Adaptation of Denitrifying Populations to Low Soil pH

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Abstract
Natural denitrification rates and activities of denitrifying enzymes were measured in an agricultural soil which had a 20-yr past history of low pH (pH .apprx. 4) due to fertilization with acid-generating ammonium salts. The soil adjacent to this site was limed and had a pH of .apprx. 6.0. Natural denitrification rates of these areas were of similar magnitude: 158 ng of N g-1 of soil day-1 for the acid soil and 390 ng of N g-1 of soil day-1 at the neutral site. Estimates of in situ denitrifying enzyme activity were higher in the neutral soil, but substantial enzyme activity was also detected in the acid soil. Rates of nitrous oxide reduction were very low, even when NO3- and NO2- were undetectable, and were .apprx. 400 times lower than the rates of N2O production from NO3-. Denitrification rates measured in slurries of the acid and neutral soil showed distinctly different pH optima (pH 3.9 and 6.3) which were near the pH values of the 2 soils. This suggests that an acid-tolerant denitrifying population was selected during the 20-yr period of low pH.