Using O<sub>2</sub> to study the relationships between soil CO<sub>2</sub> efflux and soil respiration

Abstract

Abstract. 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 CO 2 efflux is equal to the soil respiration rate. Here we challenge this assumption by combining measurements of CO 2 with high-precision measurements of O 2 . 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. Rootfree soils from the alpine sites were also incubated in the lab. We found that the ratio between the CO 2 efflux and the O 2 influx (defined as apparent respiratory quotient, ARQ) was in the range of 0.14 to 1.23 and considerably deviated from the value of 0.9 ± 0.1 expected from the elemental composition of average plants and soil organic matter. At the Mediterranean sites, these deviations are explained as a result of CO 2 dissolution in the soil water and transformation to bicarbonate ions in these high-pH soils, as well as by carbonate mineral dissolution and precipitation processes. Thus, a correct estimate of the short-term, chamber-based biological respiratory flux in such soils can only be made by dividing the measured soil CO 2 efflux by the average (efflux-weighted) soil profile ARQ. Applying this approach to a semiarid pine forest resulted in an estimated short-term biological respiration rate that is 3.8 times higher than the chamber-measured surface CO 2 . The ARQ values often observed in the more acidic soils were unexpectedly low (< 0.7). These values probably result from the oxidation of reduced iron, which has been formed previously during times of high soil moisture and local anaerobic conditions inside soil aggregates. The results reported here provide direct quantitative evidence of a large temporal decoupling between soil-gas exchange fluxes and biological soil respiration.

A considerable fraction of soil-respired CO2 is not emitted directly to the atmosphere

“…Therefore, assuming that all O 2 consumption is associated with respiration, in this semiarid forest only 1/3 of R soil is emitted directly to the atmosphere and 2/3 are removed by subsurface processes. These results are actually quite similar to those found in the only other paper that has calculated in situ ARQ for estimates of F soil 22 , which reported a mean ARQ of 0.26 and, therefore, an R soil that is 3.8 times higher than F soil estimated in their experimental site (Yatir forest). In that study, researchers collected CO 2 and O 2 samples in a pine forest overlying chalk and limestone bedrock with a mean annual precipitation of 280 mm.…”

“…The importance of these alternative CO 2 loss pathways is illustrated when considering that soil can store an order of magnitude greater CO 2 as dissolved inorganic carbon (DIC, inclusive of dissolved CO 2 , carbonic acid, bicarbonate, and carbonate) in the aqueous-filled relative to gas-filled pore space 22 . As a result, large CO 2 losses can be produced by DIC leaching in all ecosystems around the world, with increased CO 2 losses in ecosystems with higher precipitation and higher soil solution pH.…”

“…The ratio of soil CO 2 efflux to O 2 influx, known as the apparent respiratory quotient (ARQ), allows one to estimate the CO 2 losses from R soil 22 . A diagram, with the main variables involved in exchange and loss of CO 2 , is shown in Fig.…”

Environmental forcing does not induce diel or synoptic variation in the carbon isotope content of forest soil respiration

“…Because aerobic respiration involves CO 2 production that is stoichiometrically related to O 2 consumption, the ratio of these two fluxes (apparent respiratory quotient or ARQ) can be used to identify the presence of other processes in the soil that can lead to decoupling between CO 2 efflux and production. The ARQ method has highlighted the importance of CO 2 dissolution in soil water, carbonate dissolution and precipitation, and possibly the oxidation of reduced iron (Angert et al, 2015). Finally, CO 2 transported through the xylem can be similar in magnitude to soil efflux (Aubrey and Teskey, 2009).…”

On the role of soil water retention characteristic on aerobic microbial respiration

“…However, this model does not account for the distance from air-exposed soil surface. In lab incubation studies, short cores and/or cores with large exposed surfaces do not exhibit availability becomes increasingly limiting with depth as transport length increases and cumulative O2 consumption increases (Angert et al, 2015). Therefore, we add a correction term that accounts for these variations…”