Using O2 to study the relationships between soil CO2 efflux and soil respiration

Soil respiration, is the sum of respiration processes in the soil, and is a major flux in the global carbon cycle. It is usually assumed that the CO2 efflux is equal to the soil respiration rate. Here we challenge this assumption by combining measurements of CO2 with high-precision measurements of O2. These measurements were conducted on different ecosystems and soil types, and included measurements of air-samples taken from the soil profile of three Mediterranean sites, a temperate forest, and two alpine forests. Root-free soils from the alpine sites were also incubated at the lab. We found that the ratio between the CO2 efflux to the O2 influx (which we defined as apparent respiratory quotient, ARQ) was in the range of 0.14 to 1.23, which strongly deviates from 0.9 ± 0.1, which is the ratio expected from the elemental composition of average plants and soil organic matter. At the Mediterranean sites these deviations were explained as a result of CO2 dissolution in the soil water and transformation to bi-carbonate in these high pH soils, and by carbonates dissolution and precipitation processes. Thus, correct estimate of the short-term, chamber-based biological respiratory flux in such soils can only be made by dividing the measured CO2 efflux by the average (efflux weighted) soil profile ARQ. We demonstrated that applying this approach to a semiarid pine forest resulted in estimated short-term respiration rate 3.8 times higher than the chamber-measured surface CO2 efflux (8.8 μmol CO2 m−2 s−1 instead of 2.3 μmol CO2 m−2 s−1, at the time of measurement). The ARQ values that were often found for the more acidic soils were lower than 0.7, and hence surprising. These values might be the result of the oxidation of reduced iron, which could previously form during times of high soil moisture and local anaerobic conditions inside aggregates. Further research is needed to confirm that low ARQ found in non-calcareous soils, is the result of this process, which can cause additional temporal decoupling between gas fluxes and soil respiration