Effect of nitrogen fertilization and cropping system of the reference crop on estimation of N sub(2) fixation by vetch using super(15)N methodology

This study was conducted to examine the effects of varying N rates and cropping systems (mixed versus pure stand) on the suitability of oats (Avena sativa L.) for estimating N sub(2) fixed in sequentially harvested vetch (Vicia sativa) L.) over two growing seasons (1984-84 and 1985-86). The N rates were 20 and 100 kg N ha super(-1) in 1984-85 and 15 and 60 kg N ha super(-1) in 1985-86. In the 1984-85 season, vetch at maturity derived 76 and 63% N from fixation at the high and low N rates respectively. The corresponding values for the second reason were 66 and 42%. Except in the 1985-86 season when some significantly higher values of % N sub(2) fixed were estimated by using the reference crop grown at the higher (A-value approach) than at the lower N rate (isotope-dilution approach), both approaches resulted in similar measurements of N sub(2) fixed. In the 1984-85 season, similar values of N sub(2) fixed were obtained using either the pure or mixed stand oats reference crops.