Soil carbon debt of 12,000 years of human land use

Abstract

Human appropriation of land for agriculture has greatly altered the terrestrial carbon balance, creating a large but uncertain carbon debt in soils. Estimating the size and spatial distribution of soil organic carbon (SOC) loss due to land use and land cover change has been difficult but is a critical step in understanding whether SOC sequestration can be an effective climate mitigation strategy. In this study, a machine learning-based model was fitted using a global compilation of SOC data and the History Database of the Global Environment (HYDE) land use data in combination with climatic, landform and lithology covariates. Model results compared favorably with a global compilation of paired plot studies. Projection of this model onto a world without agriculture indicated a global carbon debt due to agriculture of 133 Pg C for the top 2 m of soil, with the rate of loss increasing dramatically in the past 200 years. The HYDE classes "grazing" and "cropland" contributed nearly equally to the loss of SOC. There were higher percent SOC losses on cropland but since more than twice as much land is grazed, slightly higher total losses were found from grazing land. Important spatial patterns of SOC loss were found: Hotspots of SOC loss coincided with some major cropping regions as well as semiarid grazing regions, while other major agricultural zones showed small losses and even net gains in SOC. This analysis has demonstrated that there are identifiable regions which can be targeted for SOC restoration efforts.agriculture | soil organic matter | climate change | soil degradation T he incredible rise of human civilizations and the continuing sustainability of current and future human societies are inextricably linked to soils and the wide array of services soils provide (1-3). Human population and economic growth has led to an exponential rise in use of soil resources. Roughly 50 million km 2 of soils are currently being managed to some degree by humans for food, fiber, and livestock production (4), leading to the declaration that we live on a "used planet" (5). The consequences of human domination of soil resources are far ranging (6, 7): accelerated erosion, desertification, salinization, acidification, compaction, biodiversity loss, nutrient depletion, and loss of soil organic matter (SOM).Of these soil threats, loss of SOM has received the most attention, due to the critical role SOM plays in the contemporary carbon cycle (8, 9) and as a key component of sustaining food production (10, 11). Despite the intense research interest in SOM and soil organic carbon (SOC) as the dominant component of SOM, there remain many unknowns (12) that impede progress in implementing sound land management strategies to rebuild SOC stocks (13).Conversion of native soil to agricultural uses typically leads to a decline in SOC levels (14-16). The rate and extent of decline in SOC stocks should vary greatly across the globe, due to differences in soil properties, climate, type of land-use conversion, and, importantly, the specific manage...

No silver bullet for digital soil mapping: country-specific soil organic carbon estimates across Latin America

“…Thus, larger uncertainty dominates countries with larger SOC pools probably because available data do not capture the large spatial heterogeneity of SOC stocks. We highlight that the WoSIS dataset is a unique and invaluable effort that has proven to generate global SOC predictions Sanderman et al, 2017), but there is a global need to increase information and networking capabilities for SOC (Harden et al, 2017). This study generated predictions of SOC across Latin America but also provided information about the main relationships driving the spatial distribution of SOC.…”

Technologically achievable soil organic carbon sequestration in world croplands and grasslands

“…Likely, M1 gives more plausible projections as it is based on measured SOC sequestration rates (Mg C yr −1 ) per bioclimatic zone and land cover class (i.e., cropland and grassland), whereas M2 makes inferences with respect to annual C increases vis à vis ‘present’ SOC stocks at a given location. Such stocks, however, are computed from best available (and often only available) data for organic carbon, bulk density, and proportion of coarse fragments (>2 mm) that were sampled and analysed between 1950 and 2015 (Arrouays et al, ; Batjes, ; Minasny, Malone, et al, ; Sanderman et al, ). Further, an implicit assumption of M2 is that possible C gains will be greatest where present SOC stocks are largest, which is counter‐intuitive (see Levèvre, Fatma, Viridiana, & Wiese, ; UNEP, ).…”

“…Since the 1850s, some 60 to 150 Pg C held in soil organic matter (SOM) have been lost due to land use‐conversion, agriculture, and disturbance (Canadell et al, ; Lal, ; Sanderman, Hengl, & Fiske, ). Within the next two decades, the global demand for food is projected to increase by 50%, demand for water by 35% to 60%, and demand for energy by 45% (UNEP, ).…”

No Silver Bullet for Digital Soil Mapping: Country-specific Soil Organic Carbon Estimates across Latin America

“…Thus, larger uncertainty dominates countries with larger carbon pools probably because available data does not capture the large spatial heterogeneity of SOC stocks. We highlight that the WoSIS dataset is a unique and invaluable effort that has proven to generate global SOC predictions Sanderman et al, 2017), but there is a global need to increase information and networking 25 capabilities for SOC (Harden et al, 2017).…”