Nitrogenase activity associated with three tropical grasses growing in undisturbed soil cores

Nitrogenase activity associated with the root system of three tropical grasses Axonopus compressus, Digitaria decumbens var. pangolaand Paspalum notatum was measured by C₂H₂ reduction assay of soil-plant cores. The cores were incubated in perspex chambers in which 10% of the air was replaced with C₂H₂. Gas samples were taken at 7, 24, 31, 48, 55 and 72 h. No lag before onset of C₂H₄ production was evident and good agreement was obtained between replicates. Cumulative C₂H₄ production maintained a linear trend during the 72 h incubation.The largest increase in N₂-ase activity was detected in the A. compressus—gleyed podzolic system while D. decumbens (lateritic podzolic) and P. notatum (sandy yellow podzolic) had smaller activities. Variation between sampling sites in the second year of sampling of the experiment was associated with large variations in soil moisture.N₂ fixation estimated from N₂-ase activity in soil-plant cores was similar to the amount of N accumulated in the above-ground herbage in the field during 12 weeks.Response curves relating N₂-ase activity to soil moisture and soil temperature were established for all species. P. notatum and D. decumbens responded similarly to changes in both soil temperature and soil moisture while A. compressus contrasted sharply to the other two species in its reaction to both.