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Soil respiration is the sum of respiration processes in the soil and is amajor flux in the global carbon cycle. It is usually assumed that theCO2 efflux is equal to the soil respiration rate. Here we challengethis assumption by combining measurements of CO2 with high-precisionmeasurements of O2. These measurements were conducted on differentecosystems and soil types and included measurements of air samples takenfrom the soil profile of three Mediterranean sites: a temperate forest andtwo alpine forests. Root-free soils from the alpine sites were alsoincubated in the lab. We found that the ratio between the CO2 effluxand the O2 influx (defined as apparent respiratory quotient, ARQ) wasin the range of 0.14 to 1.23 and considerably deviated from the value of0.9 ± 0.1 expected from the elemental composition of average plants andsoil organic matter. At the Mediterranean sites, these deviations areexplained as a result of CO2 dissolution in the soil water andtransformation to bicarbonate ions in these high-pH soils, as well as by carbonatemineral dissolution and precipitation processes. Thus, a correct estimate ofthe short-term, chamber-based biological respiratory flux in such soils canonly be made by dividing the measured soil CO2 efflux by the average(efflux-weighted) soil profile ARQ. Applying this approach to a semiaridpine forest resulted in an estimated short-term biological respiration ratethat is 3.8 times higher than the chamber-measured surface CO2. The ARQvalues often observed in the more acidic soils were unexpectedly low(< 0.7). These values probably result from the oxidation of reducediron, which has been formed previously during times of high soil moistureand local anaerobic conditions inside soil aggregates. The results reportedhere provide direct quantitative evidence of a large temporal decouplingbetween soil–gas exchange fluxes and biological soil respiration

biogeosciences

Using O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; to study the relationships between soil CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; efflux and soil respiration