A global study has revealed that mineral-associated organic matter (MAOM) serves as the most stable long-term reservoir of carbon across diverse ecosystems, providing new insights into how carbon moves and stabilizes in soils. 

The findings, published in Soil Biology and Biochemistry, 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.

Led by Dr. SUN Tao from the Institute of Applied Ecology of the Chinese Academy of Sciences, researchers analyzed published global datasets that employed natural abundance of the stable carbon isotope ¹³C. By examining differences in ¹³C content (Δ¹³C) across soil aggregate sizes and density fractions, they inferred microbial processing intensity and organic matter stability within various carbon pools worldwide.

Results showed that Δ¹³C values increased as aggregate size decreased and particle density increased, indicating greater microbial processing and chemical stabilization. Notably, MAOM consistently exhibited the highest Δ¹³C enrichment, confirming its role as the most chemically stable and microbially processed SOM form 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.