Carbon Sequestration in Microaggregates of No‐Tillage Soils with Different Clay Mineralogy

Identification of diagnostic soil organic matter (SOM) fractions and the mechanisms controlling their formation and turnover is critical for better understanding of C dynamics in soils. Enhanced microaggregate formation and stabilization of C due to reduced macroaggregate turnover has been proposed as a mechanism promoting C sequestration in no‐tillage (NT) compared with conventional tillage (CT) systems in temperate soils dominated by 2:1 clay mineralogy. We evaluated the contribution of macroaggregate‐protected microaggregates to total soil organic carbon (SOC) sequestration in NT relative to CT in three soils differing in clay mineralogy: a 2:1 clay‐dominated soil (2:1), a soil with mixed clay mineralogy [2:1 and 1:1] and oxides (mixed), and a soil dominated by (1:1) clay minerals and oxides (1:1). Microaggregates (mM) were isolated from macroaggregates from 0‐ to 5‐ and 5‐ to 20‐cm soil layers. Particulate organic matter (POM) located within the microaggregates (intra‐mM‐POM) was separated from POM outside of the microaggregates (inter‐mM‐POM) and the mineral fraction of the microaggregates (mineral‐mM). In all three soils, total SOC as well as microaggregate‐associated C (mM‐C) was greater with NT compared with CT. Although less than half of the total SOC under NT was associated with the microaggregate fraction, more than 90% of the total difference in SOC between NT and CT was explained by the difference in mM‐C in all three soils. Thus, we identified and isolated a fraction that explains almost the entire difference in total SOC between NT and CT across soils characterized by drastically different clay mineralogy.