Engineered Carbon Materials Could Lock Climate-Warming Carbon in Farm Soils, Review Finds

Soils hold nearly 80 percent of the carbon stored in terrestrial ecosystems, making the dirt beneath our feet one of the planet’s most important climate allies. Yet decades of intensive agriculture have stripped carbon from many farmed soils, degrading their structure and weakening their capacity to keep carbon out of the atmosphere. A sweeping new review published in the Journal of Agriculture and Food Research examines whether a family of engineered materials, collectively called artificial carbon materials, can help rebuild those carbon stocks, and its conclusions are more nuanced, and more useful, than a simple yes or no.

The review, led by Pengfei Xu and colleagues, analyzed more than 61,000 publications indexed in the Web of Science Core Collection between 2000 and 2025, of which 7,893 focused specifically on soil carbon sequestration. The bibliometric analysis reveals a research field that has grown explosively, with publication output accelerating sharply after the 2016 Paris Agreement. But it also exposes a striking imbalance: outcome-related keywords such as soil organic carbon and greenhouse gas emissions dominate the literature, while process-oriented terms like microbial community, priming effect and adsorption remain far less visible. The authors argue that this gap between what scientists measure and what they explain has left the field without a unified mechanistic framework.

Artificial carbon materials come in three main flavors, each forged from biomass through different thermochemical routes. Biochar, the best studied of the three, is produced by heating biomass above 300 degrees Celsius under oxygen-limited conditions, yielding a carbon-rich, aromatic material. Hydrochar is made through hydrothermal carbonization, which uses water and moderate temperatures and produces a more oxygenated, more microbially accessible substance. Artificial humic substances, the youngest class, are created through alkaline hydrothermal humification, a process that accelerates the chemistry of natural humification and yields oxygen-rich materials resembling natural soil humic acids. The review is careful to keep these classes distinct, noting that they differ profoundly in molecular composition, surface chemistry and likely behavior in soil.

The evidence that these materials raise measured soil organic carbon is generally positive but strikingly heterogeneous. In one 180-day incubation of maize cropland topsoil in Jilin, China, cotton straw-derived biochar applied at equivalent carbon inputs increased soil organic carbon concentration by 94.6 percent, compared with 64.4 percent for raw straw. In a paddy soil column experiment, hydrochar applications increased native soil organic carbon by 1.0 to 3.0 percent while shifting carbon from labile to stable pools. In black soil from Harbin, artificial humic acid raised total organic carbon by 21.4 grams per kilogram within 28 days. But the authors caution that these numbers cannot be pooled into a universal average, because responses depend on feedstock, production conditions, soil texture, mineralogy, pH, moisture, climate and experimental duration.

The most provocative part of the review concerns what a rise in soil carbon actually means for the climate. Soils globally release roughly 60 petagrams of carbon per year through decomposition, a gross flux that dwarfs annual fossil fuel emissions, though it is largely part of the natural carbon cycle. When an amendment is added, total carbon dioxide emissions may fall or rise, but a bulk flux measurement alone cannot tell scientists whether the carbon came from the amendment or from the soil’s native organic matter. The review draws a sharp evidentiary line: only isotope-based or otherwise source-resolved methods count as direct evidence of priming, the acceleration or suppression of native soil carbon decomposition. Many widely cited studies, it turns out, fall short of that standard.

Biochar studies report both positive and negative priming depending on pyrolysis conditions, soil pH and texture, substrate supply and duration. Hydrochar, with its soluble and aliphatic components, may boost short-term respiration, but the authors stress this is not proof of positive priming without source separation. Evidence for artificial humic substances remains thin, with few isotope-tracing studies. The message is blunt: no material class can be assumed to uniformly suppress native carbon loss, and short-term declines in carbon dioxide emissions may simply reflect transient microbial suppression or substrate limitation rather than genuine stabilization.

The mechanistic heart of the review organizes sequestration into three interacting pillars. The first is material persistence: pyrolysis creates condensed aromatic structures that resist decomposition, while hydrochar contains larger microbially accessible fractions, and artificial humic substances may gain persistence through association with minerals despite their chemical reactivity. Elemental ratios such as hydrogen-to-carbon and oxygen-to-carbon serve as rough screening tools, but the authors warn they cannot capture pore accessibility, mineral coatings or carbon-domain heterogeneity. Long-term isotope studies confirm that biochar contains persistent fractions while also releasing a measurable labile fraction shortly after incorporation, yet comparable multi-year field data for the other two classes are almost entirely absent.

The second pillar is physical and mineral protection. Soil aggregates restrict microbial access to carbon, and artificial carbon materials can promote aggregation through their pores, surface chemistry and interactions with clay minerals, roots, fungal hyphae and microbial glues. In grassland and rice paddy soils, biochar increased macroaggregates within the first 60 days, with microaggregates enriched in biochar-derived carbon developing later. Mineral type matters enormously: expanding 2:1 clays like montmorillonite offer far more sorption capacity than kaolinite, and iron and aluminum oxides can lock up carboxyl-rich compounds through ligand exchange and co-precipitation. Aging further reshapes these interfaces, with mineral bonding detected on biochar after roughly three months and organo-mineral coatings observed on particles recovered from field trials after up to 9.5 years.

The third pillar is microbial transformation. Artificial carbon materials modify habitat conditions, buffering water fluctuations and altering pH, nutrients and redox, while their soluble compounds feed microbes. Rather than simply sitting inert, amendment carbon can flow through microbial biomass into necromass, the dead residues that subsequently bind to minerals or become sealed inside aggregates. A 12-year field experiment found biochar increased microbial necromass carbon by 23.3 to 39.0 percent in topsoil but reduced it at depth, while a four-year paddy study reported decreases at all depths measured, a reminder that total soil carbon gains can coexist with losses in specific microbial carbon pools. Microspectroscopic work in a long-term pasture showed biochar particles sorbing root-derived carbon and accumulating mineral coatings that incorporated microbial necromass, effectively lifting the soil’s carbon ceiling.

The review’s practical upshot is that there is no universally optimal material. Alkaline biochars suit acidic soils but risk over-liming neutral ones; hydrochar fits wet wastes that need no pre-drying but carries soluble carbon whose fate varies; artificial humic substances offer reactive mineral interactions and may even promote biological carbon dioxide assimilation, though their long-term field persistence is unproven. The authors call for multi-year, multi-site trials using isotope source partitioning, standardized aging protocols, integrated mineralogical and microbial analyses, and full life-cycle assessments. As the 4 per mille initiative seeks to grow global soil carbon stocks by 0.4 percent annually, this review offers both a caution and a roadmap: engineered carbon materials can genuinely help soils fight climate change, but only if scientists and farmers track where the carbon comes from, where it goes, and how long it stays.

Subject of Research: Mechanisms of soil organic carbon sequestration by artificial carbon materials in agroecosystems

Article Title: Artificial carbon materials for soil organic carbon sequestration in agroecosystems: A bibliometric analysis and mechanistic review

Article References: Xu, P., Yu, H., Zhang, J., Xu, Y., & Liu, Z. (2026). Artificial carbon materials for soil organic carbon sequestration in agroecosystems: A bibliometric analysis and mechanistic review. Journal of Agriculture and Food Research, 31, Article 103343. https://doi.org/10.1016/j.jafr.2026.103343

Image Credits: AI Generated

DOI: 10.1016/j.jafr.2026.103343

Keywords: soil organic carbon, biochar, hydrochar, artificial humic substances, carbon sequestration, priming effect, soil aggregates, mineral association, microbial necromass, greenhouse gas emissions, agroecosystems, climate change mitigation