Elucidating the role of ferrihydrite and goethite in the aggregation and stability of small soil microaggregates: An experimental study on arable Luvisols under different management

• Addition of Fe oxides promoted the aggregation of < 20 μm small soil microaggregates (SMA) fractions. • Formation of 3 − 10 μm small SMA was facilitated at the expanse of < 3 and > 10 μm small SMA by addition of Fe oxides. • Differences in size and shape between ferrihydrite and goethite affected stability of formed aggregates. • Ferrihydrite generally exhibited a better ability to stabilize < 1 μm colloids in the small SMA fraction than goethite. • Addition of HA favored the dispersion of small SMA rather than aggregation. Iron oxides, exhibiting positive surface charge in most acidic to neutral soils, are key inorganic agents for microaggregate formation, especially via their electrostatic interactions with negatively charged surfaces on organic and inorganic soil compounds. Yet, little is known on the influence of Fe oxide properties, i.e., size, shape, and surface charge on the formation of soil microaggregates (SMA). Here the aggregation of < 20 μm small SMA fractions in the presence of either ferrihydrite or goethite as well as the stability of the resulting aggregates were examined. Water stable small SMA fractions were isolated from Ap-horizons of Stagnic Luvisols under different management (cropped and bare fallow), and both had an average diameter of ∼ 6 μm. Ferrihydrite and goethite, respectively, were added as suspensions to small SMA fractions at 1 or 5 wt%. For comparison, humic acid (HA), a common fraction of soil organic matter, was added at 1 wt% in solution. Laser diffraction was applied to determine changes in the hydrodynamic diameter and the stability of the resulting aggregates. Addition of Fe oxides facilitated the formation of 3 − 10 μm SMA, which probably resulted from their aggregation with < 3 μm particles in the small SMA fractions via electrostatic attraction. Moreover, changes in the particle size distribution also suggested that the addition of Fe oxides decreased the share of > 10 μm SMA, thereby increasing the abundance of 3 − 10 μm small SMA as well. Here it is likely that attachment of Fe oxides on SMA caused a rearrangement of their structure leading to a closer packing of particles. A generally higher decrease in the abundance of > 10 μm SMA in the ferrihydrite addition implied a more efficient compacting effect of ferrihydrite than that of goethite. This was presumably due to the smaller size of ferrihydrite, which can decrease the steric hindrance and provide more contact points. Changes in the size distribution of small SMA fractions were more pronounced after the addition of 5.0 wt% Fe oxides compared to the 1.0 wt% ones. In contrast, adsorption of the added HA on SMA increased their negative surface charges and steric hindrance between them, thereby favoring their dispersion rather than aggregation. In the stability test, both ferrihydrite and goethite showed a less effective stabilizing effect on SMA at the bare fallow site than the cropped one. However, ferrihydrite generally revealed a better ability to stabilize < 1 μm colloids in the small SMA fraction than goethite for both sites. Here, our study provides new insights into the abilities of different Fe oxides to form and stabilize aggregates in soil microaggregation.