Influence of arbuscular mycorrhizal fungi and simulated acid rain on the growth and coexistence of the grasses Calamagrostis villosa and Deschampsia flexuosa

Two grass species - Calamagrostis villosa (Chaix) J.F. Gmelin and Deschamps ia flexuosa (L.) Trin. - are expanding in mountain Norway spruce (Picea abi es L. Karst.) forests of Central Europe damaged by anthropogenic pollution constituted particularly of acid rain. This invasion of grasses may be caus ed by the higher irradiance reaching the forest floor after the pollution-i nduced tree defoliation. The relative abundance of the two grass species is changing during the process of forest decline. Our study investigated the effects of arbuscular mycorrhizal fungi (AMF) on the growth and coexistence of both species under simulated acid rain (SAR) and two levels of irradian ce. Three microcosm experiments were conducted to investigate how both gras ses are influenced by the AMF when grown separately or together interacting via extraradical mycelium (ERM). A positive growth response to inoculation with Glomus mosseae BEG 25 was found for both grass species when cultivate d separately and the mycorrhizal dependence and the growth benefit for D. f lexuosa was greater than for C. villosa. However, when both grass species w ere grown together in the rhizoboxes with separated root and hyphal compart ments, the growth effect of the AMF was the opposite, i.e. C. villosa benef ited more. The plants did not benefit from the AMF inoculation under the SA R treatment compared with dH(2)O treatment. The SAR also negatively influen ced root length colonised by AMF, length of the ERM, alkaline phosphatase a nd NADH diaphorase activities of the ERM. The role of the ERM in transporti ng phosphorus between these grasses was verified by applying the radioisoto pe P-32. There was a greater transport of isotopic P-32 between inoculated plants C. villosa and D. flexuosa grown in separated root compartments, as compared to non-inoculated plants. The amount of transported P-32 was low: a maximum of 3% of applied P-32 was detected in the shoots of receiver plan ts. Mechanical disturbance of the ERM significantly decreased the P-32 tran sport between plants. The P-32 transport between mycorrhizal plants was hig her in the D. flexuosa to C. villosa direction than in the opposite one. Ne ither the SAR nor the low level of irradiance influenced the amount of tran sported P-32. We discuss the role of ERM links between root systems in the coexistence of both grass species.