Long-Term Organic Amendment Application Improves Influence on Soil Aggregation, Aggregate Associated Carbon and Carbon Pools under Scented Rice-Potato-Onion Cropping System after the 9th Crop Cycle

A field experiment was conducted to investigate the influence of long-term application of organic manures on aggregate stability, associated carbon concentrations and carbon pools as an important soil-quality parameter under a scented rice-potato-onion cropping system in silt-loam textured soil in Eastern Bihar, India (subtropical climatic condition). Five treatments were used: 1) nitrogen, phosphorus and potassium (NPK) – 100% recommended dose of NPK; 2) NPK+FYM (farmyard manure) – 50%NPK+50% N as FYM; 3) FYM+VC (vermicompost)+ NC (neem cake) – different organic sources each equivalent to 1/3 of the recommended N (FYM +VC +NC); 4) FYM+VC+NC+PSB (phosphate-solubilizing bacteria – different organic sources each equivalent to 1/3 of the recommended N (FYM +VC+NC)+biofertilizers containing N and P carriers (PSB) and 5) FYM+BFN+BM+PSB – 50% N as FYM + biofertilizer for N +Bone meal to substitute P requirement of crops + PSB. The aggregate size distribution (>250 μm) at different soil depths is higher in treatment FYM+VC+NC+PSB and is at par with FYM+VC+NC, followed by FYM+BFN+BM+PSB and NPK+FYM, and the lowest in treatment receiving 100% NPK. The mean carbon concentration in each soil fraction was higher in soil depth 0–10 cm, followed by 10–20 cm and 20–30 cm. The trend of mean weight diameter (MWD) in different treatments was FYM+VC+NC+PSB ≥ FYM+VC+NC > NPK+FYM > FYM+BFN+BM+PSB > NPK. The trend of carbon concentrations in different treatments was FYM+VC+NC+PSB ≥ FYM+VC+NC > FYM+BFN+BM+PSB ≥ NPK+FYM > NPK. Non-labile pool of carbon forms the major portion (60.14%) of the total soil organic carbon (SOC) irrespective of all depths. Bulk density (BD) has a significant role in stabilizing soil aggregates as well as increasing the SOC content in soil. SOC was negatively correlated with BD (r = −0.870, p = 0.05), MWD (r = −0.911, p = 0.01) and geometric mean diameter (GMD) (r = −0.958, p = 0.05) irrespective of depth. This study took further steps toward understanding the enhancing of aggregate stability on organic manures addition for soil quality improvement.