Microaggregates

Microaggregates are small soil aggregates, typically about 20 to 250 micrometres in diameter, that form part of the microstructural fabric of many soils. They occur within the larger hierarchy of soil aggregates and contribute to the stability of soil structure and the protection of organic matter.

Microaggregation results from the interactions of clay minerals, organic matter, and microbial products. Binding agents include exopolysaccharides and mucilages produced by microbes and plant roots, humic substances, and coatings of iron or aluminum oxides. Multivalent cations such as calcium and iron help cement particles together, stabilizing microaggregates against dispersion, especially in wet conditions.

Role in carbon and nutrient cycling: Microaggregates encapsulate and stabilize organic carbon, reducing its mineralization and contributing to long-term soil carbon storage. The carbon within microaggregates often becomes part of more persistent soil organic matter and is less accessible to decomposers than carbon in larger, particulate fractions.

Ecological and management implications: Microaggregates influence soil porosity, water retention, aeration, and infiltration at small scales, and provide microhabitats for soil organisms. They are sensitive to disturbance; tillage can disrupt microaggregates and reduce soil carbon storage, while practices that add organic matter or stabilize soils can promote their formation.

Measurement and terminology: Researchers identify microaggregates using size fractionation or microscopic methods, with commonly used size ranges around 20–250 micrometres, though definitions vary by study. They are distinguished from macroaggregates, which are larger than about 0.25 millimetres.