An empirical model for quantification of symbiotic nitrogen fixation in grass-clover mixtures

Symbiotically fixed dinitrogen (N2) is an important nitrogen (N) source in many agricultural systems. In spite of this fact, the input of symbiotically fixed N to a particular cropping system is usually not known and exact measurements are costly and laborious. Consequently, there is a demand for an uncomplicated and reliable tool to estimate symbiotic N2 fixation (SNF) in leguminous crops.

A simple empirical model is proposed for quantification of SNF in forage legumes under low external input conditions. The model estimates SNF using dry matter yield as input and parameters for (i) N concentration in dry matter and the (ii) proportion of the N in the legume that is derived from the atmosphere.

Further, the model includes fixed N not included in the estimate of SNF in aboveground herbage. Thus the model operates with parameters for (iii) the ratio of fixed N in below-ground plant tissue, (iv) the ratio of fixed N transferred below-ground to the grass, (v) the ratio of fixed N transferred to the grass through the grazing animals, and (vi) the ratio of fixed N immobilised to the soil organic pool by rhizodeposition.

A general value for each parameter is suggested although the need for site-specific parameterisation is underlined.