The global magnitude (Pg) of soil organic carbon (SOC) is 677 to 0.3-m, 993 to 0.5-m, and 1,505 to 1-m depth. Thus, ~55% of SOC to 1-m lies below 0.3-m depth. Soils of agroecosystems are depleted of their SOC stock and have a low use efficiency of inputs of agronomic yield. This review is a collation and synthesis of articles published in peer-reviewed journals. The rates of SOC sequestration are scaled up to the global level by linear extrapolation. Soil C sink capacity depends on depth, clay content and mineralogy, plant available water holding capacity, nutrient reserves, landscape position, and the antecedent SOC stock. Estimates of the historic depletion of SOC in world soils, 115-154 (average of 135) Pg C and equivalent to the technical potential or the maximum soil C sink capacity, need to be improved. A positive soil C budget is created by increasing the input of biomass-C to exceed the SOC losses by erosion and mineralization. The global hotspots of SOC sequestration, soils which are farther from C saturation, include eroded, degraded, desertified, and depleted soils. Ecosystems where SOC sequestration is feasible include 4,900 Mha of agricultural land including 332 Mha equipped for irrigation, 400 Mha of urban lands, and ~2,000 Mha of degraded lands. The rate of SOC sequestration (Mg C ha year ) is 0.25-1.0 in croplands, 0.10-0.175 in pastures, 0.5-1.0 in permanent crops and urban lands, 0.3-0.7 in salt-affected and chemically degraded soils, 0.2-0.5 in physically degraded and prone to water erosion, and 0.05-0.2 for those susceptible to wind erosion. Global technical potential of SOC sequestration is 1.45-3.44 Pg C/year (2.45 Pg C/year).

Despite a 2.3% increase in world cereal production in 2019 over 2018, the number of people facing severe food insecurity may double from 135 million in January 2020 to 265 million by the end of 2020. The problem of food and nutritional insecurity is severe in urban centers, where the global population is projected to increase (%/year) by 1.

Sustainable soil carbon sequestration practices need to be rapidly scaled up and implemented to contribute to climate change mitigation. We highlight that the major potential for carbon sequestration is in cropland soils, especially those with large yield gaps and/or large historic soil organic carbon losses. The implementation of soil carbon sequestration measures requires a diverse set of options, each adapted to local soil conditions and management opportunities, and accounting for site-specific trade-offs. We propose the establishment of a soil information system containing localised information on soil group, degradation status, crop yield gap, and the associated carbon-sequestration potentials, as well as the provision of incentives and policies to translate management options into region- and soil-specific practices.

The current and projected anthropogenic global warming and the attendant increase in the severity and extent of soil degradation may exacerbate the intensity and duration of drought occurrence in agroecosystems. Restoration of the soil organic matter (SOM) content of degraded/depleted soils can increase soil water retention (SWR) more at field capacity (FC) than that at the permanent wilting point (PWP), and thus increase the plant available water capacity (PAWC). The magnitude of increase in PAWC may depend on soil texture and the initial SOM content. Thus, restoration of the SOM content of degraded/depleted soils can make them as well as agroecosystems climate-resilient. Management practices which enhance soil health by restoring SOM content include conservation agriculture, cover cropping, residue mulching, and complex farming systems involving integration of crops with trees and livestock. Such technologies must be fine-tuned under site-specific conditions. Additional research is needed to establish the cause-effect relationship between increase in SOM content and PAWC and the ameliorative effect on drought-resilience for diverse crops and cropping systems. 1 INTRODUCTION Global warming has already reached 1 • C above the preindustrial levels (Lindsey & Dahlman, 2020; Sánchez-Lugo, Berrisford, Morice, & Nicolas, 2018), which in turn has aggravated the severity of drought caused by soil moisture deficit (Berg & Sheffield, 2018). Additionally, the positive net ecosystem exchange (NEE), caused by soilwater deficit, can make soil a net source of CO 2 (Green et al., 2019), especially following some repeated summer droughts (Sowerby, Emmett, Tietema, & Beier, 2008). Further, anthropogenic perturbations may even destabilize the

The soil carbon (C) stock, comprising soil organic C (SOC) and soil inorganic C (SIC) and being the largest reservoir of the terrestrial biosphere, is a critical part of the global C cycle. Soil has been a source of greenhouse gases (GHGs) since the dawn of settled agriculture about 10 millenia ago. Soils of agricultural ecosystems are depleted of their SOC stocks and the magnitude of depletion is greater in those prone to accelerated erosion by water and wind and other degradation processes. Adoption of judicious land use and science-based management practices can lead to re-carbonization of depleted soils and make them a sink for atmospheric C. Soils in humid climates have potential to increase storage of SOC and those in arid and semiarid climates have potential to store both SOC and SIC. Payments to land managers for sequestration of C in soil, based on credible measurement of changes in soil C stocks at farm or landscape levels, are also important for promoting adoption of recommended land use and management practices. In conjunction with a rapid and aggressive reduction in GHG emissions across all sectors of the economy, sequestration of C in soil (and vegetation) can be an important negative emissions method for limiting global warming to 1.5 or 2°C This article is part of the theme issue ‘The role of soils in delivering Nature's Contributions to People’.