What is the so-called optimum pH for denitrification in soil?

Respiratory denitrification is controlled by a number of environmental and soil factors including pH, but some of the interactions between pH and denitrification in soil are still uncertain. The objective of the present study was to assess the relationship between natural soil pH and denitrifying enzyme activity (DEA), to investigate the impact of two different techniques used for determining of potential denitrification, namely short-term DEA and the long-term denitrification potential (DP), on resulting pH optimum, especially with respect to the length of measurement, and to elucidate the effect of soil pH on the nature of denitrification products (N2O, N2). For this purpose, five mineral soils similar in a texture but differing in pH, were investigated. In addition to examining the soils in their natural pH values, pHs were adjusted by amendment of sulphuric acid or sodium hydroxide solutions prior to DEA or DP determination. The study showed that there was no simple relationship between DEA and soil pH, as very high DEA was found both in acid and alkaline soil. The study also indicated that the populations of soil denitrifiers were adapted to prevailing natural soil pH in the sense that they exhibited the highest DEA at or near natural soil pH (in experiments where the pH was adjusted to values from about 4 to 11 before DEA determination). Suppression of nitrate reductase activity by introducing nitrite instead of nitrate as an electron acceptor had no effect on this interaction between the DEA and soil pH. If the soil incubation was prolonged under optimum conditions for denitrification (surplus of nitrate or nitrite and available carbon, very low partial pressure of O2) and DP was estimated, the optimum pH for evolution of denitrification products was shifted towards neutrality. It is proposed that this is either due to the development of a community of denitrifiers which can grow better at the neutral pH or due to accommodation of the existing populations to new conditions. The shift in pH optimum occurred after about 12 h, but it took about 1-2 days to fully appear. In agreement with previous studies, it was confirmed that at pH above 7, N2 is much more important denitrification product than N2O. The study also suggests that the expressions like optimum pH for denitrification should be avoided, as different denitrification characteristics obtained using different methodology can be quite differently related to the soil reaction. Such general expression should be specified by introducing the particular denitrification parameters to which it is being applied.