Effects of fertilization with MgSO4 and (NH4)2SO4 on soil solution chemistry,mycorrhiza and nutrient content of fine roots in a Norway spruce stand

This paper focuses on the short-term reaction of fine root and mycorrhiza on changes in soil solution chemistry following application of MgSO₄ (Kieserite) and (NH₄)₂SO₄ (ammonium sulfate). The experiments were conducted within the ARINUS Experimental Watershed Area near Schluchsee in the Black Forest (SW Germany). Yellowing of the older needles as related to Mg deficiency was the typical symptom observed within this 45 yr old Norway spruce stand. On the N treated plot the relative mycorrhiza frequency declined and the percentage of nonmycorrhizal root tips increased, whereas in the Mg fertilized plot these parameters did not differ from the control. The observed changes cannot be caused by A1, because elevated concentrations of potentially toxic A1 species and extremely low Ca/A1 molar ratios appeared in the soil solution of both treatments and did not result in reduced growth of long roots as reported from solution culture experiments. Moreover, the A1 content of fine roots did not increase. Therefore, it is concluded that the thresholds for A1 toxicity derived from solution culture experiments with nonmycorrhizal seedlings cannot be transferred to forest stands. A direct toxic effect of elevated NH ₄ ⁺ concentrations on mycorrhiza is unlikely, but cannot be excluded. Enhanced root growth due to a higher uptake of NH ₄ ⁺ from soil solution may provide a more plausible explanation for the observed increase in the percentage of nonmycorrhizal root tips after N application. Even though the N content of fine roots did not increase, the diminished K content gives some indirect indication for NH ₄ ⁺ uptake by the roots. This is also consistent with reduced Mg content due to NH ₄ ⁺ /Mg²⁺ antagonism. On the MgSO₄ treated plot, Mg contents of the fine roots increased thus reflecting Mg uptake by the deficient stand.