A global study has provided new insights into how carbon moves and stabilizes in soils, revealing that mineral-associated organic matter (MAOM) serves as the most stable long-term reservoir of carbon across diverse ecosystems. The findings, published in Soil Biology and Biochemistry under the title “Carbon pathways in soil: unraveled by 13C natural abundance”, offer a fresh perspective on how soil carbon stability is shaped by land use, climate, and soil chemistry.

Soil organic matter (SOM), although making up only a small portion of soil mass, plays a vital role in maintaining soil fertility, structure, and carbon storage. It consists of decomposed plant and animal residues, microbial cells, and their by-products. Despite long-standing knowledge that SOM can be protected from decomposition by binding to clay minerals, iron and aluminum oxides, or becoming trapped within soil aggregates, the specific transformation pathways of SOM within different aggregate sizes and density fractions have remained poorly understood.

Researchers from the Institute of Applied Ecology at the Chinese Academy of Sciences, led by Dr. SUN Tao, conducted a meta-analysis of published global studies that utilized natural abundance of carbon-13 (¹³C), a stable isotope of carbon, to analyze the differences in ¹³C natural abundance (Δ¹³C) across various soil aggregates and density fractions. This analysis provided insights into the transformation pathways of organic carbon within different carbon pools in soils worldwide. The difference in ¹³C content between soil pools (denoted as Δ¹³C) serves as an indicator of microbial processing and transformation intensity. Greater enrichment of ¹³C is generally associated with more intensive microbial processing, reflecting a greater stability of organic matter.

The researchers found that that as aggregate size decreased and particle density increased, Δ¹³C values increased. MAOM, the fraction of organic matter tightly bound to minerals, consistently exhibited the largest Δ¹³C enrichment, confirming its status as the most chemically stable and microbially processed form of SOM globally. 

Notably, SOM transformation pathways varied with land use. Forests and grasslands exhibited higher Δ¹³C values, whereas croplands showed lower values, likely due to more frequent soil disturbance and altered organic inputs. Climate patterns also played a significant role: humid tropical regions displayed the greatest ¹³C enrichment, while Mediterranean climates showed the least, highlighting the influence of temperature and moisture on microbial decomposition dynamics.

Soil pH and clay content emerged as key regulators of microbial activity and SOM transformation, alongside land management practices. 

These findings not only deepen our understanding of soil carbon stabilization mechanisms but also have significant implications for improving carbon sequestration strategies and nutrient management across ecosystems.

The researchers recommend tailored land-use strategies to optimize the conversion of plant litter into stable carbon forms. In temperate croplands, conservation tillage and straw return could enhance MAOM formation. In arid grasslands, maintaining perennial vegetation and improving soil moisture can boost carbon protection sequestration. In managed forests, adjusting tree species composition and improving litter quality could support microbial efficiency and increase MAOM accumulation, thereby enhancing soil resilience and carbon storage under climate change.