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 variab ility 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) (Contr ol); 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, eq uivalent to 10(6) denitrifiers g(-1) of soil (Dentreat) and 3. field s oil 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 O-2, shake n and anaerobically incubated with 10% C2H2 (v/v) for 96 h. The N2O an d CO2 production and NO3- and NO2- concentrations were monitored. Appr oximately 10, 150 and 146 mg NO3--N kg(-1) disappeared from the soil s olution in 2 h from the Control, Dentreat and Soil samples respectivel y. At the same time there was no concurrent increase in soil NO2- conc entration or N2O evolution, whilst CO2 concentrations bore no stoichio metric relationship with N disappearance until the end of the incubati on (96 h). The production of N2O did not reflect the microorganisms' c apacity to remove NO3- from the soil environment. An approximate 80% b alance of NO3- disappearance and N2O evolution was seen for the Dentre at and Soil samples at 96 h respectively. The ability to remove large quantities of NO3- quickly under non-limiting conditions and then to u tilise this at a later date, may benefit some soil microorganisms. Suc h an excessive disappearance has implications for the use of in-situ N O3- concentrations as a predictor of soil denitrification potential in the field. A proportion of soil N would be temporarily immobilised wi thin the microbial-biomass and could later be denitrified, leading to an underestimate of potential gaseous N loss. Copyright (C) 1996 Elsev ier Science Ltd