Dynamics of biologically fixed N in legume-cereal rotations: a review

Agronomically significant N yield responses of cereals following grain legumes compared with cereal monoculture are frequently measured. The positive N response of the cereal has been attributed to the transfer of biologically fixed N, to N-sparing under the antecedent legume, and to less immobilisation of nitrate during the decomposition of legume residues. Methods for estimating the transfer of biologically fixed N in rotations, and for separating the N benefit into fixed N and non-fixed N components, are reviewed. Available data indicate that both sources of N contribute to the N benefit. The role of the grain legume in the gain or drain of soil N is evaluated by considering the balance between symbiotic dependence and N harvest index, as well as long-term changes in total soil N. Several 15N-based techniques for direct estimation of inputs of biologically fixed N to the soil N pool are reviewed. N balances in grain legume-cereal rotations may be positive or negative depending on the legume species, symbiotic performance, and agronomic factors.