A dual isotope approach to isolate soil carbon pools of different turnover times

Abstract

Abstract. Soils are globally significant sources and sinks of atmospheric CO 2 . Increasing the resolution of soil carbon turnover estimates is important for predicting the response of soil carbon cycling to environmental change. We show that soil carbon turnover times can be more finely resolved using a dual isotope label like the one provided by elevated CO 2 experiments that use fossil CO 2 . We modeled each soil physical fraction as two pools with different turnover times using the atmospheric 14 C bomb spike in combination with the label in 14 C and 13 C provided by an elevated CO 2 experiment in a California annual grassland.In sandstone and serpentine soils, the light fraction carbon was 21-54 % fast cycling with 2-9 yr turnover, and 36-79 % slow cycling with turnover slower than 100 yr. This validates model treatment of the light fraction as active and intermediate cycling carbon. The dense, mineral-associated fraction also had a very dynamic component, consisting of ∼ 7 % fast-cycling carbon and ∼ 93 % very slow cycling carbon. Similarly, half the microbial biomass carbon in the sandstone soil was more than 5 yr old, and 40 % of the carbon respired by microbes had been fixed more than 5 yr ago.Resolving each density fraction into two pools revealed that only a small component of total soil carbon is responsible for most CO 2 efflux from these soils. In the sandstone soil, 11 % of soil carbon contributes more than 90 % of the annual CO 2 efflux. The fact that soil physical fractions, designed to isolate organic material of roughly homogeneous physico-chemical state, contain material of dramatically different turnover times is consistent with recent observations of rapid isotope incorporation into seemingly stable fractions and with emerging evidence for hot spots or micro-site variation of decomposition within the soil matrix. Predictions of soil carbon storage using a turnover time estimated with the assumption of a single pool per density fraction would greatly overestimate the near-term response to changes in productivity or decomposition rates. Therefore, these results suggest a slower initial change in soil carbon storage due to environmental change than has been assumed by simpler (one-pool) mass balance calculations.

Rates and radiocarbon content of summer ecosystem respiration in response to long‐term deeper snow in the High Arctic of NW Greenland

“…The 14 C content of CO 2 in soil gas, sampled below the rooting zone, provides insight to changes in microbial activity and soil physical properties Torn et al, 2013]. The 14 C values of pore space CO 2 exhibited two different depth trends ( Figure 6): (1) An increase in age with depth or (2) an increase to about 60 cm followed by a decrease toward the permafrost table.…”

Differential capacity of kaolinite and birnessite to protect surface associated proteins against thermal degradation

“…Past research into the phenomenon has concentrated on the stabilizing effects of the mineral matrix (Baldock and Skjemstad, 2000;Basile-Doelsch et al, 2007;Doetterl et al, 2015;Dungait et al, 2012;Kemmitt et al, 2008;Marin-Spiotta et al, 2008;Rasmussen et al, 2006;Schmidt et al, 2011;Torn et al, 2013), i.e. the ability of minerals to retard the decomposition of organic substrates.…”

Allocation of photosynthestically-fixed carbon in plant and soil during growth of reed (Phragmites australis) in two saline soils

“…In contrast to POC, MAOC is the residual C after POC removal and is regarded as the relatively stable C pool (Salvo et al 2010, Briedis et al 2012Martins et al 2012). However, Torn et al (2013) also report that MAOC contains a fastturnover C component, and thus it is necessary to further investigate MAOC stability. Dissolved organic C as mobile C in soil provides energy for microorganisms and is adsorbed by soil particles (Kalbitz et al 2000), and dead microorganisms release DOC or form the relative stable pools (Kalbitz et al 2003).…”

“…Dissolved organic C as mobile C in soil provides energy for microorganisms and is adsorbed by soil particles (Kalbitz et al 2000), and dead microorganisms release DOC or form the relative stable pools (Kalbitz et al 2003). Soil microorganisms can attach to soil particles (Nicolás et al 2014) and also utilize C sources from soil particles (Six et al 2002;Martins et al 2012;Torn et al 2013;Keiluweit et al 2015). Overall, DOC, MBC, POC, and MAOC together represent the chemical, biological, and physical forms of SOM.…”

“…This may be attributable to the turnover of microbial biomass induced by the rhizodeposited bio-available DOC (Torn et al 2013;Keiluweit et al 2015) or to the sorption and desorption of soil mineral-associated C by physico-chemical processes (Six et al 2002;Saidy et al 2012). Some studies also report that the heterogeneous MAOC pool contains fast-cycling C components (Six et al 2002;Torn et al 2013;Keiluweit et al 2015) and the released clay particle C further forms macroaggregate C by microbiallyderived organic cementing agents (Six et al 2002;Denef and Six 2005). Six et al (2002) also documented that fixed C in 1:1 clay had lower stability than 2:1 clay, as shown by the higher kaolinite content in HS soil than LS soil (P < 0.05, Table 1).…”