Soil organic carbon stocks are systematically overestimated by misuse of the parameters bulk density and rock fragment content

Christopher Poeplau,Axel Don,Cora Vos

doi:10.5194/soil-3-61-2017

Abstract

Abstract. Estimation of soil organic carbon (SOC) stocks requires estimates of the carbon content, bulk density, rock fragment content and depth of a respective soil layer. However, different application of these parameters could introduce a considerable bias. Here, we explain why three out of four frequently applied methods overestimate SOC stocks. In soils rich in rock fragments (> 30 vol. %), SOC stocks could be overestimated by more than 100 %, as revealed by using German Agricultural Soil Inventory data. Due to relatively low rock fragments content, the mean systematic overestimation for German agricultural soils was 2.1–10.1 % for three different commonly used equations. The equation ensemble as re-formulated here might help to unify SOC stock determination and avoid overestimation in future studies.

Highlights

Size and changes in the soil organic carbon (SOC) pool are major uncertainties in global earth system models used for climate predictions
Three out of the four SOC calculation methods produced systematically overestimated SOC stocks. These deviations are systematic errors that cannot be reduced with optimized methods to determine the parameters SOC content, BD and rock fragments content but reduce the accuracy of SOC stock estimates
Even if the rock fragment fraction might store a certain amount of organic carbon (Corti et al, 2002), which might lead to slight underestimation of SOC stocks in M4, we suggest use of this method in future studies

Summary

Introduction

Size and changes in the soil organic carbon (SOC) pool are major uncertainties in global earth system models used for climate predictions. Accurate estimation of SOC stocks is vital to understanding the links between atmospheric and terrestrial carbon (Friedlingstein et al, 2014). Estimates of global SOC stocks are based on soil inventories from regional to continental scale, involving multiplication of measured carbon content by soil bulk density (BD, oven-dry mass of soil per unit volume) and the depth of the respective soil layer (Batjes, 1996). Living root fragments > 2mm are not considered part of SOC, but usually as part of plant biomass. It is widely accepted that accurate estimates of SOC stocks should account in some way for the presence of fragments > 2 mm (Rytter, 2012; Throop et al, 2012)

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