SOIL REDOX AND pH EFFECTS ON NITRATE REDUCTION UNDER FLOODED CONDITIONS

Abstract

In an incubation experiment, both biological and chemical reduction of nitrate were studied under flooded conditions at different redox (0, 200 and 400 mV) and pH (5, 7 and 9) values in the presence and absence of HgCl2 as a sterilant.Nitrate reduction followed a linear pattern with the time of incubation at all combined pH -Eh treatments. At a given pH value, NO -3 reduction increased by 2 to 3 folds due to lowering Eh from 400 to 0 mV. The highest reduction rate was found at pH 9 combined with Eh 0 mV. At 48 hours of incubation, for instance, and pH 9 the reduction was 73% at 0 mV but didn't exceed 24% at Eh 400 mV in the absence of HgCl2. The corresponding values in the presence of HgCl2 were 80% and 42%, respectively. Catalytic action of HgCl2 under alkaline conditions was responsible for such increase by enhancing the chemical reduction. Under acid conditions, at 0 and 400 mV without HgCl2 the values were 64% and 30%, but were 25% and 14% with HgCl2. Such decreased values were attributed to the poisoning action of soluble mercury under acid conditions to soil biota. It also indicated the importance of chemical reduction under acid and well aerated conditions which are different than those suitable for nitrate reducing microorganisms.