Application of novel organically derived fulvic acids in soil aggregate size distribution, fraction, organic carbon, and nitrogen content in three distinct soils

The application of organic manure/substances in soil can directly alter soil structure and influence the soil organic carbon (SOC), soil aggregate stability, and aggregate-associated nitrogen (N). In this study, a pot experiment was conducted to evaluate the influence of three organically derived fulvic acid (FA) treatments-fulvic acid powder (S, 0.25%), natural liquid (NL, 0.50%) and plant-derived liquid (P, 0.50%)-alongside a control (CK) on aggregate size distribution (>250 μm, 250-53 μm and <53 μm sizes), SOC, total nitrogen (TN), water-stable aggregates, and mean weight diameter (MWD) of aggregates in three typical soils: albic black (AL), irrigated desert (IR) and Shahjiang black (SH). The results showed that water-stable aggregates increased by 39.6% when using 0.25% S amendment in AL soil compared with the control, while the maximum MWD (1.51) was obtained with 0.50% P in AL soil. The SOC and TN contents of >2 mm aggregates were significantly enhanced in IR soil (35-40% and 60-80%, respectively), whereas 2-0.25 mm fractions exhibited higher SOC and TN in AL soil compared with the other soils. Furthermore, SOC and TN distribution in the 0.25-0.053 mm aggregate fraction increased by 12-40% and 12-16%, respectively, with SH soil. Microaggregates (<0.053 mm) exhibited the highest TN in AL and the highest SOC in SH soil. This study demonstrates that fulvic acid application, particularly in liquid or solid forms, substantially improves soil aggregation, enhances SOC and N sequestration in different aggregate fractions, and contributes to better soil health. These findings provide new evidence supporting the use of fulvic acids as an effective amendment for improving soil structure and nutrient retention, offering a sustainable strategy for soil quality restoration and long-term agricultural productivity.