EFFECT OF ADDITION OF VARIOUS CARBON SUBSTRATES ON DENITRIFICATION IN A VERTIC MOLLISOL

Glucose, acetate, malate, and citrate were added to an agricultural soil. The pe values (-log e-; calculated from the redox potential) obtained 30 min after the addition of C were not correlated with the theoretical reducing power nor with the theoretical total energy of the C compounds. By contrast the number of electron (e-) equivalents was correlated with pe7, indicating that the proton number affected the redox potential (Eh) measurement. After 24 h of incubation, denitrification rates followed the order citrate > malate > glucose and control. No N2O production was detected with acetate. Denitrification was not correlated with the theoretical reducing power of the added C compounds but was correlated with pe+pH. Similar numbers of e- equivalents were measured with all treatments. After 72 h of incubation, the order of the denitrification rates was malate > cirate > acetate > glucose and control. The Eh values (lower than after 24 h) did not differ with treatment while the number of e- equivalents was influenced by the quality of the C source. This also demonstrates that the proton number affected the measured Eh. Our results suggest that the different C substrates did not directly influence the soil physicochemical and biological conditions through their degree of oxidation. Any effects appeared to be indirect, arising from the ability of the substrates to generate new metabolites, and consequently initiate different metabolic pathways that modified the soil physicochemical conditions, reducing power and microbial activity.