Crystalline, poorly-crystalline and organic matter-complexed Fe and Al phases in acid and calcareous temperate forest topsoils and subsoils

The importance of the crystalline and poorly-crystalline phases of Fe and Al in the stabilization of soil organic matter (SOM) is well studied mostly in acidic forest soils. As pH shifts from acidic to basic conditions, the effect of Fe and Al on SOM stabilization is declining. In addition, Fe and Al phases are suspected to influence SOM vertical distribution in soil profiles. Therefore, in this research the different Fe and Al phases of samples derived from acid and calcareous topsoil and subsoil layers were studied.

Topsoil (0–20 cm) and subsoil (30–50 cm) samples were collected from a silty Luvisol, a sandy Arenosol and three silty Cambisols from Hungary. All of the samples are derived from forest sites with a dominant oak vegetation. The pH of the samples is in the range of 5.4 to 8.1: five soils are acidic and five are neutral/alkaline. The organic carbon content of the soils is in the range of 0.8 to 6.64 %. Six soils have inorganic carbon content (0.03 to 3.98 %).

“Free” (crystalline and poorly-crystalline) Fe and Al compounds were extracted with dithionite–citrate–bicarbonate (Fe_DCB, Al_DCB) solution. “Active” (poorly-crystalline) Fe and Al compounds were extracted with acid ammonium oxalate (Fe_OX and Al_OX in case of carbonate-free soils) and sodium citrate-ascorbate (Fe_CA and Al_CA in case of calcareous soils) solutions. For extraction of Fe and Al present in organic matter complexes, sodium-pyrophosphate (Fe_Py and Al_Py) solution was used. The Fe and Al concentration of the extracts were analyzed with ICP-MS (Thermo Scientific iCAP Q). Crystalline Fe and Al phases were calculated as Fe_DCB and Al_DCB minus Fe_OX and Al_OX (or Fe_CA and Al_CA). Iron and aluminium associated with inorganic short-range order material was calculated as Fe_OX and Al_OX (or Fe_CA and Al_CA) minus pyrophosphate Fe_Py and Al_Py.

Results showed that Al was typically in organic matter-complexed form, whereas Fe was rather in crystalline form in all of the samples. Only the Luvisol samples with the lowest pH had highest amount of Fe and Al in short-range ordered minerals. The amount of crystalline Fe minerals was generally higher in subsoils than in topsoils, whereas, the amount of poorly-crystalline Fe and Al phases was higher in topsoils than in subsoils, regardless of the acidity of the samples. Organic matter-complexed Fe and Al phases were more abundant in topsoils than in subsoils without exception.

Correlation analysis showed significant positive relationship between total organic carbon content and the amount of “free”, “active” and organic matter-complexed Fe and Al phases of the samples studied. Clay content had a positive effect on the amount of “free” and “active” Fe and Al phases and on the amount of short-range ordered Fe minerals. In contrast, pH showed no effect on any of the Fe and Al forms.

This work was supported by the Ministry of Culture and Innovation of Hungary from the National Research, Development and Innovation Fund under the NRDI - Young researchers’ excellence programme - funding scheme [project no. FK 142936].