Soil organic carbon stabilization inside microaggregates within macroaggregates is the major mechanism of carbon sequestration under a long-term agroforestry system in the foot hills of the Indian Himalayas

We used a new methodology to compute stabilized soil organic carbon (SOC) within the density fractions to evaluate the alterations in SOC in fine silty hyperthermicudic haplustalf following fifteen-years of agroforestry practices under cowpea (Vigna unguiculata L.)-toria (Brassica campestris L.) based cropping systems and turmeric (Curcuma longa L.) as ground cover crop. The objective of this long-term (15 years) experiment was to quantity the carbon content within soil aggregates as affected by agroforestry practices. The results indicated that in the surface soil (0–15 cm depth), plots with mulberry (Morus alba L.) + cowpea-toria (T7) had 58, 26 and 30 % higher total SOC in bulk soil than cultivated fallow land (T9) (6.76 g kg⁻¹), sole planting of mulberry (T5) (8.47 g kg⁻¹) and cowpea-toria system (T4) (8.21 g kg⁻¹), respectively. Intra-aggregate associated particulate organic matter within the microaggregates inside macroaggregates (iPOM_mM) was maximum in plots under T7 and the value was 104 % higher than T5 and ∼90 % larger than T4 in surface soil layer. Light fraction inside microaggregates within macroaggregates (LF_mM) associated C was 18 % and 14 % more in T7 plots than T4 (13.34 g kg⁻¹) and T5 (13.80 g kg⁻¹) plots. Plots with T7 showed maximum stabilized C (5.63 g C/1000 g bulk soils) within iPOM_mM and the value was 152 % and 164 % higher than T4 (2.23 g C/1000 g bulk soil) and T5 (2.13 g C/1000 g bulk soils) plots and also had 86 % more stabilized C than T3 (3.02 g C/1000 g bulk soil) agroforestry system. About 49 % of the total stabilized C in the plots under T7 in the surface soil layer was there within the intra-aggregate particulate organic matter inside microaggregates within macroaggregates, suggesting intra-aggregate particulate organic matter formation inside microaggregates within macroaggregates was the major mechanism of C sequestration under the long-term agroforesty system in the Indian Himalayas. Morus alba L.-based agroforestry system (AFS) had more aggregates and total SOC within microaggregates inside macroaggregates which have more carbon sequestration potential in the foot hills of the Indian Himalayas. Thus, the T7 agroforestry system should be adopted for higher C stabilization.