Abstract:

Soil aggregates are the main from of soil particles, which have important effects on the enrichment and bioavailability of heavy metals in soil. Fertilization can affect the particle size distribution and physicochemical characteristics of soil aggregates. However, the effects of physicochemical characteristics of soil aggregates with different particle sizes on the enrichment and chemical forms of heavy metals are still unclear. In this study, a number of equilibrium isothermal adsorption-desorption experiments were conducted to study the adsorption-desorption characteristics and chemical forms of cadmium (Cd) in different particle sizes (< 53 μm，53?250 μm and 250?2000 μm) of water-stable aggregates fractions extracted from the agricultural soil treated with no fertilizer or conventional fertilization, and reveal the relationship between the physicochemical characteristics of soil aggregates with/without fertilization treatments and adsorption-desorption characteristics and chemical forms of Cd²⁺ in these aggregate fractions. The results showed that compared to no fertilizer treatment, conventional fertilization treatment significantly increased the proportion of 2000?250 μm aggregates and the contents of organic matter (OM), free iron oxide (especially amorphous iron oxide), total nitrogen, available phosphorus and available potassium in each aggregate component, especially 2000?250 μm aggregates. The adsorption and desorption of Cd²⁺ on different size fraction aggregate and bulk soil with conventional fertilization treatment was in accordance with Freundlich equation, and was dominated by heterogeneous multi-surface adsorption. Among them, 2000?250 μm aggregates had the strongest adsorption and retention ability for Cd²⁺. With the size fraction increase, adsorption and retention ability decreased, being significantly positively correlated with free iron oxides (especially amorphous iron oxide), OM, available phosphorus and total nitrogen. With the initial concentration of Cd²⁺ in the solution increase, the adsorption and desorption capacity of every size fraction on Cd²⁺ increased. Cd²⁺ retained in different particles were dominated by exchangeable Cd, and the proportion of exchangeable Cd in 2000?250 μm aggregates was the lowest, accounting for 66.0%. The exchangeable fraction and organic bound fraction of Cd significantly decreased, and the Fe/Mn oxide bound fraction and the residual fraction of Cd significantly increased in particle size fractions collected from soil samples with the conventional fertilization treatment. Findings above showed that the application of fertilizers could increase the ratio of large size aggregates and their organic matters and free iron oxides contents, which could decrease the migration and bioavailability of Cd²⁺ in these aggregates.

Key words: fertilization, aggregates, cadmium (Cd), adsorption, desorption, chemical forms

摘要：

土壤团聚体是土壤颗粒的主要存在形式，对重金属在土壤中的富集和生物有效性具有重要影响。施肥方式会影响土壤团聚体的粒径分布和理化特性，但是不同粒径团聚体自身理化特性对重金属富集和赋存化学形态的影响仍缺乏系统研究。选取两种施肥处理（常规施肥和不施肥）的水稳性团聚体颗粒为研究对象，采用平衡等温法研究各粒级团聚体中镉（Cd）的吸附解吸特性和赋存化学形态，探讨施肥-团聚体理化特性-Cd吸附解吸及其赋存化学形态的内在关系。结果表明：相比未施肥处理，常规施肥处理能够显著增加大粒径团聚体（>53 μm）占比及其有机质、游离氧化铁（特别是无定形氧化铁）、全氮、有效磷和速效钾含量。与未施肥处理不同，常规施肥处理原土及各粒径团聚体对Cd²⁺的吸附解吸过程符合Freundlich方程，可达到非均匀多层吸附，其中2000—250 μm粒径团聚体对Cd²⁺的吸附、固持能力最强，随着粒径的减小，对Cd²⁺的吸附、固持能力均降低，主要与其游离氧化铁（特别是无定形氧化铁）、有机质、有效磷及全氮含量呈显著正相关。随着溶液中Cd²⁺初始浓度的增大，各粒径团聚体对Cd²⁺的吸附、解吸能力均增加。原土及各粒径团聚体固持Cd²⁺的化学形态以交换态为主，其中2000—250 μm粒径团聚体交换态Cd占比最低，为66.0%。常规施肥处理使各粒径团聚体中交换态Cd和有机结合态Cd含量显著降低，铁锰氧化物结合态Cd和残渣态Cd含量显著增加。由此可以得出，适当施肥处理能够增加团粒胶结物质含量，有利于大团聚结构生成，提高土壤颗粒对Cd²⁺的固持能力，促使固持的Cd²⁺向更稳定的形态转化，降低其生物活性和迁移能力。

关键词: 施肥, 团聚体, 镉（Cd）, 吸附, 解吸, 化学形态