Empirical model for quantification of symbiotic nitrogen fixation in leguminous crops

Summary

Symbiotically fixed N2 (SNF) is an important input of nitrogen (N) in many cropping systems. In spite of this fact, the input of fixed N to a particular cropping system is usually not known, and an exact estimation of the amount of N2 fixed requires laborious measurements. Consequently, there is a demand for a simple and reliable tool to estimate N2 fixation in leguminous crops. A model is here proposed for quantification of SNF in leguminous crops in different arable systems. The model estimates SNF using dry matter yield as input and cropping system specific values for the parameters: N concentration in dry matter and the 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 fixed N in aboveground plant tissue. Thus the model operates with the ratio of fixed N in below-ground plant tissue, the ratio of fixed N transferred below-ground to the grass, the ratio of fixed N transferred to the grass through the grazing animals and the ratio of fixed N immobilised to the soil organic pool by rhizodeposition. The model is able to predict the SNF in harvested biomass precisely (r2=0.91). However, independent data for validation of the amounts of fixed N located in pools belowground is lacking.