Abstract
Quantifying soil organic carbon (SOC) changes is a fundamental issue in ecology and sustainable agriculture. However, the algorithm‐derived biases in comparing SOC status have not been fully addressed. Although the methods based on equivalent soil mass (ESM) and mineral‐matter mass (EMMM) reduced biases of the conventional methods based on equivalent soil volume (ESV), they face challenges in ensuring both data comparability and accuracy of SOC estimation due to unequal basis for comparison and using unconserved reference systems. We introduce the basal mineral‐matter reference systems (soils at time zero with natural porosity but no organic matter) and develop an approach based on equivalent mineral‐matter volume (EMMV). To show the temporal bias, SOC change rates were recalculated with the ESV method and modified methods that referenced to soils at time t1 (ESM, EMMM, and EMMV‐t1) or referenced to soils at time zero (EMMV‐t0) using two datasets with contrasting SOC status. To show the spatial bias, the ESV‐ and EMMV‐t0‐derived SOC stocks were compared using datasets from six sites across biomes. We found that, in the relatively C‐rich forests, SOC accumulation rates derived from the modified methods that referenced to t1 soils and from the EMMV‐t0 method were 5.7%–13.6% and 20.6% higher than that calculated by the ESV method, respectively. Nevertheless, in the C‐poor lands, no significant algorithmic biases of SOC estimation were observed. Finally, both the SOC stock discrepancies (ESV vs. EMMV‐t0) and the proportions of this unaccounted SOC were large and site‐dependent. These results suggest that although the modified methods that referenced to t1 soils could reduce the biases derived from soil volume changes, they may not properly quantify SOC changes due to using unconserved reference systems. The EMMV‐t0 method provides an approach to address the two problems and is potentially useful since it enables SOC comparability and integrating SOC datasets.
Highlights
Whether a given terrestrial soil functions as a sink or source of atmospheric carbon (C) depends on a precise quantification of the stock and accumulation rate of soil organic carbon (SOC)(Dixon et al 1994; Richter et al 1999; Jobbágy & Jackson 2000; Lal 2004; Stockmann et al.2013)
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
Temperate, and tropical forests, the increase in volume of OM contributed to 1.4 - 11.8%, 0.8 - 8.9%, and 1.1 - 9.4% of the soil volume expansions, respectively, and the increase volume in soil porosity (SP) contributed to 1.2 - 17.9%, 3.7 - 25.1%, and 5.5 - 28.8% of the soil volume expansions (Table S7)
Summary
Whether a given terrestrial soil functions as a sink or source of atmospheric carbon (C) depends on a precise quantification of the stock and accumulation rate of soil organic carbon (SOC)(Dixon et al 1994; Richter et al 1999; Jobbágy & Jackson 2000; Lal 2004; Stockmann et al.2013). Whether a given terrestrial soil functions as a sink or source of atmospheric carbon (C) depends on a precise quantification of the stock and accumulation rate of soil organic carbon (SOC). There are still large uncertainties in the estimation of SOC accumulation rate which hampers reliable assessments of the response and feedback of terrestrial ecosystems to global changes. The stock of SOC is conventionally calculated by multiplying soil mass with. Changes in SOC stocks are estimated across space or over time. The conventional approach fails to define the total soil mass because it ignores changes in soil volume (ΔV). Major problems arise when the conventional method for calculating SOC stock (Post & Kwon 2000; Jandl et al 2014; Schuur et al 2015) is used to compare SOC across space or over time (Table 1).
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