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The carbon balance of peatlands is predicted to shift from a sink to a source this century. However, peatland ecosystems are still omitted from the main Earth system models that are used for future climate change projections, and they are not considered in integrated assessment models that are used in impact and mitigation studies. By using evidence synthesized from the literature and an expert elicitation, we define and quantify the leading drivers of change that have impacted peatland carbon stocks during the Holocene and predict their effect during this century and in the far future. We also identify uncertainties and knowledge gaps in the scientific community and provide insight towards better integration of peatlands into modelling frameworks. Given the importance of the contribution by peatlands to the global carbon cycle, this study shows that peatland science is a critical research area and that we still have a long way to go to fully understand the peatland–carbon–climate nexus.

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.

We have shown previously that soil with historical (>150 years) applications of charcoal had larger recent (C4-maize derived) carbon content than adjacent soil; however, we could not determine whether there was an effect on older, C3-plant-derived, soil organic carbon (SOC). Therefore, we assessed the effect of historical additions of charcoal on the sequestration of recent and older SOC with a combination of delta C-13 analysis and different quantification techniques for black carbon (BC): dichromate oxidation (Cr2O7), chemo-thermal oxidation (CTO-285) and differential scanning calorimetry (DSC). Topsoils cropped with maize (Zea mays) under former charcoal production sites (N = 12) were identified in the field as black spots and had a larger (3.5%, P < 0.05) percentage of organic carbon (OC) contents than adjacent soil outside these spots (2.0%). The charcoal content varied with the detection technique used as follows: CTO-285 > DSC > Cr2O7. Black spots contained 1.6-1.7 times more (P < 0.05) maize-derived OC content than adjacent soil, irrespective of the BC quantification technique. The content of non-charcoal OC was 1.0-1.4 times larger in black spots than in adjacent soil, but differences were significant only for the Cr2O7 method. Soil physicochemical fractionation showed that at charcoal production sites more OC was recovered in the particulate organic matter and silt and clay fractions. The delta C-13 analysis suggested that additional maize-OC in black spots was in the physically more protected silt and clay fraction. Overall, this study shows that historical charcoal amendment in soil enhances the accumulation of recent maize-derived OC in a temperate climate without replacing the older C stocks.

Research on biochar has increased, but its long-term effect on the fertility of temperate agricultural soil remains unclear. InWallonia, Belgium, pre-industrial charcoal production affected former forested areas that were cleared for cultivation in the nineteenth century. The sites of traditional charcoal kilns, largely enriched in charcoal residues, are similar to soil amended with hardwood biochar more than 150 years ago. We sampled 17 charcoal kiln sites to characterize their effect on soil properties compared with adjacent reference soils. Charcoal-C content was estimated by differential scanning calorimetry. The kiln soil contains from 1.8 to 33.1 g kg−1 of charcoal-C, whichmarkedly increases organic C:N and C:P ratios. It also contains slightly more uncharred soil organic carbon (SOC) than the reference soil, which accords with larger total N content.We measured a small increase in nitrates in the kiln soil that might relate to greater mineralization and nitrification of organic N. Frequent application of lime raised the pH to values close to neutral, which offset the residual effect of charcoal production on soil acidity. A cation exchange capacity (CEC) of 414 cmolc kg−1 was estimated for charcoal-C, whereas that of uncharred SOC was 213 cmolc kg−1. Despite the large CEC of the kiln soil, exchangeable K+ content was no different from the adjacent soil, whereas exchangeable Ca2+ and Mg2+ contents were considerably larger. Charcoal enrichment has little effect on available, inorganic and total P, but it can form strong complexes with Cu, which reduces the availability of the metal. Biochar is very persistent in soil; therefore, long-term implications should not be overlooked.

Below ground carbon (BGC) inputs by agricultural plants into the soil are an important variable in soil carbon (C) modelling. The sources for BGC inputs are dead root biomass and C release by living roots (C rhizodeposition). Since management effects on BGC inputs are not profoundly understood, we address the following research questions in this research project: (i) What are the proportions of root biomass and C rhizodeposition of the total BGC input in the topsoil and subsoil under maize cultivation at different sites? (ii) Does long-term fertilization practice affect total root biomass, root distribution, shoot/root ratios, and C rhizodeposition of maize? Results obtained from field experiments in 2013 on two Swiss long-term experimental sites (“DOK” near Basel and “ZOFE” in Zurich) reveal no significant differences between total root biomasses and total C rhizodeposition of maize in different management treatments. While the proportion of topsoil (0-0.25 m) root biomass of the total (0-0.75 m) root biomass increases on the “DOK” site, the below ground/above ground C ratios decrease on both sites with increasing management intensity (trends only).