Colloidal iron and organic carbon control soil aggregate formation and stability

Several beneficial soil functions are linked to aggregates, but how the formation and stability depend on the presence of colloidal building blocks is still understood poorly. Here, we sampled subsites from an arable toposequence with 190 and 340 g kg^-1 clay, and isolated small soil microaggregates (small SMA; < 20 µm) from larger macroaggregate units (> 250 µm) using an ultrasonic dispersion energy of 60, 250, and 440 J mL^-1 , respectively. We then allowed these small SMA to reaggregate after chemical removal of organic carbon (OC) as well as of Fe- and Al (hydr)oxides, respectively. The size distribution of the reaggregated small SMA and fine colloids (< 0.45 µm) was analyzed via laser diffraction and asymmetric flow field-flow fractionation coupled to inductively coupled plasma mass spectrometry and OC detection, respectively. We found elevated amount of both fine colloids and stable SMA at subsites with larger clay contents. The size distribution of small SMA was composed of two distinct fractions including colloids (< 1 µm) and SMA with an average size of 5 µm. The removal of Fe with Dithionite-Citrate-Bicarbonate (DCB) shifted the size of the small SMA to a larger equivalent diameter, while destruction of OC with NaOCl reduced it. After three wetting and drying cycles, the concentration of colloids declined, whereas the small SMA without chemical pre-treatments reaggregated to particles with larger average diameters up to 10 µm, with the size depending on the clay content. Intriguingly, the gain in size was more pronounced after Fe removal, but it was not affected by OC removal. We suggest that Fe (hydr)oxides impact the stability of small SMA primarily via cementing the aggregates to smaller size. In contrast, the effect of OC was restricted to the size of colloids, gluing them together to small SMAs within defined size ranges when OC was present but releasing these colloids when OC was absent.