Coaction of dissolved organic matter and electrolyte fluctuations on Al and Fe dynamics in matured podzols – what matters more?

The release and translocation of organic matter and metal cations, particularly Al and Fe, from topsoil and their immobilization in the subsoil is the key pedogenetic process active in podzols. In the Fichtelgebirge mid-mountain range, Northern Bavaria (Germany), strong acidic conditions have been reported since the 1980s. Independent of the dominant parent material, the soils are characterized by distinct podsolization reflected by elevated concentrations of Fe and Al in the soil solutions, the subsoil horizons, and even in groundwater. Previous research on these matured podzols revealed organometallic complexes as primary forms of Fe and Al in those soils. We hypothesize that the transport fate of Fe and Al is governed by the dynamics of organic matter rather than the dissolution and mobilization of Fe and Al-bearing minerals.

In this experimental pedogenesis study, we conducted a series of soil column experiments using materials from Fichtelgebirge topsoil (Ae) and subsoil (Bs) horizons to investigate the role of organometallic complexes as well as the interplay of (im)mobilization of Al, Fe, and organic matter. Over a period of more than 4 months, we analyzed the effect of different inflow solutions, including artificial rainwater (ARW) and organic matter-enriched ARW, on the composition and properties of the effluent at high temporal resolution combining complementary instrumental analytical techniques.

The results showed that flow variations and changes in ionic strength during tracer application significantly increased the mobilization of particles, elements, and organic matter. During the application of high ionic strength influent solution, we observed the release of iron species from cation exchange sites, leading to a decrease in aggregate stability and particle release, thus peptization.  In contrast, aluminum is released in association with organic matter after conditions of low ionic strength re-established, clearly showing that Fe and Al follow distinct release dynamics. Furthermore, calcium ions replaced protons during the addition of ARW finally increasing effluent pH. However, the short-term application of organic matter compensated for this leaching by providing additional protons. This renders the supply of organic matter from litter a decisive source of protons and main contributor to soil acidification and podzolization.