An Objective Approach to Select Soil CO2 Concentration Measurement Depths When Using the Gradient Method to Measure Soil Respiration.

Soil CO₂ efflux has traditionally been measured using chambers. However, over recent years the gradient method has been increasingly used to calculate CO₂ fluxes based on measurements of the soil CO₂ concentration profile and soil CO₂ diffusivity. Soil CO₂ concentration measurement depths should be chosen to meet the assumptions of the gradient method and remain within the sensors’ measurement range, but at most sites relevant information is not available, which makes selecting depths challenging. Here, we present the approach used by the National Ecological Observatory Network (NEON) to select three site-specific measurement depths at locations throughout the US. The approach was designed to fulfil four aims: 1) the shallowest depth should be as close to the soil surface as possible so that the entire soil-atmosphere CO₂ flux can be determined; 2) the measurement depths should correspond to CO₂ concentrations that do not exceed the sensors’ range (0-20,000 µmol mol^-1); 3) the depths meet the assumption that CO₂ production rates are similar between any two measurement depths; and 4) the depths maximize the extent of the soil CO₂ profile, since this allows CO₂ production rates at different depths to be determined over the largest possible portion of the soil profile. The approach uses three types of site-specific information: 1) a modeled estimate the soil CO₂ concentration profile, 2) estimates of the soil CO₂ production profile based on root biomass distribution, and 3) a soil profile description to ensure deployment depths reflect major changes in soil composition. Applying this approach to 6 initial sites located in Florida, Massachusetts, New Mexico, and North Dakota yielded measurement depths of 2 cm for the shallowest sensors, 4-9 cm for the middle depth, and 9-21 cm for the deepest depth, which are similar to depths used successfully at other sites.