EFFECTS OF RHIZOBIUM, ARBUSCULAR MYCORRHIZAL FUNGI AND ANION CONTENT OF SIMULATED RAIN ON SUBTERRANEAN CLOVER

An experiment was concluded to determine the extent to which rhizobia, mycorrhizal fungi, and anions in simulated rain affect plant growth r esponse to acid deposition. Germinating subterranean clover seeds were planted in steam-pasteurized soil in pots and inoculated with Rhizobi um leguminosarum, Glomus intraradices, Glomus etunicatum, R. leguminos arum + G. intraradices, R. leguminosarum + G. etunicatum, or no microb ial symbionts. Beginning 3 weeks later, plants and the soil surface we re exposed to simulated rain in a greenhouse on 3 days week(-1) for 12 weeks. Rain solutions were deionized water amended with background io ns only (pH 5.0) or also adjusted to pH 3.0 with HNO3 only, H2SO4 only , or a 50/50 mixture of the two acids. Glomus intraradices colonized p lant roots poorly, and G. intraradices-inoculated plants responded lik e nonmycorrhizal plants to rhizobia and rain treatments. Variation in plant biomass attributable to different rain formulations was stronges t for G. etunicatum-inoculated plants, and the effect of rain formulat ion differed with respect to nodulation by rhizobia. The smallest plan ts at the end of the experiment were noninoculated plants exposed to r ains (0.38 g mean dry weight total for 3 plants pot(-1)). Among nonnod ulated plants infected by G. etunicatum, those exposed to HNO3 rain we re largest, followed by plants exposed to HNO3 + H2SO4, pH 5.0, and H2 SO4 rain, in that order. Among plants inoculated with both R, legumino sarum + G. etunicatum, however, the greatest biomass occurred with pH 5.0 rains, resulting in the largest plants in the study (1.00 g/3 plan ts). Treatment-related variation among root and shoot biomass data ref lected those for whole-plant biomass. Based on quantification of bioma ss and N concentrations in shoot and root tissues, total N content of plants inoculated with G. etunicatum alone and exposed to the HNO3 + H 2SO4 rains was approximately the same as plants inoculated with R. leg uminosarum + G, entunicatum and exposed to pH 5 rains. Thus, the acid- mixture pains and rhizobia under no acid deposition provided approxima tely equal amounts of N in biomass. The significant interactions among rain formulation and the symbiotic status of the plants suggest that conclusions concerning the impact of acid deposition on plants in the environment cannot be considered reliable because most experiments on which such assessments are based have not tested confounding influence s of microorganisms and precipitation characteristics.