Extramatrical mycelial growth, biomass allocation and nitrogen uptake in ectomycorrhizal systems in response to collembolan grazing

The effects of Collembola grazing activities on the ectomycorrhizal symbiosis were studied in laboratory microcosms. Pinus contorta seedlings in association with Paxillus involutus were grown in a sandy soil and the Collembola Onychiurus armatus was added in different densities. To study effects on nutrient uptake by the extramatrical mycorrhizal mycelium, cups containing 15NH4+ and phytin amended soil were evenly distributed in the microcosms. These cups were covered with a net that allowed the mycelium to penetrate but neither Collembola nor plant roots.

Extramatrical hyphal growth was impeded at a high density of O. Armatus. Low densities of O. armatus increased the extramatrical hyphal growth, the colonization rate of side plants and the biomass of P. involutus. However, the amount of P. involutus on/in the plant roots was not affected. Thus, low densities of collembolans induced a shift towards a larger proportion of P. involutus growing extramatrically. The presence of O. armatus in low numbers enhanced the uptake and transfer of 15N by P. involvus to the plants by up to 76%. The Collembola population growth was not higher in mycorrhizal compared with non-mycorrhizal microcosms while nematode population size was reduced in mycorrhizal compared with non-mycorrhizal microcosms.