Co-Inoculation with Azospirillum brasilense and Bradyrhizobium sp. Enhances Nitrogen Uptake and Yield in Field-Grown Cowpea and Did Not Change N-Fertilizer Recovery

Abstract

This study was designed to investigate the effects of <i>Azospirillum brasilense</i> and <i>Bradyrhizobium</i> sp. co-inoculation coupled with N application on soil N levels and N in plants (total N, nitrate N-NO₃^- and ammonium N-NH₄⁺), photosynthetic pigments, cowpea plant biomass and grain yield. An isotopic technique was employed to evaluate ¹⁵N fertilizer recovery and derivation. Field trials involved two inoculations-(i) single <i>Bradyrhizobium</i> sp. and (ii) <i>Bradyrhizobium</i> sp. + <i>A. brasilense</i> co-inoculation-and four N fertilizer rates (0, 20, 40 and 80 kg ha^-1). The co-inoculation of <i>Bradyrhizobium</i> sp. + <i>A. brasilense</i> increased cowpea N uptake (an increase from 10 to 14%) and grain yield (an average increase of 8%) compared to the standard inoculation with <i>Bradyrhizobium</i> sp. specifically derived from soil and other sources without affecting ¹⁵N fertilizer recovery. There is no need for the supplementation of N via mineral fertilizers when <i>A. brasilense</i> co-inoculation is performed in a cowpea crop. However, even in the case of an NPK basal fertilization, applied N rates should remain below 20 kg N ha^-1 when co-inoculation with <i>Bradyrhizobium</i> sp. and <i>A. brasilense</i> is performed.