Hierarchical saturation of soil carbon pools near a natural CO₂ spring

Abstract

Soil has been identified as a possible carbon (C) sink to mitigate increasing atmospheric CO 2 concentration. However, several recent studies have suggested that the potential of soil to sequester C is limited and that soil may become saturated with C under increasing CO 2 levels. To test this concept of soil C saturation, we studied a gley and organic soil at a grassland site near a natural CO 2 spring. Total and aggregate-associated soil organic C (SOC) concentration showed a significant increase with atmospheric CO 2 concentration. An asymptotic function showed a better fit of SOC and aggregation with CO 2 level than a linear model. There was a shift in allocation of total C from smaller size fractions to the largest aggregate fraction with increasing CO 2 concentration. Litter inputs appeared to be positively related to CO 2 concentration. Based on modeled function parameters and the observed shift in the allocation of the soil C from small to large aggregate-size classes, we postulate that there is a hierarchy in C saturation across different SOC pools. We conclude that the asymptotic response of SOC concentration at higher CO 2 levels indicates saturation of soil C pools, likely because of a limit to physical protection of SOC.

Hypoxic and acidic — Soils on mofette fields

“…These studies show a negative effect of geogenic CO 2 on the extent of interactions between SOM and soil minerals. Kool et al (2007) showed that SOM allocated in aggregates increased with CO 2 at only slightly enhanced concentrations (up to 0.4%), but SOM was shifted from smaller to larger aggregates. They explained it with limits to physical protection, which may be caused by decreased formation of pedogenic oxides that build up small aggregates.…”

“…Accumulation of SOM in soils on mofette fields relative to reference sites has been frequently reported (e.g., Beaubien et al, 2008;Beulig et al, 2015Beulig et al, , 2016Kerpen, 1960;Kool et al, 2007;Nowak et al, 2015;Rennert et al, 2011;Saßmannshausen, 2010). Accumulation of SOM is not only expressed by increased contents of soil organic carbon (SOC) in the topsoils, but also by the growth of hummocks, similar to those in peat lands, and the preservation of unaltered plant debris (Flechsig et al, 2008;Kerpen, 1960;Pätzold, 2009).…”

Comparison of the impacts of elevated CO 2 soil gas concentrations on selected European terrestrial environments

“…Biological monitoring techniques were reviewed by Noble et al (2012). Other work has examined long-term ecosystem responses to elevated CO 2 concentrations from nearby natural CO 2 seepages from springs and vents in, for example, New Zealand (Kool et al, 2007: Stock et al, 2005, Iceland (Cook et al, 1998) and South Africa (Ross et al, 2000). However, these studies only report time-averaged atmospheric concentrations so it is difficult to determine if the observed effects were directly influenced by elevated CO 2 concentrations in the root zone.…”

The impact of a naturally occurring CO2 gas vent on the shallow ecosystem and soil chemistry of a Mediterranean pasture (Latera, Italy)

“…A number of other studies have also been conducted to examine long-term ecosystem responses to elevated atmospheric CO2 concentrations induced by nearby natural CO2 vents and springs, such as at Hakanoa Springs, New Zealand (Kool et al, 2007;Rillig et al, 2000;Ross et al, 2000), Olafsvik, W. Iceland (Cook et al, 1998), and Pleasant View Farm, South Africa (Stock et al, 2005). Unfortunately these works only report time-averaged atmospheric CO2 concentrations, and thus it is difficult to understand if the observed effects were also influenced by high concentrations of un-measured or unreported root zone CO2.…”