Nitrogen use efficiency in hydroponic plant growth chambers is reduced by microbial denitrification. A computer-controlled bioreactor system was developed to investigate methods for controlling denitrification in hydroponic plant growth chambers envisioned for use on long duration space missions. Through regulation of O2 and NO3- levels in the bioreactor, the rate of denitrification of pure cultures was controlled. This control strategy may be used to favor plant nitrate uptake in the presence of denitrifying bacteria in hydroponic systems. Reducing denitrification provides advantages to advanced life support (ALS) systems by improving crop nitrogen use efficiency, reducing fixed-nitrogen demand, and reducing gaseous wastes (N2O and N2).