A practical approach for uncertainty quantification of high-frequency soil respiration using Forced Diffusion chambers

This paper examines the sources of uncertainty for the Forced Diffusion (FD) chamber soil respiration (R-s) measurement technique and demonstrates a protocol for uncertainty quantification that could be appropriate with any soil flux technique. Here we sought to quantify and compare the three primary sources of uncertainty in R-s: (1) instrumentation error; (2) scaling error, which stems from the spatial variability of R-s; and (3) random error, which arises from stochastic or unpredictable variation in environmental drivers and was quantified from repeated observations under a narrow temperature, moisture, and time range. In laboratory studies, we found that FD instrumentation error remained constant as R-s increased. In field studies from five North American ecosystems, we found that as R-s increased from winter to peak growing season, random error increased linearly with average flux by about 40% of average R-s. Random error not only scales with soil flux but scales in a consistent way (same slope) across ecosystems. Scaling error, measured at one site, similarly increased linearly with average R-s, by about 50% of average R-s. Our findings are consistent with previous findings for both soil fluxes and eddy covariance fluxes across other northern temperate ecosystems that showed random error scales linearly with flux magnitude with a slope of similar to 0.2. Although the mechanistic basis for this scaling of random error is unknown, it is suggestive of a broadly applicable rule for predicting flux random error. Also consistent with previous studies, we found the random error of FD follows a Laplace (double-exponential) rather than a normal (Gaussian) distribution.