DOI: 10.5061/dryad.b5mkkwhp0
title: Land use drives the distribution of free, physically protected, and chemically protected soil organic carbon storage at a global scale

description: Soil organic carbon (SOC) sequestration is increasingly emphasized as a
 climate mitigation solution, as scientists, policy makers, and
 land-managers prioritize enhancing belowground C storage. To identify key
 underlying drivers of total SOC distributions, we compiled a global
 dataset of soil C stocks held in three chemical forms, reflecting
 different mechanisms of organic C protection: free particulate organic C
 (fPOC), physically protected particulate organic C (oPOC), and
 mineral-protected soil organic C (mSOC). In our dataset, these three SOC
 pools were differentially sensitive to effects of climate, soil
 mineralogy, and ecosystem type, emphasizing the importance of
 distinguishing between physical and chemical C protection mechanisms. C
 stocks in all three pools varied among ecosystems: cropland soils stored
 the least amount in each pool, with forest and grassland soils both
 containing significantly more fPOC (40-60% greater in each ecosystem) than
 croplands. oPOC stocks did not significantly differ from zero in
 croplands, but were substantial in forest and grassland soils. Meanwhile,
 mSOC stocks were the greatest in grasslands and shrublands (90-100%
 greater than croplands). In cropland soils, there were no major effects of
 tillage on C storage in any of the three pools, while manure addition
 enhanced mSOC stocks, especially when added with inorganic N. Thus, the
 human land use intensity in croplands appears to reduce SOC storage in all
 major pools, depending upon management; retaining native vegetation should
 be emphasized to maintain current global SOC stocks.
description: We collected data from papers which utilized a density
 fractionation methodology to measure soil organic C (SOC) within the three
 fraction pools: free particulate organic fraction (fPOC), occluded
 particulate organic fraction (oPOC), and mineral soil organic carbon
 (mSOC). To filter through published studies reporting SOC stocks, we chose
 to extract data from papers which cited one of the following rigorous and
 influential fractionation methodologies: (Golchin et al., 1994;
 Shaymukhametov et al., 1984; Six et al., 1998; Sollins et al., 1984, 2006,
 2009; Spycher et al., 1983; Steffens et al., 2009). On Google Scholar,
 there were 3915 papers citing one of the above papers through September
 2020. We examined each publication and only extracted data from empirical
 studies, published in a peer-review journal in English, that measured SOC
 via density fractionation. When recording oPOC or mSOC data, we only
 collected data from papers which utilized sonication, sodium
 hexametaphosphate dispersion, or physical disruption (e.g., from glass
 beads) to separate oPOC from mSOC (Figure S1). Unless it was specified for
 a particular soil and reported in the respective study, we set a density
 cutoff of 1.85 g cm<sup>-3</sup> for classifying mSOC. Since
 some studies isolated C pools along a density gradient, we summed SOC
 stored in fractions above or below the density cutoff.
 To make meaningful global comparisons, we chose to only include
 papers which reported enough information to calculate the C stock in units
 of g C m<sup>2</sup>. In cases where C concentrations but not
 stocks were reported, we used soil depth and bulk density (BD, g
 cm<sup>-3</sup>) to calculate the C stock in a given pool
 (fPOC, oPOC, mSOC). Data were recorded separately for different locations,
 ecosystem types, treatments, soil depths, and fractions. Because there was
 a wide range in soil depth increments used within studies, we summed C
 stocks from specific depth intervals (e.g. 0-10 cm, 10-30 cm) into two
 categories: ‘topsoil’ (0-30cm) or subsoil (31+cm). Soil volume associated
 with each observation was calculated assuming a 1x1 m sampling area and
 the sampling depth increment reported by the study authors (e.g. 0-10 cm,
 15-30 cm).
