Plant growth chamber design for subambient pCO2 and δ13C studies

Rationale Subambient pCO2 has persisted across the major Phanerozoic ice ages, including the entire late Cenozoic (ca 30 Ma to present). Stable isotope analysis of plant‐derived organic matter is used to infer changes in pCO2 and climate in the geologic past, but a growth chamber that can precisely control environmental conditions, including pCO2 and δ13C value of CO2 (δ13CCO2) at subambient pCO2, is lacking. Methods We designed and built five identical chambers specifically for plant growth under stable subambient pCO2 (ca 100 to 400 ppm) and δ13CCO2 conditions. We tested the pCO2 and δ13CCO2 stability of the chambers both with and without plants, across two 12‐hour daytime experiments and two extended 9‐day experiments. We also compared the temperature and relative humidity conditions among the chambers. Results The average δ13CCO2 value within the five chambers ranged from −18.76 to −19.10‰; the standard deviation never exceeded 0.14‰ across any experiment. This represents better δ13CCO2 stability than that achieved by all previous chamber designs, including superambient pCO2 chambers. Every pCO2 measurement (n = 1225) was within 5% of mean chamber values. The temperature and relative humidity conditions differed by no more than 0.4°C and 1.6%, respectively, across all chambers within each growth experiment. Conclusions This growth chamber design extends the range of pCO2 conditions for which plants can be grown for δ13C analysis of their tissues at subambient levels. This new capability allows for careful isolation of environmental effects on plant 13C discrimination across the entire range of pCO2 experienced by terrestrial land plants.