The ecological significance of the coarse soil fraction for Picea abies (L.) Karst. seedling nutrition

Abstract

Recent studies showed that the coarse fraction of soils can have a considerable stock of short-term available nutrients. However, a direct proof that coarse soil fragments have significant nutritional functions in forest ecosystems is missing. In a 23 week microcosm experiment with controlled in- and output, mycorrhizal (Laccaria bicolor S238N) and non-mycorrhizal Norway spruce seedlings (Picea abies (L.) Karst.) were grown on either coarse soil fragments (dark paragneiss, Black Forest, S-W Germany) or quartz only. The microcosms were irrigated with a solution adjusted to the mean ionic composition of the soil solution in the reference profile, but devoid of Ca and Mg. The uptake rates of Ca and Mg by spruce seedlings were sufficient to allow normal root and shoot growth. Mycorrhization was low and did not have a significant influence on seedling nutrition. The oxalic anion was found in concentrations below 1.5 mg L−1 in the leachate, approximately corresponding to a C2O42−-saturation in equilibrium with precipitated calcium oxalate. Cation budgets and relationships among the cation species suggest that exchange processes were the main trigger of Ca and Mg mobilisation and uptake rather than protolytic weathering by exudation of carboxylic acids. The exchange processes may be attributed to weathering cracks filled with fine material of high base saturation. Therefore it is concluded that the short-term nutritional functions of coarse soils in forest sites are significant, and exchange mechanisms in the “stone protected fine earth” are in principal the same as they are known for the “free fine earth”.