Rhizobial inoculation and phosphorus and zinc nutrition for annual medics adapted to Mediterranean environments

Abstract

Cereal production in the Mediterranean region (where rainfall averages 200 to 600 mm yr-1) has traditionally used fallowing in alternate years to conserve crop-limiting soil moisture. Self-regenerating pasture medics (Medicago spp.) were introduced to provide forage for livestock in the alternate year and to reduce the cereal's need for fertilizer N; their adaptation depends on compatible Rhizobium meliloti bacteria for N2 fixation and on climatic conditions. In addition, nutrients such as P and Zn are potentially limiting factors. Therefore, in a greenhouse experiment (60 d) using a P-deficient, air-dried, nonsterilized clay soil (Calcixerollic Xerochrept) treated with P (0, 15, 45, and 135 mg kg-1) and Zn (0 and 5 mg kg-1), we assessed growth of four annual medic species with and without rhizobial inoculation and fertilizer N: Medicago polymorpha L., M. rotata Boiss., M. rigidula (L.) All., and M. noeana Boiss. Except for M. noeana, growth of all species responded significantly to applied P and to Zn only with adequate P and N levels (or rhizobial inoculation). Pod number and root biomass were also increased by P application, with differences occurring between species. Thus, when a medic species is newly introduced, inoculation has a role to play where no compatible rhizobia exist. Phosphorus fertilization and medic seed inoculation can easily be done, while the use of adapted biotypes in low-Zn soils is more problematic