Soil structure exerts important influences on the edaphic conditions and the environment. It is often expressed as the degree of stability of aggregates. Aggregation results from the rearrangement, flocculation and cementation of particles. It is mediated by soil organic carbon (SOC), biota, ionic bridging, clay and carbonates. The complex interactions of these aggregants can be synergistic or disruptive to aggregation. Clay-sized particles are commonly associated with aggregation by rearrangement and flocculation, although swelling clay can disrupt aggregates. Organo-metallic compounds and cations form bridges between particles. The SOC originates from plants, animals and microorganisms, and their exudates. It enhances aggregation through the bonding of primary soil particles. The effectiveness of SOC in forming stable aggregates is related to its decomposition rate, which in turn is influenced by its physical and chemical protection from microbial action. Soil inorganic carbon (SIC) increases aggregation in arid and semi-arid environments, and the formation of secondary carbonates is influenced by the presence of SOC and Ca2+ and Mg2+. Soil biota release CO2 and form SOC which increase dissolution of primary carbonates while cations increase precipitation of secondary carbonates. The precipitation of (hydr)oxides, phosphates and carbonates enhances aggregation. Cations such as Si4+, Fe3+, Al3+ and Ca2+ stimulate the precipitation of compounds that act as bonding agents for primary particles. Roots and hyphae can enmesh particles together while realigning them and releasing organic compounds that hold particles together, a process with a positive impact on soil C sequestration. Soil structure can be significantly modified through management practices and environmental changes. Practices that increase productivity and decrease soil disruption enhance aggregation and structural development.

Soil carbon (C) sequestration is important to the mitigation of increasing atmospheric concentration of CO2. This study was conducted to assess soil aggregation and C concentration under different management practices. The effects of crop rotation, manure application and tillage were investigated for 0–5 and 5–10cm depths on two silt loam soils (fine-loamy, mixed, active, mesic Aquic Fragiudalfs and fine-loamy, mixed, active, mesic Aeric Fragiadalf) in Geauga and Stark Counties, respectively, in northeastern Ohio, USA. Wet sieve analysis and gravity fractionation techniques were used to separate samples in aggregate and particle size groups, respectively. In the Stark County farms water stable aggregate (WSA) is higher in wooded (W) controls (WSA=94.8%) than in cultivated soils with poultry manure (PM, 78.7%) and with chemical fertilizers (CF, 79.0%). Manure applications did not increase aggregation compared to unmanured soils. The C concentrations (%) within aggregates (Cagg) are higher in W than in cultivated soils (W=5.82, PM=2.11, CF=1.96). Soil C (%) is enriched in the clay (W=9.87, PM=4.17, CF=4.21) compared to silt (4.26, 1.04 and 0.98, respectively) and sand (0.93, 0.14 and 0.32, respectively) fractions. In the Geauga County farm, continuous corn (CC) with conventional tillage has lower WSA (83.1%) than soils with rotations (R) (93.9%), dairy manure (DM) application (93.2%) and no-till (NT) (91.1%). The C concentrations within macroaggregates (Cagg) were higher in W soils (4.84%) than in cultivated soils (ranging from 2.65 to 1.75%). The C (%) is enriched in clay (W=8.56, CC=4.18, R=5.17, DM=5.73, NT=4.67) compared to silt (W=2.35, CC=0.90, R=0.96, DM=1.57, NT=1.06) and sand (W=0.44, CC=0.33, R=0.13, DM=0.41, NT=0.18). Cultivation decreased C concentration whereas reduced tillage, rotation and manure enhanced C concentration in soil.