Centennial black carbon turnover observed in a Russian steppe soil

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Abstract

Black carbon (BC), from incomplete combustion of fuels and biomass, has been considered highly recalcitrant and a substantial sink for carbon dioxide. Recent studies have shown that BC can be degraded in soils. We use two soils with very low spatial variability sampled 100 years apart in a Russian steppe preserve to generate the first whole-profile estimate of BC stocks and turnover in the field. Quantities of fire residues in soil changed significantly over a century. Black carbon stock was 2.5 kg m-2, or about 7-10% of total organic C in 1900. With cessation of biomass burning, BC stocks decreased 25% over a century, which translates into a centennial soil BC turnover (293 years best estimate; range 182-541 years), much faster than so-called inert or passive carbon in ecosystem models. The turnover time presented here is for loss by all processes, namely decomposition, leaching, and erosion, although the latter two were probably insignificant in this case. Notably, at both time points, the peak BC stock was below 30 cm, a depth interval, which is not typically accounted for. Also, the quality of the fire residues changed with time, as indicated by the use benzene polycarboxylic acids (BPCA) as molecular markers. The proportions of less-condensed (and thus more easily degradable) BC structures decreased, whereas the highly condensed (and more recalcitrant) BC structures survived unchanged over the 100-year period. Our results show that BC cannot be assumed chemically recalcitrant in all soils, and other explanations for very old soil carbon are needed.

Figures

  • Table 1. Selected characteristics of soil profiles. The site sampled in 1900 and 1997 is compared to three other soil profiles about 1 km away sampled in 1997 (1997-2) and 2004. All of the soils were developed on the same loess parent material.
  • Fig. 1. (a) Black carbon concentrations (g kg−1 dry weight soil) of the 1900 soil (thin black line, open squares) and 1997 soil (thick black line, filled circles). The data are without the 2.27 correction factor often used with the BPCA method to compensate for the fact that not all BC is converted to BPCA (Brodowski et al., 2005). Error bars show the standard deviation of replicate laboratory analyses (n=2–3 per soil sample). (b) Black carbon as proportion of organic carbon for the 1900 and 1997 soils. (c) Black carbon stock of the 1900 and 1997 soils.
  • Table 2. Modeling parameters for the sensitivity analysis, depicting three scenarios of black carbon turnover time: best estimate, minimum and maximum (see text and Fig. 5 caption for details).
  • Fig. 2. Total profile BC stocks of steppe soil sampled in 1900, 1997 and 2004. Error bars show the standard deviation of replicate laboratory analyses (n=2–3 per soil sample). The BC stock decreased 25% between 1900 and 1997. The comparative 2004 soils are also about 25% lower.
  • Fig. 3. Mellitic acid (B6CA) is a molecular marker that is only released from the highly condensed, aromatic (and recalcitrant) core of BC structures. Relative contributions of B6CA to the total BPCA for 1900 (thin black line, open squares), and 1997 (thick black line, filled circles). Error bars show the standard deviation of replicate laboratory analyses (n=2–3 per soil sample). After one century of soil BC loss relative contributions of B6CA increased indicating that mostly the recalcitrant BC is left behind (see also Fig. 4).
  • Fig. 4. Benzene polycarboxylic acid (BPCA) pattern of black carbon for whole soil profiles (1900 and 1997) as influenced by time for whole soil profiles of 1900 and 1997 under pristine steppe (B3CA= ∑ hemimellitic, trimellitic, trimesic acids; B4CA= ∑ pyromellitic, prehnitic, mellophanic acids; B5CA=benzene pentacarboxylic acid; B6CA=mellitic acid). Error bars show the standard deviation of replicate laboratory analyses (n=2–3 per soil sample).
  • Fig. 5. Estimated turnover time of black carbon in the Russian steppe soil versus uncertainty in bulk density of the 1900 soil, shown as % change from the 1997 bulk density. The solid circle shows the best estimate of 293 y, calculated with Eq. 3 and previously published values for original sampling date (1903) and bulk density (no change from present), and assuming BC inputs from fires post-1900 were reduced to 10% of those pre-1900. The lines show the sensitivity of turnover time to the estimated uncertainty range in time since sampling (t), reduction of BC inputs post 1900 (e.g., f =0.1 denotes inputs reduced to 10% of pre-1900 value), and bulk density of the 1900 soil, which affects the estimate of change in stock (b; see Table 2). Each line shows τ for a different combination of t and f while bulk density varies on the x-axis. Lines A and B bracket the minimum and maximum possible τ . Our estimate of τ is much more sensitive to uncertainty in bulk density (two-fold effect) than it is to uncertainty in sampling year (lines A vs. C) or reduction in BC inputs (lines A vs. B or lines C vs. D) which each contribute about 12% uncertainty in τ . Line E shows that if BC inputs have not been reduced as much as we assume, the true turnover may be even faster than we have estimated here.
  • Fig. 6. Range of turnover times for the (a) fast BPCA pool (sum of B3CA, B4CA, B5CA) and (b) the slow BPCA pool (B6CA). The solid circles show the best estimate for the fast pool (a) of 83 y and the slow pool (b) of 812 y. Scenarios A to E are described in Sect. 2 and Fig. 5.

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APA

Hammes, K., Torn, M. S., Lapenas, A. G., & Schmidt, M. W. I. (2008). Centennial black carbon turnover observed in a Russian steppe soil. Biogeosciences, 5(5), 1339–1350. https://doi.org/10.5194/bg-5-1339-2008

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