Quantitative assessment of Soil nitrate disappearance and N2O evolution during denitrification: Nitrate disappearance during denitrification

Nitrate dynamics in the soil environment often confound investigations into the denitrification process and contribute to the extreme variability witnessed. NO3− dynamics and evolution of N2O were investigated in a soil chemically-treated to control the amount and form of carbon, nitrogen and the microbial population. This was achieved by removing soil organic matter with hydrogen peroxide, leaving the soil inorganic skeleton. Treatments were: 1. chemically-treated soil (low in C and N) amended with 500 mg NO3−N kg−1 and 600 mg glucose-C kg−1 (Control); 2. chemically treated soil amended with 500 mg NO3−N kg−1 and 600 mg glucose-C kg−1 and a culture of Pseudomonas denitrificans, equivalent to 106 denitrifiers g−1 of soil (Dentreat) and 3. field soil amended with 500 mg NO3−N kg−1 and 600 mg glucose-C kg−1 and a culture of P. denitrificans (Soil). Flasks were purged of O2, shaken and anaerobically incubated with 10% C2H2 (v/v) for 96 h. The N2O and CO2 production and NO3− and NO2− concentrations were monitored. Approximately 10, 150 and 146 mg NO3−N kg−1 disappeared from the soil solution in 2 h from the Control, Dentreat and Soil samples respectively. At the same time there was no concurrent increase in soil NO2− concentration or N2O evolution, whilst CO2 concentrations bore no stoichiometric relationship with N disappearance until the end of the incubation (96 h). The production of N2O did not reflect the microorganismsʹ capacity to remove NO3− from the soil environment. An approximate 80% balance of NO3− disappearance and N2O evolution was seen for the Dentreat and Soil samples at 96 h respectively. The ability to remove large quantities of NO3− quickly under non-limiting conditions and then to utilise this at a later date, may benefit some soil microorganisms. Such an excessive disappearance has implications for the use of in-situ NO3− concentrations as a predictor of soil denitrification potential in the field. A proportion of soil N would be temporarily immobilised within the microbial-biomass and could later be denitrified, leading to an underestimate of potential gaseous N loss.