Estimation of biological N2-fixation in a grass-clover system by the 15 N-dilution method and the total N difference method

A correct estimation of the amount of biologically fixed atmospheric derived nitrogen (N) is important for evaluating the environmental impact of a clover-grass system.

The total-N difference method was compared with the 15N dilution method for estimating biologically fixed nitrogen in a cut and carry clover-grass system managed with three or six cuts per growth season and 0 or 400 kg N ha−1.

The two methods were found not to differ statistically. However, as fertilizer recovery must be identical for grass in monoculture and in a clover-grass system in order to get identical estimations of biologically fixed nitrogen when using the two methods, it was indicated that under a high N regime the total-N difference method underestimated the amount of fixed N in the harvested clover material by up to 33%. The choice of method will depend on the environmental conditions and the purpose of the experiment, as well as the resources available.

Three sinks of biologically fixed nitrogen were identified: first, the amount of fixed N bound in the existing root system, secondly, the amount of fixed N in root and stubble derived organic matter and, thirdly, the amount of mineralized fixed N. A method is proposed to estimate the amount of fixed N, mineralized and bound in newly dead organic matter, using fertilizer recovery to characterize the dynamics under stubble height. When only measuring the harvested leaf- material the biologically fixed nitrogen was underestimated by 19%—25%.