Water‐Stable Aggregates and Organic Matter Fractions in Conventional‐ and No‐Tillage Soils

Abstract No‐tillage (NT) practices can improve soil aggregation and change the distribution and retention of soil organic matter (SOM) compared with conventional tillage (CT), but the relationships between aggregates and SOM fractions are poorly known. The effects of long‐term (13‐yr) CT and NT management on water‐stable aggregates (WSA) and aggregate‐associated SOM were investigated on a Hiwassee sandy clay loam (clayey, kaolinitic, thermic Rhodic Kanhapludult). Samples were collected at two depths in replicated CT and NT plots and separated into five aggregate size classes by wet sieving. The stability of intact WSA was measured turbidimetrically. The C and N content of total, particulate (POM), and mineral‐associated organic matter was determined for each size class. Whole‐soil organic C was 18% higher in NT (30.7 Mg C ha −1 ) than in CT (26.1 Mg C ha −1 ). In CT, macroaggregates (>250 µm) were fewer and less stable than those of NT. The POM C made up ≈36% of whole soil C regardless of tillage, but the quantity of POM was nearly 20% higher in NT than in CT. The POM comprised a higher percentage of total aggregate N in surface soils of NT than in CT and values increased with increases in aggregate size. In NT, concentrations of total and mineral‐associated C and N were higher in the 106‐ to 250‐µm WSA than in the other size classes but, in CT, the concentrations were similar among size classes. An alternative view of aggregate organization is discussed in which microaggregates are formed in association with POM at the center of macroaggregates, helping to explain relationships between SOM storage and aggregate size distributions under different management practices.