Soil Carbon Regulating Ecosystem Services in the State of South Carolina, USA

: Sustainable management of soil carbon (C) at the state level requires valuation of soil C regulating ecosystem services (ES) and disservices (ED). The objective of this study was to assess the value of regulating ES from soil organic carbon (SOC), soil inorganic carbon (SIC), and total soil carbon (TSC) stocks, based on the concept of the avoided social cost of carbon dioxide (CO 2 ) emissions for the state of South Carolina (SC) in the United States of America (U.S.A.) by soil order, soil depth (0–200 cm), region and county using information from the State Soil Geographic (STATSGO) database. The total estimated monetary mid-point value for TSC in the state of South Carolina was $124.36B (i.e., $124.36 billion U.S. dollars, where B = billion = 10 9 ), $107.14B for SOC, and $17.22B for SIC. Soil orders with the highest midpoint value for SOC were: Ultisols ($64.35B), Histosols ($11.22B), and Inceptisols ($10.31B). Soil orders with the highest midpoint value for SIC were: Inceptisols ($5.91B), Entisols ($5.53B), and Alﬁsols ($5.0B). Soil orders with the highest midpoint value for TSC were: Ultisols ($64.35B), Inceptisols ($16.22B), and Entisols ($14.65B). The regions with the highest midpoint SOC values were: Pee Dee ($34.24B), Low Country ($32.17B), and Midlands ($29.24B). The regions with the highest midpoint SIC values were: Low Country ($5.69B), Midlands ($5.55B), and Pee Dee ($4.67B). The regions with the highest midpoint TSC values were: Low Country ($37.86B), Pee Dee ($36.91B), and Midlands ($34.79B). The counties with the highest midpoint SOC values were Colleton ($5.44B), Horry ($5.37B), and Berkeley ($4.12B). The counties with the highest midpoint SIC values were Charleston ($1.46B), Georgetown ($852.81M, where M = million = 10 6 ), and Horry ($843.18M). The counties with the highest midpoint TSC values were Horry ($6.22B), Colleton ($6.02B), and Georgetown ($4.87B). Administrative areas (e.g., counties, regions) combined with pedodiversity concepts can provide useful information to design cost-efﬁcient policies to manage soil carbon regulating ES at the state level.


Introduction
Economic valuation of soil carbon is vital for achieving the United Nations (UN) Sustainable Development Goals (SDGs), especially SDG 13: "Take urgent action to combat climate change and its impacts on future climate" [1]. The ecosystem services (ES) framework is often used in connection with UN SDGs because it is focused on the economic valuation of benefits (ES) and/or disservices (ED) people obtain from nature [2]. The ES framework includes three general categories of services: provisioning, regulating/maintenance, and cultural supporting services [2]. Although TSC is composed of SOC Table 1. Total soil carbon: soil organic matter (SOM), soil organic carbon (SOC), soil inorganic carbon (SIC), and carbon sequestration pathway (adapted from Mikhailova et al., 2019 [8]).

Biotic Abiotic
Soil organic matter (SOM) of <2 mm particle size fraction Soil organic carbon (SOC) Soil inorganic carbon (SIC) -Fresh residue, decomposing organic matter, stable organic matter (humus), and living organisms. or -"Continuum of organic material in all stages of transformation and decomposition or stabilization [12]." The ES framework is increasingly being used to address environmental concerns (e.g., global warming, climate change, etc.), but because of "the difficulty in relating soil properties to ES, soil ES are still not fully considered in the territorial planning decision process" [16]. According to Fossey et al., 2020 [16], soil databases play an essential role in assessing ES/ED in territorial planning. For sustainable soil C management decisions at the state level and its counties, it is critical to determine soil C and the distribution of its social costs within the state overall and by individual counties linked to biophysical units (e.g., soil orders). This type of analysis will allow prioritization of soil C management within the state based on this distribution. The hypothesis of this study is that pedodiversity concepts overlayed with administrative units (Figures 1 and 2) can be used to identify spatial patterns of soil carbon hotspots for sustainable management.
The specific objective of this study was to assess the value of SOC, SIC, and TSC in the state of South Carolina (U.S.A.) based on the social cost of carbon (SC-CO 2 ) and avoided emissions provided by carbon sequestration, which the U.S. Environmental Protection Agency (EPA) has determined to be $46 per metric ton of CO 2 , which is applicable for the year 2025 based on 2007 U.S. dollars and an average discount rate of 3% [17]. This study provides the monetary values of SOC, SIC, and TSC for soil depth (0-200 cm) across the state and by considering different spatial aggregation levels (i.e., region, county) using State Soil Geographic (STATSGO) database, and information previously reported by Guo et al. (2006) [18].

