Nutrient Dynamics in a Long‐Term Cereal‐Based Rotation Trial in a Mediterranean Environment: Nitrogen Forms

Abstract

Mediterranean agriculture is mainly rainfed, with drought being the main crop production constraint, and is based on cereals, wheat (Triticum spp), and barley (Hordeum vulgare). Fallow was a traditional practice to conserve soil moisture, but because of land‐use pressure it is giving way to cereal monoculture, which is unsustainable. The substudy reported here was part of a long‐term rotation trial that sought to examine alternative crop rotation options, that is, durum wheat (T. durum var durum) in rotation with fallow, summercrop (melon, Citrullus vulgaris), wheat (continuous cropping), chickpea (Cicer arietinum), lentil (Lens culinaris), vetch (Vicia sativa), and medic (Medicago spp). Ancillary treatments involved nitrogen (N) applied to the cereal phase and variable stubble grazing intensity (stubble retention, medium grazing, and heavy grazing or complete stubble removal). This substudy, conducted in the final 3 years of the 14‐year trial, involved sampling soil and plants within the cropped rotation plots and sampling soil within bare microplots in selected larger rotation plots. We measured N forms in soil samples at different depths and throughout the seasons. Despite variation within and between seasons, the rotation effect of enhanced N was significant and consistent, being highest for vetch and medic, intermediate for chickpea and lentil, and least for continuous cereal, summer crop, and fallow. Therefore, legume‐based cereal rotations can enhance soil N and thus save on N fertilizer. In bare microplots, total N decreased, labile N was inconsistent, mineral N increased, and biomass N increased and remained stable during the cropping season and then sharply declined. The nutrient dynamic data complemented the crop yield, water‐use efficiency, and soil aggregation data from the trial to support the argument for using legumes in cereal rotations in place of fallow and continuous cereal cropping.