Effects of Different Organic Amendments on Aggregate-Associated Humus Carbons and Nutrients in a Paddy Soil

The degradation of soil structure in paddy fields is critical, and the application of organic amendments is an effective way to enhance soil structure and function. However, the mechanisms by which different organic amendments influence soil aggregate-associated humus carbon and nutrients remain unclear. Considering this, four treatments were employed in a randomized complete block design with three replications: (1) chemical fertilizer (CK); (2) chemical fertilizer plus organic amendment (MC); (3) chemical fertilizer plus organic amendment containing Bacillus subtilis (FT); and (4) Chemical fertilizer plus organic amendment containing polyacrylamide (PM). The results showed that all soil improvement measures significantly increased the proportion of macroaggregates (>2 mm and 2–0.25 mm), primarily the 2–0.25 mm fraction (34.53–48.46%), and the mean weight diameter (MWD), compared to CK. Soil organic carbon (SOC), humic acid carbon (HAC), fulvic acid carbon (FAC), humin carbon (HUC), total nitrogen (TN), and total phosphorus (TP) were predominantly concentrated within the macroaggregates. Relative to CK, the PM increased the HUC content in large aggregates (>2 mm) and significantly enhanced HAC by 19.53% within the same fraction, while the FT significantly boosted FAC by 31.78% in the >2 mm fraction. Furthermore, MC, FT, and PM treatments significantly enhanced SOC, TN, and TP contents within large macroaggregates compared to CK, with PM generally showing the highest SOC and TN levels, and FT being the highest in terms of TP in large aggregates (though differences among treatments were non-significant). Correlation analysis revealed that only in large aggregates did SOC show significant positive correlations with humus carbon fractions (except HAC), as well as with TN and TP. The amendments, particularly PM, effectively enhanced nutrient and humus carbon accumulation within large aggregates and improved aggregate stability. Notably, PM strengthened the direct pathways for the formation of SOC and humus carbon. In summary, the combined application of chemical fertilizer and organic amendments, containing polyacrylamide positively influenced aggregate stability and nutrient accumulation in paddy soil.