Influence of pH on production and consumption of NO by slurries of an agricultural soil under denitrifying conditions

Net NO production from anaerobic slurries of an agricultural sandy loam soil was >99.9% biological and strongly inversely related to pH (r2=0.974). The rate of net NO production at pH 6 (6.95 nmol g–1 dry soil h–1) was 20 times greater than that at pH 8. Nitrate-depleted sandy loam soil actively consumed NO under anaerobic conditions and could consume headspace NO concentrations of approximately 20 ppmv within 5 h. There was also a strong positive linear relationship between pH and NO consumption (r2=0.971). First-order rate constants for NO consumption were lowest at pH 6 (–0.47 h–1) and increased twofold at pH 8. In the presence of 100 µM carbonyl cyanide m-chlorophenylhydrazone (CCCP) net NO production decreased between 77% and 94% and was also inversely related to pH (r2=0.787). First-order rate constants for NO consumption were inhibited by 46–54% in the presence of 100 µM CCCP and were linearly related to pH (r2=0.999). Autoclaving effectively prevented net NO production from the sandy loam but decreased the rate constants for NO consumption by only 42–53%. HgCl2 was more effective at preventing NO consumption, decreasing the rate constants by 88% at pH 7. We conclude that environmental conditions which increase pH would decrease the net release of NO from the sandy loam and suggest that the existence of high capacity biological and abiological NO-consuming systems is a factor limiting NO emissions from soil.