Abstract: The study aims to investigate the effects of application of the modified lignite on the distribution of typical northern calcareous soil aggregates and distribution of heavy metal cadmium (Cd) in different particle-size aggregates. Different types of modified lignite materials were introduced into soils contaminated with Cd, followed by a 150-day incubation period. Soil aggregates at different grain levels were extracted by dry sieving and wet sieving, and the contents of calcareous soil aggregates and heavy metal cadmium and their stability were investigated in response to different modified lignite application amounts (0, 1%, 3%, and 5%), and the distribution of different Cd forms in aggregates. The distribution of various forms of Cd in soil aggregates was analyzed across different particle size fractions. The results obtained from dry and wet sieving demonstrated a gradual increase in the content of water-stable aggregates in the soil due to the application of modified lignite. More specifically, the content of water-stable aggregates exhibited an increase ranging from 0.9% to 25.2%, 5.5% to 24.5%, 12.1% to 46.5%, 98.4% to 156.1%, and 127.8% to 174.4% under treatments of lignite (LI), lignite-based humic acid (LIH), demineralized lignite (LID), humic acid resin (HAR), and humic acid-grafted copolymer (HAG), respectively. The content of water-stable agglomerates, average weight diameter, geometric mean diameter and particle size greater than 0.25 mm agglomerates of the soil increased significantly after the application of amendment. Among the different treatments, the coefficient of enrichment of Cd in coarse sand-sized aggregates (r1) and fine sand-sized microaggregates (r3) exhibited higher enrichment factors, while the coefficient of enrichment of water-stable agglomerates at the grain level (r2) was the smallest. In conclusion, the application of modified materials can not only effectively increase the water-stable aggregates of soil, but also effectively enrich the ion exchange state, carbonate binding state and iron and manganese oxidation state in the aggregates larger than 0.25 mm and the water-stable micro-aggregates smaller than or equal to 0.053 mm. The strong organic, weak organic and residual Cd are enriched in the water-stable microaggregates smaller than or equal to 0.053 mm. Particularly, lignite-based humic acid and demineralized lignite exhibited the most favorable effects on the formation of water-stable aggregates and the enrichment of Cd.

Key words: calcareous soil, lignite, water stable aggregate, soil particle size, cadmium enrichment