A Systematic Review of Soil Amendments Using Biochar and Enhanced Rock Weathering (ERW) for Soil Carbon Sequestration

This review provides a comparative critical synthesis of biochar and enhanced rock weathering (ERW), identifies key trends and gaps in soil carbon research, and outlines pathways for improving carbon sequestration and monitoring in soil systems. From a global perspective, materials and agricultural studies were read to examine the properties of these amendments and their effects in cropland and forest soils. The main research question guiding this literature review was as follows: What are common trends in published biochar and ERW studies? Major themes were derived from the stated question and structure the Discussion. The Web of Science provided access to relevant literature for both biochar and ERW, and a total of 38 articles (biochar: 17; ERW: 21) were read and covered in this paper. The findings conveyed the growing number of Chinese studies on these amendments to resolve climate-related soil quality affecting crop yields and potential for carbon sequestration, namely carbon dioxide removal or CDR—which sequesters CO2 that is already in the atmosphere. Studies commonly used application rates of <5% for biochar and 5 or 50 t/ha for ERW, with (wood) biochar commonly processed at temperatures of 500–550 °C. Finer powders were known to be more effective due to their increased surface area, although there were emissions trade-offs to consider for climate change mitigation. There were options for using glacial rock flour (GRF) as an alternative. For ERW, the type of minerals matters, with basaltic amendments being most investigated and minerals like zeolite, for example, having quick responses and potential to filter out heavy metals. Depth of analysis was an issue in the studies, especially affecting ERW work—which needs to adopt greater depths (>60 cm) and both soil organic carbon (SOC) and soil inorganic carbon (SIC) or total carbon need address, particularly for ERW since studies only provided selective coverage. Biochar studies tended to focus more on crop yields and were not as concerned as ERW studies in CDR. Many studies agreed that these are promising products that need to be economically compared before being applied at a large scale. More field studies are needed to test biochar, while limitations imposed by soil pH (acidification affecting dissolution and nutrient availability) and climate need consideration for ERW—especially since it works best in warm, humid climates. The application rate and duration are important variables to also consider for ERW, and both SOC and SIC dynamics are subsystem components requiring consideration. Ultimately, studies call for field trials executed in the long term at greater depth and in different climates and representing different soil types.