Materials and Methods The Accounting Framework
This study used both biophysical (science-based, Figure 1) and administrative (boundarybased, Figure 3) accounts to calculate monetary values for SOC, SIC, and TSC (Tables 2 and 3). Table 2. A conceptual overview of the accounting framework used in this study (adapted from Groshans et al., 2018 [19]).

County (Region)
Total Area (km 2 ) (Rank) The present study is based on the SOC [21], SIC [21], TSC estimated values for the SOC, SIC, and TSC storage (in Mg or metric tons) and content (in kg m −2 ) in the contiguous U.S. from Guo et al. (2006) [18]. A monetary valuation for TSC was calculated using the social cost of carbon (SC-CO 2 ) of $46 per metric ton of CO 2 , which is applicable for 2025 based on 2007 U.S. dollars and an average discount rate of 3% [17]. According to the EPA, the SC-CO 2 is intended to be a comprehensive estimate of climate change damages. Still, it can underestimate the true damages and cost of CO 2 emissions due to the exclusion of various important climate change impacts recognized in the literature [17]. Soil carbon (SC) storage and content numbers were then converted to U.S. dollars and dollars per square meter in Microsoft Excel using the following equations, with a social cost of carbon of $46/Mg CO 2 : (2) Table 4 presents area-normalized content (kg m −2 ) and monetary values ($ m −2 ) of soil carbon, which were used to estimate total soil carbon storage and total soil carbon value by multiplying corresponding content (values) numbers by an area of a particular soil order within a county (region) ( Table 3). For example, for the soil order of Entisols, Guo et al. (2006) [18] reported an area-normalized midpoint SOC content number of 8.0 kg·m −2 in the upper 2 m (Table 4), which was used to calculate the total SOC storage in soil order by multiplying its area in particular county or region. Then, the reported area-normalized midpoint SOC content number of 8.0 kg·m −2 in the upper 2 m (Table 4) was converted to monetary values ($ m −2 ) of soil organic carbon using a social cost of carbon (SC-CO 2 ) of $46 per metric ton of CO 2 (2007 U.S. dollars with an average discount rate of 3% [17]), which is $1.35 m −2 to calculate the total monetary value of SOC storage.

Results
The total estimated monetary mid-point value for TSC in the state of South Carolina was $124.36B (i.e., $124.36 billion U.S. dollars, where B = billion = 10 9 ), $107.14B for SOC, and $17.22B for SIC. The state of South Carolina ranked 31st for TSC, 25th for SOC, and 32nd for SIC. Figure 3 shows the distribution of soil carbon by South Carolina regions.             [17]).

