Soil aggregation dynamics and carbon sequestration

The quantity and quality of residues determine the formation and stabilization of aggregate structure for soil organic carbon (SOC) sequestration. Plant roots and residues are the primary organic skeleton to enmesh the inorganic particles together and build macro- and microaggregates while sequestering SOC. There are three major organic binding agents of aggregation: temporary (plant roots, fungal hyphae, and bacterial cells), transient (polysaccharides), and persistent (humic compounds and polymers). Conversion of natural ecosystems into agricultural lands for intensive cultivation severely depletes SOC pools. Magnitude of SOC sequestration in the soil system depends on the residence time of SOC in aggregates. Microaggregates are bound to old organic C, whereas macroaggregates contain younger organic material. Many techniques have been used to assess the SOC distribution in aggregates. Classical methods include SOC determination in aggregate fractions by wet and dry sieving of bulk soil. Isotopic methods including the determination of 13C and 14C with mass spectrometry are techniques to quantify the turnover and storage of organic materials in soil aggregates. Other techniques involve the use of computed tomography, X-ray scattering, and X-ray microscopy to examine the internal porosity and interaggregate attributes of macro- and microaggregates. Current state-of-knowledge has not unravelled completely the underlying complex processes involved in the sequestration, stability, dynamics, and residence times of SOC in macro- and microaggregates. There is a need to develop a unique conceptual model of aggregate hierarchy.