1. Studies were made on the decomposition of a substrate containing glucose, ammonia, and nitrate in soil held under differing aeration conditions.
2. When water slurries were incubated with substrate, the loss of total-N equalled the loss of nitrate plus nitrite nitrogen.
3. Under percolation conditions, with small amounts of substrate and an oxygen partial pressure of 15.2 cm of mercury, there was little change in nitrate or nitrite concentrations. Loss of nitrate only occurred under conditions of reduced aeration but, when it did occur, the sum of nitrate plus atmospheric oxygen utilized by the soil was approximately the same, irrespective of the loss of nitrate. Under an atmosphere of oxygen-free nitrogen, gas output was proportional to loss of nitrate plus nitrite nitrogen. In all cases immobilisation of ammonia was similar.
4. Soils which had been percolated under anaerobic conditions with substrate, when put under aerobic conditions and with fresh substrate added, did not lose nitrate. Soils that had been percolated under aerobic conditions, when put under anaerobic conditions and with fresh substrate added, lost nitrate after a lag phase. The period of the phase was decreased by using small amounts of substrate for the aerobic percolation.
5. It is concluded that analyses for nitrate and nitrite, or measurements of oxygen uptake, can be used to give approximate measures of nitrate dissimilation.