Abstract
The N2-ase activities of field-grown Brazilian grasses were measured with C2H2 reduction by soil cores containing the plants. C2H2 and C2H4 were observed to diffuse at similar rates through soil and equilibrated across the Brazilian soil in 3 h, but could take up to 30 h or more with some British soils. The diurnal fluctuation in the rates of N2-ase activity by Brachiaria mutica and Sorghum vulgare were similar and the variation in rate was correlated with soil temperature. Estimates of N2-fixation by measurement of C2H2 reduction by soil cores ranged from 14.7 to 51.4 g N ha−1 day−1 and were much lower than with “pre-incubated” excised roots from the cores or taken directly from the field. The merits of the soil core and the “pre-incubated” excised root assays are discussed. ft∗|Present address: U.S. Department of Agriculture, Science and Education Administration, Agricultural Research, Northeastern Region, Room 309, Building 001, Beltsville, MD 20705, U.S.A.