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Restricting global warming below 2 °C to avoid catastrophic climate change will require atmospheric carbon dioxide removal (CDR). Current integrated assessment models (IAMs) and Intergovernmental Panel on Climate Change scenarios assume that CDR within the energy sector would be delivered using bioenergy with carbon capture and storage (BECCS). Although bioenergy-biochar systems (BEBCS) can also deliver CDR, they are not included in any IPCC scenario. Here we show that despite BECCS offering twice the carbon sequestration and bioenergy per unit biomass, BEBCS may allow earlier deployment of CDR at lower carbon prices when long-term improvements in soil fertility offset biochar production costs. At carbon prices above $1,000 Mg −1 C, BECCS is most frequently ( P >0.45, calculated as the fraction of Monte Carlo simulations in which BECCS is the most cost effective) the most economic biomass technology for climate-change mitigation. At carbon prices below $1,000 Mg −1 C, BEBCS is the most cost-effective technology only where biochar significantly improves agricultural yields, with pure bioenergy systems being otherwise preferred. Prior mitigation assessments of atmospheric CO 2 removal rely on bioenergy carbon capture and storage (BECCS), excluding bioenergy-biochar systems (BEBCS). Here, Woolf et al . find that BEBCS offers an alternative cost-effective solution, and may allow earlier CO 2 removal at a lower carbon price.

Global warming necessitates urgent action to reduce carbon dioxide (CO 2 ) emissions and remove CO 2 from the atmosphere. Biochar, a type of carbonized biomass which can be produced from crop residues (CRs), offers a promising solution for carbon dioxide removal (CDR) when it is used to sequester photosynthetically fixed carbon that would otherwise have been returned to atmospheric CO 2 through respiration or combustion. However, high‐resolution spatially explicit maps of CR resources and their capacity for climate change mitigation through biochar production are currently lacking, with previous global studies relying on coarse (mostly country scale) aggregated statistics. By developing a comprehensive high spatial resolution global dataset of CR production, we show that, globally, CRs generate around 2.4 Pg C annually. If 100% of these residues were utilized, the maximum theoretical technical potential for biochar production from CRs amounts to 1.0 Pg C year −1 (3.7 Pg CO 2 e year −1 ). The permanence of biochar differs across regions, with the fraction of initial carbon that remains after 100 years ranging from 60% in warm climates to nearly 100% in cryosols. Assuming that biochar is sequestered in soils close to point of production, approximately 0.72 Pg C year −1 (2.6 Pg CO 2 e year −1 ) of the technical potential would remain sequestered after 100 years. However, when considering limitations on sustainable residue harvesting and competing livestock usage, the global biochar production potential decreases to 0.51 Pg C year −1 (1.9 Pg CO 2 e year −1 ), with 0.36 Pg C year −1 (1.3 Pg CO 2 e year −1 ) remaining sequestered after a century. Twelve countries have the technical potential to sequester over one fifth of their current emissions as biochar from CRs, with Bhutan (68%) and India (53%) having the largest ratios. The high‐resolution maps of CR production and biochar sequestration potential provided here will provide valuable insights and support decision‐making related to biochar production and investment in biochar production capacity.

Soil organic carbon (SOC) models currently in widespread use omit known microbial processes, and assume the existence of a SOC pool whose intrinsic properties confer persistence for centuries to millennia, despite evidence from priming and aggregate turnover that cast doubt on the existence of SOC with profound intrinsic stability. Here we show that by including microbial interactions in a SOC model, persistence can be explained as a feedback between substrate availability, mineral protection and microbial population size, without invoking an unproven pool that is intrinsically stable for centuries. The microbial SOC model based on this concept reproduces long-term data (r 2  = 0.92; n = 90), global SOC distribution (rmse = 4.7 +/− 0.6 kg C m −2 ), and total global SOC in the top 0.3 m (822 Pg C) accurately. SOC dynamics based on a microbial feedback without stable pools are thus consistent with global SOC distribution. This has important implications for carbon management, suggesting that relatively fast cycling, rather than recalcitrant, SOC must form the primary target of efforts to build SOC stocks.

The application of biochar to soil is a highly durable nature‐based carbon dioxide removal (CDR) pathway. It provides certifiable climate‐change mitigation, with mean carbon residence times exceeding 1,000 years, and additional co‐benefits for soil health and fertility. Biochar persistence in soil depends on both intrinsic material properties and environmental factors. Its longevity is determined not only by the polyaromatic structure of the biochar itself but also by soil mineralogy, biological activity, and climatic conditions. Biochar aging involves both decomposition and stabilization processes. The complementary mechanisms of decomposition and stabilization include interactions of biochar with minerals and native organic matter, as well as aggregations with soil particles that maintain its long‐term persistence. Biochars and inertinite‐ranked fossil coals cannot be equated. Inertinite has been protected from biotic and abiotic oxidation for millions of years through burial in sediments and inclusion in minerals under high pressure and temperature. Biochar produced today in modern pyrolysis facilities is a fundamentally different material. No carbonaceous material is completely inert. Field and laboratory studies consistently show measurable, though small, mineralization across a wide range of biochar types. Declaring that soil‐applied biochar carbon persists at 100% over millennia is inconsistent with current scientific understanding. Analytical proxies indicate relative, but not absolute, biochar persistence. Policy definitions of biochar CDR should reflect climate‐relevant timescales. The degree of persistence should be estimated on the order of centuries rather than millennia, supported by registered material properties, traceable application data, conservative modeling, and continued long‐term field experiments for model validation.

