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Available Nitrogen and Nitrogen Cycling in Forest Soils Exposed to Simulated Acid Rain¹

ABSTRACT

The formation of NO^-₃ and NH⁺₄ was measured in columns containing samples from the surface horizons of 12 forest soils both during and after exposure to simulated rain applied at three times the ambient deposition rates for 116 d. The relative responses to increased acidity were correlated with organic matter and N levels of the soils. The average inhibition for the 12 soils was linearly related to the amount of acidity added. The quantity of N mineralized was less in some soils after their exposure to simulated rain at pH 3.5 than at pH 5.6 and greater in other soils, but the average amount mineralized after exposure of the 12 soils was not significantly affected by the pH of the simulated rain during the treatment period. The mean percentage of the inorganic N produced in the 12 soils that was in the NO^-₃ form was lower during but not after the exposure to simulated rain at pH 3.5 than at 5.6. The amount of inorganic N added in the simulated acid rain exceeded the diminished supply arising because of the inhibition of mineralization during the exposure. The suppression of N mineralization in Crary soil (Aquic Fragiorthods) containing white pine (Pinus strobus L.) seedlings and in Mardin soil containing red oak (Quercus rubra L.) seedlings was less than in unplanted soil. The simulated rain at pH 3.5 altered the amounts of K, Ca, Mg, Al, Fe, and Mn leached from the soil. It is suggested that the inhibition of inorganic N formation by microorganisms in short periods of acid precipitation may be compensated by the N added with the precipitation, but the impact on available N over long periods of exposure is unknown.

¹ Contribution from the Dep. of Agronomy, Cornell Univ, Ithaca, NY, 14853. This research was supported by Cooperative Agreement 58-32U4-2-409 from the USDA.

² Graduate Research Assistant and Professor of Soil Science.

Received for publication March 4, 1985. Accepted for publication July 24, 1985.