Grain legumes in crop rotations under low and variable rainfall: are observed short-term N benefits sustainable?

Increased grain yields are frequently obtained when a cereal follows a grain legume in sequence compared with a cereal-cereal rotation. The biotic (disease break) and abiotic (N supply) components of the observed benefits are identified and methods for differentiating the components are discussed. Annual measurements of the N balance of grain legume-cereal rotations are extremely variable when measured over short time periods and are therefore not useful as indicators of cropping system N sustainability. While measurement of long-term changes in total soil N is a valid index of N sustainability, this approach is impractical. We suggest an alternative avenue for the assessment of N sustainability by using simulation modelling, after validation, which takes climatic and biophysical parameters into account. [ABSTRACT FROM AUTHOR]