N-15 ESTIMATES OF NITROGEN-FIXATION BY WHITE CLOVER (TRIFOLIUM-REPENS L) GROWING IN A MIXTURE WITH RYEGRASS (LOLIUM-PERENNE L)

The N-15 isotope dilution technique and the N difference method were used to estimate N2 fixation by clover growing in a mixture with ryegrass, in a field experiment and a controlled environment experiment. Values obtained using N difference were approximately 25% lower than those estimated using N-15 isotope dilution. In the field experiment there was a measured N benefit to grass growing with clover, equivalent to 42.7 kgN ha-1. The grass in the mixture had a lower atom % N-15 content and a higher N content than grass in a monoculture; therefore values for N2 fixation were different depending on choice of control plant i.e. monoculture or mixture grass. In the controlled environment experiment there were no significant differences between either the atom % N-15 contents or the N contents of monoculture grass and grass growing in a mixture with clover. It is concluded that there is a long term indirect transfer of N from clover to associated grass which can lead to errors in estimates of N2 fixation.