The influence of glucose and nitrate concentrations upon denitrification rates in sandy soils

Abstract

Dentrification rates in two soils were assessed separately as a function of NO 3 − concentration while providing a constant initial glucose concentration, and as a function of glucose concentration while providing a constant initial NO 3 −-N concentration. Of the soils used, a Hanford sandy loam and a Coachella fine sand, the bacteria in the former produced higher rates of denitrification with a maximum loss of 1500 μg NO 3 −-N/ml day −1 as compared to a loss of 150 μg NO 3 −-N/ml day −1 from the latter. Rates of loss closely approximated Michaelis Menten kinetics in the Coachella sand, and K m values for glucose-C and NO 3 −-N were 500 μg/ml and 170 μg/ml, respectively. Rates of loss of NO 3 −-N from the Hanford soil did not approximate Michaelis-Menten kinetics, and this was attributed to failure to saturate enzyme systems in the denitrifying bacteria with glucose and nitrogen when each was held constant. C/N ratios around 2 appeared to provide the greatest rates of denitrification. High C/N ratios or high glucose concentrations (1.8 per cent) retarded denitrification, with fungal growth and a subsequent drop in pH occuring. A Pseudomonas was incubated aerobically for 24 h followed by a 72 h anaerobic incubation with nitrate as the sole nitrogen source at 0, 10, 50, 100, 250 and 500mg N/ml concentrations. Assimilatory nitrate reduction never exceeded 75 mg N/ml, and it was concluded that this mode of nitrate reduction is insignificant at higher nitrate concentrations by comparison to dissimilatory nitrate reduction, i.e. denitrification.