<i>Frankia</i> in acid soils of forests devoid of actinorhizal plants

Abstract

The capacity of some acid forest soils to induce nodulation on a hybrid between Alnus incana (L.) Moench and A. glutinosa (L.) Gaertn. was investigated. Soil was sampled from tree stands devoid for decades of actinorhizal hosts. Seven‐week‐old Alnus seedlings growing m liquid culture were inoculated with soil dilutions. The nodules were counted after 6 weeks and classified as Sp − , if they lacked spores, or as Sp + . if spores were present, according to microscopy of microtome sections. Frankia was found in all the forest soils studied, apart from a soil from a Betula swamp. The highest nodulation capacities on Alnus , caused predominantly by Frankia of the Sp − type. were observed in mineral soil sites with Betula stands — even higher than in soil from an A. incana stand. A positive correlation was found between the pH and the noduiation capacity of the soil.