Nitrogenase activity associated with some wisconsin prairie grasses

Abstract

Yearly rates of nitrogen fixation associated with seven species of grass were measured on two artificially-established prairies. The C2H2 reduction method was used to measure the activity of soil cores taken within the stands of grass. Nitrogenase activity was specifically associated with Panicum virgatum and Sporobolus heterolepis, which had activities estimated at 3.6 and 2.9 kg N ha-1 yr-1. Fixation in stands of the other grasses ranged between 0.2 and 1.8 kg N ha-1 yr-1; free-living organisms might have fixed the N2 without specific association with the grasses, which were Andropogon gerardi, Andropogon scoparius, Spartina pectinata, Stipa spartea, and Poa pratensis. Three relic prairies were also examined, but the rates of fixation were no higher, except for S. heterolepis, which at one relic prairie had rates that extrapolated to 9 kg N ha-1 yr-1. The choices made for core location, size, depth, length of C2H2 incubation, and the time of day of sampling did not appear to have a substantial effect on the accuracy of the measurements. The organisms associated with S. heterolepis required O2 for N2 fixation, and they were located in the soil or on the smaller roots which remained when the major roots were removed from the soil. re]19750217