The Development and Use of a Flow-through Apparatus for Measuring Nitrogenase Activity and Photosynthesis in Field Crops

A flow-through apparatus for measuring nitrogenase activity in field-grown crops is described. The apparatus was used to measure nitrogenase activity in a sainfoin crop throughout a growth season. A marked seasonal variation was observed which was partly the consequence of defoliation. The acetylene-based estimate of the nitrogen fixed annually by sainfoin was less (146 kg N ha−1) than the nitrogen accumulated in the shoots. Reasons for this difference are discussed with special emphasis on the validity of the technique and alternative sources of nitrogen. There were no treatment differences between the rates of growth of either sainfoin or lucerne when grown with or without fertilizer nitrogen in the field. This result is in marked contrast with results from a glasshouse experiment where sainfoin plants utilizing nitrate-nitrogen exhibited faster growth rates than similar plants that were solely dependent on symbiotic nitrogen fixation.