Gunda Matschonat

Quantification of ammonium sorption in acid forest soils by sorption isotherms

Quantity/Intensity relations of NH4+ were established to evaluate the role of NH4+ sorption as a N-sink as well as for the transport of NH4+ within the soil profile. Two haplic podzols (originating from Phyllite and Granite) from the German Fichtelgebirge were used. Exchange isotherms were obtained from batch experiments with NH4+ concentrations between 0.01 to 0.6 mmol NH4+ L-1. Background solutions had a composition close to the natural soil solutions to identify the cations displaced by NH4+. Organic layers sorbed a maximum amount of 0.3–0.5 mmolc NH4+ 100 g-1, A horizons 0.1–0.2 mmolc NH4+ 100 g-1 and B horizons 0.09 mmolc 100 g-1, corresponding to less than 1% of the cation exchange capacities. NH4+ displaced K+ and Ca2+ throughout the whole profile, and additionally Al in the subsoil horizons and H3O+ in the organic and A horizons. “Initial mass isotherms” (Quantity/Quantity isotherms) were established with distribution coefficients ranging from 0.17 to 0.35 for NH4+ with highest values in the O horizons. While exchangeable NH4+ in acid forest soils is considered to be only a minor long-term sink for deposited NH4+, it has its relevance in the seasonal dynamics and transport of NH4+.

Equilibrium solution composition and exchange properties of disturbed and undisturbed soil samples from an acid forest soil

Microscalic heterogeneity of soil chemical properties caused by soil structure has been reported for several soils. We investigated exchange properties and soil solution composition of disturbed and undisturbed samples of an acid forest soil lacking visible structure. Cation concentrations in the soil solution resulting from two extraction procedures and two analytical methods were compared. The effective cation exchange capacity (CECe) of the undisturbed sample represented 56–69% of the bulk soil CECe. Base saturation of undisturbed samples equalled that of disturbed samples for EA, Bhs, and Bsh horizons, and was higher for the Bw horizon. Contradicting the results of other authors, soil pore solution obtained by percolating soil cores under conditions of low water tension offered more favourable conditions for plant roots when compared to the equilibrium soil solution of the bulk soil sample in all except the Bsh horizon. Ca2+/Al3+ molar ratios were higher and fractions of H+ + Al3+ on total cationic charge were lower in the soil pore solution. These results were obtained employing soil: solution ratios of about 1:0.5 during the extraction of soil pore solution, and by determination of free cations. Other authors used a water extraction with soil:solution ratios up to 1:2 and took total metal for ion concentrations. The combination of the latter extraction and analytical method in our study, too, led to unfavourable Ca2+/Al3+ ratios and high tractions of H+ + Al3+. The choice of analytical and extraction method are thus decisive for the valuation of the soil solution composition in view of plant nutrition.