Microbial biomass is one of the most common microbial parameters used in land carbon (C) cycle models, however, it is notoriously difficult to measure accurately. To understand the consequences of mismeasurement, as well as the broader importance of microbial biomass abundance as a direct driver of ecological phenomena, greater quantitative understanding of the role of microbial biomass abundance in environmental processes is needed. Using microcosms, we manipulated the initial biomass of numerous microbial communities across a 100-fold range and measured effects on CO2 production during plant litter decomposition. We found that the effects of initial biomass abundance on CO2 production was largely attenuated within a week, while the effects of community type remained significant over the course of the experiment. Overall, our results suggest that initial microbial biomass abundance in litter decomposition within an ecosystem is a weak driver of long-term C cycling dynamics.

Observed increases in the mineralization rate of labile organic carbon (LOC) in the presence of black carbon (BC) have led to speculation that corresponding decreases in non-pyrogenic (i.e. non-BC) soil organic carbon (npSOC) could significantly reduce or negate the C sequestration benefit of BC in soils. Here we show that the potential effect of an increased LOC decomposition rate on long-term npSOC stocks is negligible, even when using assumptions that would favour large losses, potentially causing no more than 3–4 % loss of npSOC over 100 years if 50 % of above-ground crop residues were converted to BC annually. Conversely, if the BC-stimulated enhanced stabilization of npSOC that has been observed in laboratory trials is extrapolated to the long-term, it would greatly increase soil carbon stocks by 30–60 %. Annual additions of BC derived from crop residues would increase total SOC (including both BC and npSOC) by an amount five times greater than the potential increase from enhanced stabilization and an order of magnitude greater than losses of npSOC caused by annual removals of biomass to provide BC feedstock.

Soil organic carbon sequestration has been promoted to combat climate change while improving soil fertility. However, its quantitative contribution to crop productivity has proven elusive. Using data from 13,662 controlled field trials with 66,593 treatments across a broad range of soils, climates and management practices, we here show that yields increase with increased soil organic carbon, until no further increase (p < 0.05) occurs above mean optimum soil organic carbon of 43.2–43.9 g kg−1 for maize, 12.7–13.4 g kg−1 for wheat and 31.2–32.4 g kg−1 for rice. Sequestering soil organic carbon is one-fifth as effective (that is, 80% less) as nitrogen fertilization for improving crop yield where soil management is optimized. By increasing soil organic carbon beyond current technology to optimum levels, global production of the three most important staple crops increases by 4.3% (sufficient to provide calories for 640 million people). However, currently available management practices would increase crop production by only 0.7% once other production constraints have already been addressed. Therefore, yield improvements under currently available technologies are unlikely to drive adoption of soil organic carbon sequestration globally, except in hot-spot regions where crop production benefits most, or unless novel practices that allow greater soil organic carbon sequestration beyond current limitations can further increase yields cost-effectively.Increasing soil organic carbon can, under optimum management only, enhance global production of maize, wheat and rice by up to 0.7% with important regional differences, according to 13,662 field trials across a broad range of soils, climates and management practices.

The adoption of natural climate solutions in crop-lands, such as cover crops, no tillage and residue retention, is widely assumed to provide both climate change mitigation and crop yield benefits. We find important spatially variable trade-offs between these outcomes and demonstrate that safeguarding crop yields will substantially lower the mitigation potential of natural climate solutions.

Climate change mitigation not only requires reductions of greenhouse gas emissions, but also withdrawal of carbon dioxide (CO 2 ) from the atmosphere. Here we review the relationship between emissions reductions and CO 2 removal by biochar systems, which are based on pyrolysing biomass to produce biochar, used for soil application, and renewable bioenergy. Half of the emission reductions and the majority of CO 2 removal result from the one to two orders of magnitude longer persistence of biochar than the biomass it is made from. Globally, biochar systems could deliver emission reductions of 3.4–6.3 PgCO 2 e, half of which constitutes CO 2 removal. Relevant trade-offs exist between making and sequestering biochar in soil or producing more energy. Importantly, these trade-offs depend on what type of energy is replaced: relative to producing bioenergy, emissions of biochar systems increase by 3% when biochar replaces coal, whereas emissions decrease by 95% when biochar replaces renewable energy. The lack of a clear relationship between crop yield increases in response to fertilizer and to biochar additions suggests opportunities for biochar to increase crop yields where fertilizer alone is not effective, but also questions blanket recommendations based on known fertilizer responses. Locally specific decision support must recognize these relationships and trade-offs to establish carbon-trading mechanisms that facilitate a judicious implementation commensurate with climate change mitigation needs. Climate change mitigation strategies based on biochar generation—and its application to agricultural soils—can effectively sequester carbon, although biogeochemical and economic trade-offs must be considered.

Production of biochar (the carbon (C)-rich solid formed by pyrolysis of biomass) and its storage in soils have been suggested as a means of abating climate change by sequestering carbon, while simultaneously providing energy and increasing crop yields. Substantial uncertainties exist, however, regarding the impact, capacity and sustainability of biochar at the global level. In this paper we estimate the maximum sustainable technical potential of biochar to mitigate climate change. Annual net emissions of carbon dioxide (CO 2 ), methane and nitrous oxide could be reduced by a maximum of 1.8 Pg CO 2 -C equivalent (CO 2 -C e ) per year (12% of current anthropogenic CO 2 -C e emissions; 1 Pg=1 Gt), and total net emissions over the course of a century by 130 Pg CO 2 -C e , without endangering food security, habitat or soil conservation. Biochar has a larger climate-change mitigation potential than combustion of the same sustainably procured biomass for bioenergy, except when fertile soils are amended while coal is the fuel being offset. The storage in soils of biochar, the product of biomass pyrolysis, has been proposed as an attractive option to mitigate climate change. Amonette and co-workers model the potential impact of biochar and find that it could eliminate more carbon from the atmosphere than using the same biomass for biofuel.