Water stable aggregates and the associated active and recalcitrant carbon in soil under rubber plantation

Rehabilitation of the degraded soil is imperative to minimize the effects of soil degradation. It is in this context that stable soil aggregates, essential to providing physical protection to the organic residues, are important indicators of soil restoration or degradation. Thus, the present study was aimed at determining the soil aggregate stability and associated carbon fractions under rubber (Hevea brasiliensis) plantations. The study was conducted on 10, 15, 25, and 34-year-old rubber plantation established on Imperata grassland. Soil samples were collected from 0 to 10, 10-20, 20-50, 50-100 cm depths from different aged rubber plantation and native forest (NF) using a soil core of 5.6 cm inner diameter. Soil aggregates from each depth were separated by the wet-sieving technique, and grouped into three fraction size classes: macro-aggregates (>2 mm), meso-aggregates (0.25-2 mm), and micro-aggregates (<0.25 mm), and analyzed for carbon concentrations. The results showed that macro-aggregates dominated soil under different plantation ages and decreased with an increase in soil depth. The Mean Weight Diameter (MWD) and the Geometric Mean Diameter (GMD) increased with an increase in the age of the plantation and decreased with increase in soil depth. The MWD was the highest in the forest soil (5.8 mm) and the lowest (3.0 mm) under 10-year-old rubber plantation. The highest GMD was found under 34-year-old rubber plantation (2.1 mm) and the lowest under 10-year (1.4 mm) plantation. The SOC concentration under the recalcitrant pool increased with the increase in plantation age, and the highest amount was observed under 34-year old plantation. The increase in aggregate stability, recalcitrant carbon pool, and SOC stock with age chronosequence suggests the ecological role of mature rubber plantations in soil rehabilitation by minimizing the process of soil degradation.