A novel plant growth chamber for separate and continuous monitoring of above-ground and below-ground gas exchange

A large part of photosynthetically fixed carbon is translocated below-ground in order to construct and maintain the roots needed to supply the shoot with adequate water and nutrients. However, the amount of carbon translocated below-ground is not easily quantified, as an unknown part is lost as root respiration, which is not easily distinguished from microbial soil respiration.

Here we present a novel plant growth chamber enabling continuous and separate monitoring of above-ground and below-ground gas exchange. The above-ground compartment is separated from the soil compartment by an impermeable fat layer, and a custom-developed carbon-free soil substrate is used to eliminate CO₂ release due to microbial decomposition of pre-existing soil carbon. Each compartment of the growth chamber is connected to an infrared gas analyzer, enabling simultaneous monitoring of above-ground and below-ground fluxes. A novel experimental approach using chemical agents was employed to test if CO₂ uptake and release was adequately quantified in each compartment over several days.

In a pilot experiment performed to identify a suitable carbon-free soil, maize plants grown at a 20% volumetric water content, 1.3g/cm³ bulk density and a 14h/10h day/night regime showed a correlation between evapo-transpiration and root length but not with root biomass, suggesting that the cost/benefit ratio of root allocation may be more related to root respiration and mechanical energy expenditure than accumulated root biomass. In fact, our preliminary results suggest that cumulative root respiration over 2 weeks was of a similar order of magnitude as the carbon stored in the root system at the end of the experiment, and that root respiration rates were relatively similar to nocturnal shoot respiration rates. A detailed analysis is underway and will be presented during the conference.