We need both natural and energy solutions to stabilize our climate.

We respond to concerns raised by Baldocchi and Penuelas who question the potential for ecosystems to provide carbon sinks and storage, and conclude that we should focus on decarbonizing our energy systems. While we agree with many of their concerns, we arrive at a different conclusion: we need strong action to advance both clean energy solutions and natural climate solutions (NCS) if we are to stabilize warming well below 2°C. Cost-effective NCS can deliver 11.3 PgCO2 e yr-1 or ~30% of near-term climate mitigation needs through protection, improved management, and restoration of ecosystems, as we increase overall ambition.

Dynamics and climate change mitigation potential of soil organic carbon sequestration.

When assessing soil organic carbon (SOC) sequestration and its climate change (CC) mitigation potential at global scale, the dynamic nature of soil carbon storage and interventions to foster it should be taken into account. Firstly, adoption of SOC-sequestration measures will take time, and reasonably such schemes could only be implemented gradually at large-scale. Secondly, if soils are managed as carbon sinks, then SOC will increase only over a limited time, up to the point when a new SOC equilibrium is reached. This paper combines these two processes and predicts potential SOC sequestration dynamics in agricultural land at global scale and the corresponding CC mitigation potential. Assuming that global governments would agree on a worldwide effort to gradually change land use practices towards turning agricultural soils into carbon sinks starting 2014, the projected 87-year (2014-2100) global SOC sequestration potential of agricultural land ranged between 31 and 64 Gt. This is equal to 1.9-3.9% of the SRES-A2 projected 87-year anthropogenic emissions. SOC sequestration would peak 2032-33, at that time reaching 4.3-8.9% of the projected annual SRES-A2 emission. About 30 years later the sequestration rate would have reduced by half. Thus, SOC sequestration is not a C wedge that could contribute increasingly to mitigating CC. Rather, the mitigation potential is limited, contributing very little to solving the climate problem of the coming decades. However, we deliberately did not elaborate on the importance of maintaining or increasing SOC for sustaining soil health, agro-ecosystem functioning and productivity; an issue of global significance that deserves proper consideration irrespectively of any potential additional sequestration of SOC.

A global clustering of terrestrial food production systems.

Food production is at the heart of global sustainability challenges, with unsustainable practices being a major driver of biodiversity loss, emissions and land degradation. The concept of foodscapes, defined as the characteristics of food production along biophysical and socio-economic gradients, could be a way addressing those challenges. By identifying homologues foodscapes classes possible interventions and leverage points for more sustainable agriculture could be identified. Here we provide a globally consistent approximation of the world's foodscape classes. We integrate global data on biophysical and socio-economic factors to identify a minimum set of emergent clusters and evaluate their characteristics, vulnerabilities and risks with regards to global change factors. Overall, we find food production globally to be highly concentrated in a few areas. Worryingly, we find particularly intensively cultivated or irrigated foodscape classes to be under considerable climatic and degradation risks. Our work can serve as baseline for global-scale zoning and gap analyses, while also revealing homologous areas for possible agricultural interventions.

Effects of moisture and physical disturbance on pore-scale oxygen content and anaerobic metabolisms in upland soils.

Soils are the largest dynamic stock of carbon (C) on Earth, and microbial respiration of soil organic C accounts for over 25% of global carbon dioxide (CO2) emissions. Zones of oxygen depletion in upland soils (anaerobic microsites) are increasingly recognized as an important control on soil microbial respiration rates, but the factors governing the volume and distribution of anaerobic microsites are relatively unknown. We measured the dissolved oxygen (DO) content of porewater from incubated soil cores of varying moisture contents (<80% and >80% water saturation) and degrees of disturbance (undisturbed, conventionally tilled, and physically disturbed). Porewater was extracted sequentially from pores constrained by three effective pore diameters, ≥3.0 µm, 3.0-1.0 µm, and 1.0-0.6 µm, from cores incubated for 7, 14, or 28 days, using a modified Tempe cell extraction system. We observed a parabolic pattern in mean dissolved oxygen (DO) concentrations across pore sizes, independent of soil moisture and degree of disturbance. Specifically, DO values within the largest and smallest pore domains were relatively depleted (155 ± 10 µM and 160 ± 11 µM, respectively), while DO values within medium pores were closer to saturation (214 ± 8 µM). The observed DO pattern provides insight into the balance of microbial oxygen demand versus oxygen supply across pore domains within upland soils. Additionally, we observed iron and manganese reduction in all soils except samples subjected to disturbance and incubated at <80% water saturation, suggesting that disturbance enhances aeration and diminishes anaerobic metabolisms within upland soils. Our findings highlight the influence of soil moisture and management on soil redox and CO2 efflux rates.

The concept and future prospects of soil health.

Soil health is the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals and humans, and connects agricultural and soil science to policy, stakeholder needs and sustainable supply chain management. Historically, soil assessments focused on crop production, but today soil health also includes the role of soil in water quality, climate change and human health. However, quantifying soil health is still dominated by chemical indicators, despite growing appreciation of the importance of soil biodiversity, due to limited functional knowledge and lack of effective methods. In this Perspective, the definition and history of soil health are described and compared to other soil concepts. We outline ecosystem services provided by soils, the indicators used to measure soil functionality, and their integration into informative soil health indices. Scientists should embrace soil health as an overarching principle that contributes to sustainability goals, rather than only a property to measure. TOC BLURB: Soil health is essential to crop production, but is also key to many ecosystem services. In this Perspective, the definition, impact and quantification of soil health are examined, and the needs in soil health research are outlined.

Global Sequestration Potential of Increased Organic Carbon in Cropland Soils.

The role of soil organic carbon in global carbon cycles is receiving increasing attention both as a potentially large and uncertain source of CO2 emissions in response to predicted global temperature rises, and as a natural sink for carbon able to reduce atmospheric CO2. There is general agreement that the technical potential for sequestration of carbon in soil is significant, and some consensus on the magnitude of that potential. Croplands worldwide could sequester between 0.90 and 1.85 Pg C/yr, i.e. 26-53% of the target of the "4p1000 Initiative: Soils for Food Security and Climate". The importance of intensively cultivated regions such as North America, Europe, India and intensively cultivated areas in Africa, such as Ethiopia, is highlighted. Soil carbon sequestration and the conservation of existing soil carbon stocks, given its multiple benefits including improved food production, is an important mitigation pathway to achieve the less than 2 °C global target of the Paris Climate Agreement.

Global Tree Cover and Biomass Carbon on Agricultural Land: The contribution of agroforestry to global and national carbon budgets.

Agroforestry systems and tree cover on agricultural land make an important contribution to climate change mitigation, but are not systematically accounted for in either global carbon budgets or national carbon accounting. This paper assesses the role of trees on agricultural land and their significance for carbon sequestration at a global level, along with recent change trends. Remote sensing data show that in 2010, 43% of all agricultural land globally had at least 10% tree cover and that this has increased by 2% over the previous ten years. Combining geographically and bioclimatically stratified Intergovernmental Panel on Climate Change (IPCC) Tier 1 default estimates of carbon storage with this tree cover analysis, we estimated 45.3 PgC on agricultural land globally, with trees contributing >75%. Between 2000 and 2010 tree cover increased by 3.7%, resulting in an increase of >2 PgC (or 4.6%) of biomass carbon. On average, globally, biomass carbon increased from 20.4 to 21.4 tC ha(-1). Regional and country-level variation in stocks and trends were mapped and tabulated globally, and for all countries. Brazil, Indonesia, China and India had the largest increases in biomass carbon stored on agricultural land, while Argentina, Myanmar, and Sierra Leone had the largest decreases.