Release of complexing organic acids by rhizosphere fungi as a factor in Norway spruce yellowing in acidic soils

A field experiment in the Vosges (a low mountain range in north-eastern France) had shown that a CaMg amendment of an acidic forest soil reduced the total number of fungal propagules per gramme dry soil, while the number of species considerably increased. Some rhizosphere fungi isolated from the amended and/or control plots were grown in three media with ammonium or nitrate as the sole source of nitrogen, and with or without aluminium. Complexing organic acids released by the fungi and likely to interfere with tree nutrition were analyzed in the culture medium after fourteen days incubation. The largest amounts of complexing organic acids are released by fungi isolated from the acid control plot. These rhizospheric fungi take part in soil acidification, which results in the long-term decrease of soil mineral reserves in a forest soil subjected to permanent leaching. Therefore, we conclude from this work that fungi might contribute to the deficient mineral nutrition of the Norway spruce on acidic soils, by impoverishment of the root environment in essential cations. However, among the causes of the Norway spruce yellowing, rhizosphere deleterious fungi act as secondary agents or contributing factors rather than primary causes of decline.