Aluminum fate in forest soils developed from magmatic and metamorphic rock of mid-mountain areas in Germany

Climate and land use change affect weathering and pedogenesis with potential consequences for the fate of Al-bearing minerals and the potential export of Aluminum to groundwater resources. These changes might result in strong acidification, originally known for “acid rain” affecting these areas until the second but last decade of the past century. To explore the fate of Al in areas now affected by climate and land use change, we investigated two sites of different geology in North-Bavaria. Site 1 is located on granitic rocks under a reforested 6-year-old Norway spruce forest. Site 2 is a hilltop site located on metamorphic rocks under a 60-80-year-old spruce forest. Soil samples (< 2mm) and clay fractions were analyzed by hydrochemical and spectroscopic techniques. Zero tension controlled lysimeter and automated tension controlled lysimeters were installed for monitoring the soil solution volume and composition at the topsoil-subsoil and the subsoil-regolith boundary. Monitoring started in June 2018. Since then, 85 sampling campaigns have been completed that amounted to 1500 individual lysimeter samples. Analysis comprised among others EC, pH, elemental composition major anions and cations, and carbon sum parameters (DOC, TOC, DIC, TIC).

Recent climate at the sites differs markedly from the 1961-1990 period, indicating a transient climate at the sites. Mean soil pH ranged from 3.2 to 4.7 at both sites and was comparable to values published in 1995 by Franken et al. (3.4 to 4.2). Thus, recent soil pH is as low as used to be under the conditions of strong acid precipitation of the last century. Soils developed from magmatic rock showed higher contents of variable Al phases than those developed from metamorphic rocks.

At both sites pyrophosphate extractable Al is the dominant Al pool accounting 19.4% of total Al in site 1(14.1 g/kg in Bs horizon), and 6.9% of total Al in site 2 (4.9 g/kg in Bs horizon).

Noteworthy, hydrological summer was more important for seepage generation than the hydrologic winter: Roughly 68% of the total annual seepage volume was found in the hydrological summer. As a result, the TOC flux from the subsoil in summer is 35.66 ± 20 mg/year, and only 13.88 ± 13.8 mg/year in winter. Similarly, the Al flux in summer is 1.02 ± 0.7 mg/year and only 0.43 ± 0.4 mg/year in winter.

Variation partitioning analysis showed that the seasonal variation and the difference between topsoil and subsoil combined explained less than 5 % of the particle-related soil solution properties ((pH, ∑LMWO, TOC, Al and Si(mg/L)) and less than 1% of the hydrochemical properties (TIC, Cl⁻, SO₄²⁻, Ca, Mg, Na (mg/L)). Difference between the two sites explained 13.84% and 6.48% of the two sets, respectively and the sampling year explained 4.52% and 4.74%. We conclude that the Al system at our sites is controlled by climatic conditions and site properties (lithology, slope, vegetation..). There are no indications that the released Al is immobilized in any secondary immobile Al-phase in the subsoil or downstream, pointing to the potential transport of Al and other unwanted substances to the aquifers.