Soil colloids as binding agents in the formation of soil microaggregates in wet-dry cycles: A case study for arable Luvisols under different management

• Published in: Geoderma Vol. 443; p. 116830
• Main Authors: Tang, Ni, Dultz, Stefan, Gerth, Daniel, Klumpp, Erwin
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.03.2024; Elsevier
• Subjects: A horizons; case studies; centrifugation; Elemental composition; fallow; Field flow fractionation; fractionation; Luvisols; microaggregates; organic carbon; Size distribution of aggregates; Soil microaggregation; spectrometers; Size distribution of aggregates; Field flow fractionation; Elemental composition; Soil microaggregation
• ISSN: 0016-7061; 1872-6259
• DOI: 10.1016/j.geoderma.2024.116830

•In Ap horizons of arable Luvisols, colloids control the aggregation of small soil microaggregates (<20 μm) in wet-dry cycles.•The presence of <450 nm colloids favored the formation of 1–40 μm soil microaggregates.•In absence of <1 μm colloids, >40 μm soil micro- and macroaggregates were preferentially formed.•The particle size is decisive for the aggregate formation under the current experimental condition. In the hierarchical model of soil aggregates, small soil microaggregates (small SMA; <20 μm) are often considered to be fundamental building units at the micron scale. Below which, soil colloids (<1 µm) have recently been proposed as binding agents of (micro)aggregates. However, the way in which soil colloids contribute to the formation and stability of soil micro- and macroaggregates remains largely unknown. For clarification, we evaluated potential impacts of the colloidal content, particularly the <450 nm colloids, on the aggregation of small SMA. Free water stable small SMA and <450 nm colloids were isolated from Ap-horizons of Stagnic Luvisols under different management (cropped and bare fallow). The size-resolved elemental composition of the <450 nm colloids was analyzed by asymmetric flow field-flow fractionation in combination with an inductively coupled plasma mass spectrometer and an organic carbon detector. To vary the colloidal content in small SMA, (1) suspensions containing different amounts of <450 nm colloids were added in small SMA, or (2) <1 µm colloids were removed from small SMA by centrifugation. In the maximum colloidal addition treatment, the mass ratios of added colloids to small SMA were 3.0 and 5.1 wt% for the cropped and bare fallow soil samples, respectively. Aggregation of small SMA with different colloidal amounts was performed in three successive wet-dry cycles. Afterwards, the size distribution of the resulting aggregates was measured by laser diffraction. Our results indicated that, in wet-dry cycles, colloids were important binding agents for the formation of SMA. Their presence, especially those <450 nm, was likely to support the formation of solid bridges during drying at particle contacts of 1–10 µm small SMA, favoring hereby SMA build-up in a relatively small size range of 1–40 µm. In contrast, the absence of <1 μm colloids in small SMA led to a preferential generation of relatively large aggregates in wet-dry cycles, i.e., typically with sizes >40 μm up to 1700 μm in maximum. Our study on aggregation in wet-dry cycles revealed that the colloidal content has a controlling effect on the size distribution of resulting aggregates by acting as a binding agent and provides hereby new insights into the evolvement of aggregate hierarchy in soils.