Comparing the closed static versus the closed dynamic chamber flux methodology: Implications for soil respiration studies

Soil respiration is the largest C-flux component in the terrestrial carbon (C) cycle, yet in many biomes this flux and its environmental responses are still poorly understood. Several methodological techniques exist to measure this flux, but mostly there remain comparability uncertainties. For example, the closed static chamber (CSC) and the closed dynamic chamber (CDC) systems are widely used, but still require a rigorous comparison. A major issue with the CSC approach is the generally long manual gas sampling periods causing a potential underestimation of the calculated fluxes due to an asymptotic increase in headspace CO2 concentrations. However, shortening the sampling periods of the static chamber approach might provide comparable results to the closed dynamic chamber system. We compared these two different chamber systems using replicated CSC cover boxes and a Li-Cor 8100 CDC system under field conditions, and performed tests on both, mineral and peat soil. Whereas the automated CDC system calculated fluxes during the first two minutes, the CSC approach considered either all seven manual sampling points taken over 75 min, or only the first three sampling points over 15 min. Although flux variation was fairly large, there were considerable and statistically significant differences between the calculated fluxes considering the two chamber systems, yet this depended on soil type and the number of CSC sampling time points. The cover-box approach underestimated the chamber-based fluxes by 30% for combined samples, 21% for mineral and 39% for peat soils when calculated over 75 min but was comparable over the first 15 min. The chamber flux comparison demonstrates that the CSC approach can provide CO2 flux measurements comparable to the CDC system when sampling at an appropriate initial frequency, preventing flux underestimation due to a build up of CO2 headspace concentrations.