WUE increased strongly (p< 0.0001) at higher [CO₂] in both species (species x [CO₂] ns), due to a combination of decreasing E and increasing A_CO2 (p ≤ 0.005). Biomass of S. nutans was not related to [CO₂], however biomass of S. canadensis increased strongly at higher [CO₂] on Bastrop and Houston soils (soil x [CO₂] p = 0.0003). We conclude that 1) [CO₂] was the primary control on leaf-level photosynthetic water use efficiency, 2) soil type was the primary control on growth, and 3) there was not a consistent association between WUE and biomass responses to [CO₂] in these species.