Impact of elevated CO2, flooding, and temperature interaction on heterotrophic nitrogen fixation in tropical rice soils
Small samples of soil amended with 2% (w/w) of glucose were preincubated either aerobically or anaerobically and then assayed (N2 15 and C2H2-C2H4) either aerobically or anaerobically for different time periods. One-hour C2H2-C2H4 assays showed greatest activity when anaerobic assay followed anaerobic preincubation. During the anaerobic preincubation a lag of 12–24 h occurred before rapid increase in one-hour assay activity was observed. When aerobic assay followed aerobic preincubation a longer lag was observed and lower activities were obtained. When anaerobic assay followed aerobic preincubation (orvice versa) negligible activities were observed in short assays, and longer assays showed increasing activity related to changes in atmosphere and/or microbial population in the closed system. Preincubation of soil on a diffusion gradient at a series of different partial pressures of oxygen confirmed the above pattern and showed that as preincubation pO2 increased, the anaerobic assay activity rapidly decreased. As preincubation pO2 decreased from 0.2 atm the aerobic assay activity decreased but less rapidly. The activities observed were related to the sizes of the Azotobacter and Clostridium populations. There was no evidence of aerobic or anaerobic C2H2 reduction in any cultures of ‘oligonitrophiles’ isolated. Incorporation of N2 15 was related to C2H2 reduction activity in the soil system studied. However, observed C2H4/N2 molar ratios ranged from 10 to 22 and appeared to be highest in samples which were preincubated anaerobically.