Faster carbon accumulation in global forest soils

Comparing soil organic carbon (SOC) stocks across space and time is a fundamental issue in global ecology. However, the conventional approach fails to determine SOC stock in an equivalent volume of mineral-soil, and therefore, SOC stock changes can be under- or overestimates if soils swell or shrink during forest development or degradation. Here, we propose to estimate SOC stock as the product of mineral-soil mass in an equivalent mineral-soil volume and SOC concentration expressed as g C Kg-1 mineral-soil. This method enables researchers to compare SOC stocks across space and time. Our results show an unaccounted SOC accumulation of 2.4 - 10.1 g C m-2 year-1 in the 1m surface mineral-soils in global forests. This unaccounted SOC amounts to an additional C sink of 0.12 – 0.25 Pg C year-1, which equals 30 – 62% of the previously estimated annual SOC accumulation in global forests. This finding suggests that forest soils are stronger C sinks than previously recognized.


INTRODUCTION
Whether a given terrestrial soil functions as a sink or source of atmospheric carbon (C) depends 99 on a precise quantification of the stock and accumulation rate of soil organic carbon (SOC) 100 (Dixon et al. 1994;Richter et al. 1999 where Modified Cdensity and Conventional Cdensity refers to SOC stock estimated by the 189 conventional method and our modified method, respectively (g C m -2 soil); ΔCdensity refers to 190 the unaccounted SOC stock (g C m -2 soil) for a given sampled volume of soil if comparing 191 SOC in EMSV ; BD is g soil cm -3 soil; V is the sampled soil volume for a given soil horizon, 192 cm 3 m -2 ; OM is the organic matter concentration, g OM kg -1 soil; "i" refers to the number of 193 soil horizon for a given soil profile; 0.50 is the conversion factor from OM to C (Pribyl 194 2010). BDm is the BD of mineral soil (g mineral soil cm -3 soil); OMm is the organic matter 195 associated with each unit of mineral soil (g OM kg -1 mineral soil); ΔVi refers to the ΔV of the 196 "i"th horizon in the profile (cm 3 m -2 ); "n" refers to the last (deepest) soil horizon for a given

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If the soil profile contains several horizons (n > 1), VOM and ΔVSP in the "i"th horizon (i <= n 215 -1) can be calculated as:

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For the last soil horizons (i = n and n > 1) or if the soil profile contains only one horizon (n = 220 1), VOM and ΔVSP in the "n" horizon can be calculated as: Where ΔV refers to the total soil volume change for a sampled soil profile; "i" refers to the 229 number of soil horizon in the soil profile; BDm is the BD of mineral soil (g mineral soil cm -3 230 soil); Vi is the sampled soil volume of the "i"th horizon, cm 3 m -2 ; OMm refers to the organic 231 matter content (g OM kg -1 mineral soil); ΔVi is the soil volume change in the "i"th horizon;  (Table S1). Thereby, BDmn+1 and OMmn+1 were calculated with those equations, where "h" 270 equals the original depth of a given soil profile plus its expanded depth (Δh). Afterwards, the 271 unaccounted C for each soil profile, plot, and site were estimated by equation 5.

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New estimation of forest SOC accumulation rate at a long-term study site. In order to 273 quantify the method-derived uncertainty in SOC accumulation rate, we re-analyzed the soil C  calculations suggest that this mature forest is even a stronger C sink than previously estimated.

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On the global scale, our new calculations suggest that the conventional method profoundly  (Table S6). Consequently, a more   None 581 §: "I" indicates error source results from the use of SOC unit of g C kg -1 soil; "II" refers to error source from soil volume change.        Table S7. Theoretical change patterns of soil volume and unaccounted SOC in the standardized 10 cm mineral soil across biomes.