Changes in soil aggregate fractions under long-term double rice cropping system with application of inorganics and organics

Studying the aggregate fractions of soil is important for understanding the C-stabilization. Carbon sequestration in soils has the potential to curb global warming besides maintaining sustainability of agricultural system under tropical and subtropical climate. Thus a 14-year old experiment was adopted to assess the impact of double rice cropping system with manuring and grades of fertilization on aggregate stability and aggregate-associated carbon in an inceptisol in southern India. Among the treatments FYM had the highest value of different size distributions of water-stable aggregates (WSA) (>2000, 2000–1000 and 250–100 μm) followed by RDF + FYM, fallow, N, 50 per cent RDF, RDF and control. The meso-aggregates (MesoA 2000-250 μm) comprised 7.6–12.0 per cent of total WSA compared to 79.0–86.7 per cent as coarse macro-aggregates (CMacA >2000 μm) and 4.8–13.3 per cent as coarse micro-aggregates (CMicA 250–100 μm). The values of mean weight diameter (MWD) and geometric mean diameter (GMD) ranged between 2.0 and 2.3 and 1.42 and 1.65 mm respectively. Irrespective of the treatment a maximum amount of C was found with meso-aggregate fractions (250–2000 μm, 22.63–37.21 g/kg) followed by silt + clay fraction (<53 μm, 9.27–12.45 g/kg), fine micro-aggregates (53–100 μm, 5.68–16.69 g/kg), coarse macro-aggregates (>2000 μm, 7.54–8.47 g/kg) and coarse micro-aggregates (100–250 μm, 3.94–7.31 g/kg) in a decreasing order constituting on an average 43.1, 18.0, 16.7, 13.1 and 9.0 per cent respectively of the total aggregate-associated C.