Using large, automated, light and dark chamber systems to directly measure rates of ecosystem gross primary productivity (GPP) and respiration (Reco)

Abstract

The biological processes of carbon (C) uptake via plant photosynthesis (gross primary productivity, GPP) and carbon loss by autotrophic and heterotrophic respiration (ecosystem respiration, Reco) each constitute a C flux of approx. 130 Gt C per year, equal to 1/7 of the atmospheric C pool. Still, because the biological processes driving GPP and Reco are both active during daytime, they are intrinsically difficult to measure directly. The eddy covariance technique, which is effectively the gold standard for measuring net ecosystem exchange (NEE), relies on partitioning models of NEE to estimate GPP and Reco, but these methods remain debated because other processes, such as inhibition of leaf-level respiration during daytime, are not accounted for.   In ecosystems with short-stature vegetation like grasslands, shrublands, tundra, and many agricultural systems, light and dark closed chamber measurements at the ecosystem scale enable direct daytime measurements of NEE (under light conditions) and Reco (under dark conditions) while GPP can be directly estimated as NEE - Reco. Long-term data series of automated light and dark chamber measurements are, however, very rare. Here, we present data of > 50,000 measurements over six years from a novel, automated light and dark gas exchange measurement chamber that was tested in heathland, wetland, and agricultural vegetation types. In the heathland, we applied standard eddy covariance gap-filling methods to estimate annual NEE across the six years of observations. The results show annual NEE rates ranging from -96 (net uptake) to 21 (net release) g C m-2y-1 over the different years. We further applied standard eddy covariance nighttime and daytime methods to partition the observed NEE measurements into GPP and Reco. Using the nighttime method, GPP ranged from 966 to 1355 g C m-2y-1 while Reco ranged from 867 to 1372 g C m-2y-1. On average, this was only 0-4% higher than observed rates from the chamber measurements. In comparison, the daytime method yielded GPP and Reco rates that were approximately 11-30% higher than observed rates. The slightly to moderately lower direct measurements with the automatic light and dark chamber could indicate that the chamber observations are able to account at least partially for the daytime leaf-level inhibition of respiration and thus may provide a sound method for measuring the actual rates of GPP and Reco. While potential biases cannot be ruled out and will be discussed, our results indicate that automated light and dark chambers may provide an additional and highly useful tool for estimating rates of GPP and Reco in short-stature vegetation and may further serve to help constrain methods for partitioning NEE fluxes observed with other techniques, such as the eddy covariance methodology.