Soils' dirty little secret: Depth‐based comparisons can be inadequate for quantifying changes in soil organic carbon and other mineral soil properties

Abstract

Quantifying changes in soil organic carbon (SOC) stocks and other soil properties is essential for understanding how soils will respond to land management practices and global change. Although they are widely used, comparisons of SOC stocks at fixed depth (FD) intervals are subject to errors when changes in bulk density or soil organic matter occur. The equivalent soil mass (ESM) method has been recommended in lieu of FD for assessing changes in SOC stocks in mineral soils, but ESM remains underutilized for SOC stocks and has rarely been used for other soil properties. In this paper, we draw attention to the limitations of the FD method and demonstrate the advantages of the ESM approach. We provide illustrations to show that the FD approach is susceptible to errors not only for quantifying SOC stocks but also for soil mass‐based properties such as SOC mass percent, C:N mass ratio, and δ13C. We describe the ESM approach and show how it mitigates the FD method limitations. Using bulk density change simulations applied to an empirical dataset from bioenergy cropping systems, we show that the ESM method provides consistently lower errors than FD when quantifying changes in SOC stocks and other soil properties. To simplify the use of ESM, we detail how the method can be integrated into sampling schemes, and we provide an example R computer script that can perform ESM calculations on large datasets. We encourage future studies, whether temporal or comparative, to utilize sampling methods that are amenable to the ESM approach. Overall, we agree with previous recommendations that ESM should be the standard method for evaluating SOC stock changes in mineral soils, but we further suggest that ESM may also be preferred for comparisons of other soil properties including mass percentages, elemental mass ratios, and stable isotope composition.

Decreased land use intensity improves surface soil quality on marginal lands

“…For deep core analyses, the equivalent soil mass (ESM) method was used to account for significant differences in soil masses due to management impacts on BD (Mikha et al, 2013;von Haden et al, 2020). Fixed depth concentrations of SOC, STN, NO 3 , P, and K were transformed to fixed mass stocks in each depth using an R (Version 1.3.1093; RStudio, PBC) computer script provided by von Haden et al ( 2020).…”

“…SOC, soil organic C; STN, soil total N; P, plant-available P; K, plant-available K. The p values are for the main effects of land use on ESM soil properties sliced by soil depth. Even though SOC, STN, NO 3 , P, and K are presented by sampled depth, they are actually ESM soil properties based on "ESM depth" rather than the sampled depth; therefore, the ESM soil properties do not necessarily pertain to the sampled depth, but instead they are the estimated values that would have been obtained if the soils were sampled according to the reference soil mass rather than the sampled depth(von Haden et al, 2020). BC, business as usual-row crop; BP, business as usual-pasture; CN, Conservation Reserve Program-new; CO, Conservation Reserve Program-old.…”

Carbon sequestration in Mediterranean soils following afforestation of abandoned crops: Biases due to changes in soil compaction and carbonate stocks

“…For longer time spans (millennia), more complex approaches are needed, such as the VMC method (Sollins & Gregg, 2017), which also considers erosion, leaching, and bioturbation. Regardless of the chosen method, the changes in soil compaction can no longer be ignored in the research about soil OC sequestration, as stressed recently (Von Haden et al, 2020).…”

Long-Term Effects of Organic Amendments on Soil Organic Matter Quantity and Quality in Conventional Cropping Systems in Switzerland

“…Soil bulk density was measured with the ring method by vertically inserting rings (100 cm 3 ) at 3-7 and 13-17 cm depths at the center of each plot and averaged to approximate bulk density at 0-20 cm depth [11]. To avoid bias on SOC stocks due to the correlation between SOC and bulk density, treatments were compared on equivalent soil mass basis using the bulk density (1.47 g cm −3 ) of NPK treatment as reference [44]. SOC stocks (Mg•ha −1 within the first 20 cm of depth) were calculated as follows:…”