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 soi l was > 99.9% biological and strongly inversely related to pH (r(2)=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 activ ely consumed NO under anaerobic conditions and could consume headspace NO c oncentrations of approximately 20 ppmv within 5 h. There was also a strong positive linear relationship between pH and NO consumption (r(2)=0.971). Fi rst-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 muM carbonyl cyanide m-chlorophenylhydrazone (CCCP) net NO production decreased between 77% and 94% and was also inversely related to pH (r(2)=0.787). First-order rate co nstants for NO consumption were inhibited by 46-54% in the presence of 100 muM CCCP and were linearly related to pH (r(2)=0.999). Autoclaving effectiv ely 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 pr eventing NO consumption, decreasing the rate constants by 88% at pH 7. We c onclude that environmental conditions which increase pH would decrease the net release of NO from the sandy loam and suggest that the existence of hig h capacity biological and abiological NO-consuming systems is a factor limi ting NO emissions from soil.