description: # Land use drives the distribution of free, physically protected, and
 chemically protected soil organic carbon storage at a global scale
 [https://doi.org/10.5061/dryad.b5mkkwhp0](https://doi.org/10.5061/dryad.b5mkkwhp0) ### Files and variables #### File: Willard\_GCB\_2024\_processed\_data.csv **Description:**  ##### Variables * UniqueObsCodeThreePool: Code to denote each unique datapoint featuring: Study ID, simplified ecosystem, USDA soil order, Treatment name, fractionation type (fLF, oLF, mSOC), and topsoil or subsoil * Study_ID: Author name and published year * Citation: Author name, published year, journal name, and edition information * Year_sampled: Sampled year * Field_or_Lab: Study system * Latitude: Latitude of sampled location * Longitude: Longitude of sampled location * Country: Country of sampled location * Elevation: Sample elevation * Ecosystem_type: Ecosystem of sampled location * Ecosystem_simplified: Simplified ecosystem of sampled location * Crop_or_forest_type_reported: Cropping or forest type of sampled location * Cropland_addition: Nutrient addition of sampled soil * Cropland_addition_N.kg.ha.yr: Inorganic nitrogen addition of sampled soil in units of kg/ha/yr. * Cropland_addition_Manure.Mg.ha.yr: Manure addition of sampled location in units of kg/ha/yr. * Cropland_tillage: Tillage regimen of sampled soil * Soil_order_reported: Reported soil order including USDA and World Reference Base * Soil_order_simplified: USDA simplified soil order * Soil_texture: Reported soil texture * Horizon: Sampled soil horizon * Soil_volume: Calculated soil volume (100cm **100cm* * soil depth) * Top_depth: Top soil depth (cm) * bottom_depth: Bottom soil depth (cm) * Soil_pH_reported: Reported soil pH * Treatment: Treatment name * Rotation: Reported cropland rotation information * Fraction_reported: SOC fraction reported in study * Fraction_harmonized: Harmonized fraction name for dataset unification * Floatation_density: Reported density flotation in units of g/cm^3 * Two_pool: Harmonized fraction for two pool sorting (HF and LF) * Three_pool: Harmonized fraction for three pool sorting (fLF, oLF, mSOC; note manuscript uses terms fPOC, oPOC, and mSOC to convey geochemical characteristics) * Method_used_C: Reported methodology used to differentiate SOC fraction * Primary_C_pool: Primary C pool data reported from study: used to calculate data found in C_stock_for_analysis_three_pool. Obsoleted in Willard_GCB_2024_processed_data.csv due to data recombining. * Units_Primary: Units of primary data (e.g.: g/kg, %) * Units_Primary_Annotated: Annotation of primary units * Secondary_C_pool: Secondary C pool data reported from study (used to calculate to g m^2) : used to calculate data found in C_stock_for_analysis_three_pool. Obsoleted in Willard_GCB_2024_processed_data.csv due to data recombining * Units_Secondary: Units of secondary data (e.g.: g/kg, %) * MAT_reported: Mean annual temperature (degrees Celsius) reported (used when available) * MAP_reported: Mean annual precipitation (mm) reported (used when available) * MAT_added: Mean annual temperature (degrees Celsius) extracted from Terra Climate * MAP_added: Mean annual precipitation (mm) extracted from Terra Climate * MAT_used: Mean annual temperature (degrees Celsius) used * MAP_used: Mean annual precipitation (mm) used * T_PH_H2O: HWSD topsoil pH; extracted from Harmonized World Soil Database * S_PH_H2O: HWSD subsoil pH; extracted from Harmonized World Soil Database * T_CLAY: HWSD topsoil clay percentage; extracted from Harmonized World Soil Database * S_CLAY: HWSD subsoil clay percentage; extracted from Harmonized World Soil Database * T_BULK_DEN: HWSD topsoil bulk density (g/cm^3); extracted from Harmonized World Soil Database * S_BULK_DEN: HWSD subsoil bulk density (g/cm^3); extracted from Harmonized World Soil Database * BD_reported: Bulk density reported (g cm^3; used when available) * BD_used: Bulk density used (g cm^3) * Calculated_C_pool: Calculated C pool size (Primary_C_pool multiplied by Secondary_C_pool C * BD_used * summed_vol where appropriate to units of g m^2; calculated prior to combining along UniqueObsCodeThreePool) * layer: topsoil (0-30cm) or subsoil (31+cm) * UniqueObsCodeTwoPool: Code to denote each unique datapoint featuring: Study ID, simplified ecosystem, USDA soil order, Treatment name, fractionation type (HF or LF), and topsoil or subsoil * C_stock_for_analysis_three_pool: Combined C pool along UniqueObsCodeThreePool: used as response C pool data in manuscript models and in units of g m^2 * PH_three_pool: Combined soil pH along UniqueObsCodeThreePool * summed_vol: summed soil volume along UniqueObsCodeThreePool and in units of cm^3 **Note: NA's denote unreported data from publications** #### File: Willard\_GCB\_2024\_raw\_data.csv **Description:**  ##### Variables * Study_ID: Author name and published year * Citation: Author name, published year, journal name, and edition information * Year_sampled: Sampled year * Field_or_Lab: Study system * Latitude: Latitude of sampled location * Longitude: Longitude of sampled location  * Country: Country of sampled location * Elevation: Sample elevation * Ecosystem_type: Ecosystem of sampled location * Ecosystem_simplified: Simplified ecosystem of sampled location * Crop_or_forest_type_reported: Cropping or forest type of sampled location * Cropland_addition: Nutrient addition of sampled soil * Cropland_addition_N kg/ha/yr: Inorganic nitrogen addition of sampled soil in units of kg/ha/yr. * Cropland_addition_Manure Mg ha yr: Manure addition of sampled location in units of kg/ha/yr. * Cropland_tillage: Tillage regimen of sampled soil * Soil_order_reported: Reported soil order including USDA and World Reference Base * Soil_order_simplified: USDA simplified soil order * Soil_texture: Reported soil texture * Horizon: Sampled soil horizon * Soil_volume: Calculated soil volume (100cm*100cm* * soil depth) * Top_depth: Top soil depth (cm) * bottom_depth: Bottom soil depth (cm) * Soil_pH_reported: Reported soil pH * Treatment: Treatment name * Rotation: Reported cropland rotation information * Fraction_reported: SOC fraction reported in study * Fraction_harmonized: Harmonized fraction name for dataset unification * Floatation_density: Reported density flotation in units of g/cm^3 * Two_pool: Harmonized fraction for two pool sorting (HF and LF) * Three_pool: Harmonized fraction for three pool sorting (fLF, oLF, mSOC; note manuscript uses terms fPOC, oPOC, and mSOC to convey geochemical characteristics) * Method_used_C: Reported methodology used to differentiate SOC fraction * Primary_C_pool: Primary C pool data reported from study: used to calculate data found in C_stock_for_analysis_three_pool. Obsoleted in Processed_data file due to data recombining. * Units_Primary: Units of primary data (e.g.: g/kg, %) * Units_Primary_Annotated: Annotation of primary units * Secondary_C_pool: Secondary C pool data reported from study (used to calculate to g m^2) : used to calculate data found in C_stock_for_analysis_three_pool. Obsoleted in Processed_data file due to data recombining. * Units_Secondary: Units of secondary data (e.g.: g/kg, %) * MAT_reported: Mean annual temperature (degrees Celsius) reported (used when available) * MAP_reported: Mean annual precipitation (mm) reported (used when available) * MAT_added: Mean annual temperature (degrees Celsius) extracted from Terra Climate * MAP_added: Mean annual precipitation (mm) extracted from Terra Climate * MAT_used: Mean annual temperature (degrees Celsius) used  * MAP_used: Mean annual precipitation (mm) used * T_PH_H2O: HWSD topsoil pH; extracted from Harmonized World Soil Database * S_PH_H2O: HWSD subsoil pH; extracted from Harmonized World Soil Database * T_CLAY: HWSD topsoil clay percentage; extracted from Harmonized World Soil Database * S_CLAY: HWSD subsoil clay percentage; extracted from Harmonized World Soil Database * T_BULK_DEN: HWSD topsoil bulk density (g/cm^3); extracted from Harmonized World Soil Database * S_BULK_DEN:HWSD subsoil bulk density (g/cm^3); extracted from Harmonized World Soil Database * BD_reported: Bulk density reported (g cm^3; used when available) * BD_used: Bulk density used (g cm^3) * Calculated_C_pool: Calculated C pool size (Primary_C_pool multiplied by Secondary_C_pool C * BD_used * summed_vol where appropriate to units of g m^2; calculated prior to combining along UniqueObsCodeThreePool) and in units of g m^2 * layer: topsoil (0-30cm) or subsoil (31+cm) Note: NA's denote unreported data from publications ## Code/software Data analysis was done in R (version 4.3.1). Packages for statistics and graphing included: lme4, lmerTest, MuMIn, plotrix, stringr, dplyr, tidyverse, ggplot2, and lsmeans. ## Access information Data was derived from the following sources: * Provided is the list of publications from which data was extracted and utilized within the analysis: 1.         Alcántara, V., Don, A., Vesterdal, L., Well, R. & Nieder, R. Stability of buried carbon in deep-ploughed forest and cropland soils - Implications for carbon stocks. *Sci Rep* **7**, 1–12 (2017). 2.         Alhameid, A., Ibrahim, M., Kumar, S., Sexton, P. & Schumacher, T. E. Soil Organic Carbon Changes Impacted by Crop Rotational Diversity under No-Till Farming in South Dakota, USA. *Soil Science Society of America Journal* **81**, 868–877 (2017). 3.         Alvarez, C. R., Alvarez, R., Grigera, M. S. & Lavado, R. S. Associations between organic matter fractions and the active soil microbial biomass. *Soil Biol Biochem* **30**, 767–773 (1998). 4.         Alvarez, R. & Alvarez, C. R. 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