EFFECTS OF ACID RAIN ON MYCORRHIZAL INFECTION AND NITROGEN CYCLING IN FOREST SOILS (NITRIFICATION, PHOTOSYNTHESIS, ABSIDIA CYLINDROSPORA, OZONE)

Increasing the acidity of simulated rain from pH 5.6 to 3.0 reduced the number of mycorrhizal roots on white pine seedlings by 20% after 16 weeks of exposure. Mycorrhizal infection of red oaks was 25% less at a rain pH of 3.5 than at pH 5.6. Simulated acid rain also caused increases in the N contents, net photosynthesis, and growth of seedlings, as well as decreases in root:shoot ratios and in the concentration of sucrose in the roots.; To measure the effects of acid rain on N mineralization, nitrification, and total inorganic N, columns containing samples from the surface horizons of 12 forest soils were exposed to simulated rain at 3 times ambient deposition rates for 16 weeks. The effects on N mineralization varied between soils, with the greatest inhibitions being observed in soils with low organic matter contents. The apparent protection by organic matter was associated with an increase in short-term buffering capacity. The amount of dissolved organic C (DOC) in leachates and in soil solutions was reduced by pH 3.5 simulated rain vs. pH 5.6, with the length of time that DOC was depressed being a function of the organic matter content. Although N mineralization during the exposure period was reduced an average of 7 and 20% at pH 4.0 and 3.5, respectively, the total inorganic N in the soils increased 11 and 32% at pH 4.0 and 3.5, respectively, because of the inorganic N added in the simulated acid rain. The average amount of N mineralized after exposure was not significantly affected by rain pH. Similarly, nitrification was inhibited during exposure to simulated rain at pH 3.5, but was unaffected after exposure.; Enrichments from an acid forest soil failed to show the presence of autotrophic nitrifiers, and the effects of temperature and selective inhibitors indicated that fungi were primarily responsible for nitrification in this soil. A fungus capable of heterotrophic nitrification at pH 4.0 was isolated and identified as Absidia cylindrospora Hagem. Nitrite was produced intracellularly by A. cylindrospora, after growth had ceased, and the fungus could produce up to 20 mg of nitrite-N/L when grown on (beta)-alanine or glycine as C and N source.