Determining Soil Bulk Density for Carbon Stock Calculations: A Systematic Method Comparison

Abstract

Accurate and effective determination of soil bulk density (BD) is needed to monitor soil organic C (SOC) stocks and SOC stock changes. However, BD measurements are often lacking in soil inventories and BD is estimated by pedotransfer functions with substantial uncertainty. In a systematic method comparison, we evaluated different methods for BD determination in the field by comparing the performance of MINI (5 cm 3 ) and BIG (250 cm 3 ) sample rings and of three driving hammer probes differing in diameter, material, and extraction method. Bulk density determined with 100-cm 3 sample rings was defined as the reference method (REF). All methods were tested at five depth increments in nine subplots at four sites with differing soil texture and SOC content. All methods determined BD in the depth increments with low systematic error (8% for probes and 2% for sample rings). The random error of the probe samples was, on average, 50% higher than that of the ring samples when the cores of the probes were adequately corrected for compaction or stretching. The BD was significantly overestimated (by 2%) when determined with MINI rings, and the variation in BD was not reduced with BIG sample rings rather than the smaller REF sample rings. The performance of the driving hammer technique varied widely among probe types and sites. The sheath probe had the smallest systematic error of all probes tested and is recommended for soil inventories. All methods for estimating BD had smaller errors than pedotransfer functions.Abbreviations: BD, bulk density; CV, coefficient of variation; MPE, mean prediction error; SDPE, standard deviation of the prediction error; SOC, soil organic carbon. C arbon storage in soils exceeds that in vegetation and the atmosphere (Ciais et al., 2013). Thus, small changes in soil organic C (SOC) stocks could have severe impacts on the global C cycle. Reliable measurements of C concentration are an important prerequisite for detecting such small changes in SOC stocks (Goidts et al., 2009). Information on soil bulk density (BD) is essential in converting weight-based concentration data to volume-or area-based stock data. However, BD is a parameter that is only partly or never sampled in many soil inventories (Gruneberg et al., 2014;Reynolds et al., 2013;Saby et al., 2008). Pedotransfer functions are often applied instead to predict soil BD on the basis of SOC or soil organic matter content and soil texture data (Arrouays et al., 2012). It has been shown that most pedotransfer functions are suitable only for the agro-pedo-climatic conditions prevailing at the sites used to fit these functions (Martin et al., 2009). Under different conditions, they lead to substantial systematic errors (De Vos et al., 2005;Nanko et al., 2014;Vasiliniuc and Patriche, 2015). For a soil with a BD of 1.4 g cm −3 , a systematic measurement error of −0.01 to −0.51 g cm −3 (De Vos et al., 2005) would result in SOC stocks being underestimated by 1 to 36%. Katja Core Ideas• Little is known about the methodological errors of bulk densit...

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

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