Corrected calculations for soil and ecosystem measurements of CO2 flux using the LI-COR 6200 portable photosynthesis system

The LI-COR 6200 portable photosynthesis (LI-6200) is commonly used in combination with large chambers to measure ecosystem level CO2 flux in ecosystems with small-statured canopies (agriculture, tundra, grasslands, forest understory, etc.). Two problems with the methodology lead to artifactually low estimates of rates of net ecosystem assimilation of CO2 (or overestimates of ecosystem respiration). The first is that accuracy of the equations used by the LI-6200 to calculate photosynthesis depends on a constant vapor pressure in the chamber. This assumption is commonly violated with large ecosystem chambers when evapotranspiration rates are high. We provide equations that correct this problem and permit recalculation of the LI-COR fluxes. The second problem is that of boundary layer formation under still conditions, such as at night. As high concentrations of CO2 close to the ground surface become mixed by chamber fans, exceptionally high values of net ecosystem respiration result. Substantial mixing time is necessary for rates to stabilize. As ecologists attempt to understand how global change might affect whole-ecosystem carbon balance, both of these technical problems must be addressed to get accurate results.