Discussion
Pedodiversity (soil diversity) in South Carolina is a source of various ES goods, services, and disservices (ED). This study demonstrates the value of regulating ES/ED in the state and its regions and counties. According to Mikhailova et al. (2021) [22], taxonomic pedodiversity (e.g., soil order) "provides a general description of the stock, its type, and spatial distribution," which is often rereferred to as a "portfolio" to describe the link between pedodiversity and its stocks. South Carolina soil "portfolio" is composed of seven soil orders: Entisols (9% of the total state area), Inceptisols (9%), Histosols (1%), Alfisols (9%), Mollisols (0%), Spodosols (2%), and Ultisols (70%) (Figure 4, Table 11). Highly weathered Ultisols have the highest proportion of the total area of the state (Figure 4a), which contributes to the highest SOC and TSC storage and their associated social costs of carbon. The contribution of SIC to associated social costs of carbon is small at the state level and primarily associated with Inceptisols, Entisols, and Alfisols. Soil "portfolio" differs within each county, and Figure 5 illustrates this concept using three counties from different regions: Anderson (Upstate), Newberry (Midlands), and Colleton (Low Country). In all three cases, Ultisols occupy the largest proportion of the area in each county. The type of soil order influences the value of SOC storage. In Colleton County, the soil order of Histosols contributes to the social costs of C as much as the Ultisols even though its area is much smaller ( Figure 5) because of high SOC content of 142.5 kg m −2 . Figures 4 and 5 represent social costs of soil C from different point of views: "avoided" versus "realized" social costs. Soil carbon stored in the soil represents the "avoided social cost" of soil C if not converted to CO 2 and released into the atmosphere. When CO 2 is released into the atmosphere, it becomes the "realized social cost" because of the damages from global warming. In South Carolina, Histosols and Alfisols are particularly sensitive to climate change because of relatively high soil C content, which is most likely to experience higher decomposition rates due to increases in temperature and precipitation. All soils in the state of South Carolina have low recarbonization potential. Table 11. Distribution of soil carbon regulating ecosystem services in the state of South Carolina (U.S.A.) by soil order (photos courtesy of USDA/NRCS [23]) in the upper 2-m depth based on avoided or realized the social cost of CO 2 (SC-CO 2 ) of $46 (USD) per metric ton of CO 2 [17]. Amelung et al. (2020) [24] proposed linking soil C sequestration to food security using soil-and site-specific potentials and opportunities for soil C sequestration. In this respect, the state of South Carolina faces serious limitations in both soil-(dominated by highlyweathered soil order, Ultisols) and site-specific (high demand for soil C due to rapid urbanization and population growth; rapid changes in coastal areas, etc.) potentials. Soil order Histosols (which often contains organic soils) is located in the coastal areas of the state and can be drained for agriculture and urbanization, leading to high losses of soil C into the atmosphere [24]. Recarbonization of soils in the state of South Carolina may not be economically feasible due to past excessive levels of soil degradation [25], high fertilization and liming costs (including transportation) associated with increasing soil C in mostly highly-weathered and acid soils in the state. It should be noted that the reported soil survey-based C values may be an overestimate of actual soil C measured in the field, but the overall trends for the soil orders should be similar [10]. Soil C should be regularly monitored to quantify soil contributions to ES and its flows [26,27].

Conclusions
This study examined the application of soil diversity (pedodiversity) concepts (taxonomic) and its measures to value soil C regulating ES/ED in the state of South Carolina (U.S.A.), its administrative units (regions, counties), and the systems of soil classification (e.g., U.S. Department of Agriculture (USDA) Soil Taxonomy, Soil Survey Geographic (SSURGO) Database) to be considered in territorial planning. Pedodiversity provides a critical context (e.g., "portfolio-effect," "distribution-effect," "evenness-effect," etc.) for analyzing, interpreting, and reporting ES/ED within the ES framework for sustainable management of soil carbon within the state. Taxonomic pedodiversity in South Carolina exhibits high soil diversity (7 soil orders: Entisols, Inceptisols, Histosols, Alfisols, Mollisols, Spodosols, and Ultisols), which is not evenly distributed within the state, regions, and counties. In general, pedodiversity tends to increase from the Upstate to Low Country, where three counties (Beaufort, Colleton, and Jasper) have all seven orders. Similarly, soil carbon storage and its associated social costs tend to increase in a similar geographic direction. Ultisols occupy the highest proportion of the state area (70%) and have the highest SOC storage and related social costs of carbon ($64.30B). The contribution of SIC to associated social costs of carbon is small ($17.22B) at the state level and primarily associated with Inceptisols ($5.91B), Entisols ($5.53B), and Alfisols ($5.00B). In the state of South Carolina, Histosols and Alfisols are particularly sensitive to climate change because of relatively high soil C content, which is most likely experience higher rates of decomposition due to increases in temperature and precipitation. All soils in the state of South Carolina have low recarbonization potential. Administrative areas (e.g., counties, regions) combined with pedodiversity concepts can provide useful information to design cost-efficient policies to manage soil carbon regulating ES at the state level.