Impact of different rhizobial strains and reduced nitrogen supply on growth, yield and nutrient uptake in cowpea grown hydroponically

Abstract

One of the strategies that contributes to more sustainable crop production in greenhouses is the reduction of nitrogen input without compromising yield. In legume crops, this goal can be achieved by using efficient rhizobial strains capable of biologically fixing atmospheric nitrogen to inoculate the seeds or the young seedlings. In soilless culture, inoculation of legumes with rhizobia can considerably reduce the input of inorganic nitrogen when preparing nutrient solutions. However, the practical application of this approach faces some difficulties. On the one hand, the supply of plant-available nitrogen is important at the early growth stage, when the rhizobia are still not functional in terms of N(2) fixation. On the other hand, inorganic nitrogen and especially nitrate N inhibits rhizobium colonization. To cope with these two contrasting needs, fine tuning of the nitrogen supply is needed, whereby legumes grown in soilless culture are inoculated with relevant rhizobial strains. In the present study, two different indigenous rhizobia, Bradyrhizobium sp. VULI1.1 and Ensifer sp. VUKA2, and a mixture of them were used to inoculate cowpea [Vigna unguiculata (L.) Walp.] grown in Perlite and supplied with different nitrogen levels to test their inoculation and nitrogen-fixation ability. Plants were supplied with either full-N (total-N 11.2 mM) or 60% of full-N until the flowering stage. Afterwards, the nitrogen supply was reduced to either 30 or 0% until the end of the cultivation period. Nodule number and weight per plant were also estimated. The reduction of nitrogen supply to 60% of the full requirements during the vegetative stage and 0% during the reproductive stage led to increased nodule weight but decreased number of nodules per plant, which resulted in reduced growth and yield. However, supplying 60% of the full nitrogen requirements up to the flowering stage and 30% thereafter increased both nodulation and yield in rhizobium-inoculated cowpea plants in comparison with 100% nitrogen supply without rhizobium inoculation throughout the cropping period. These results indicate that applying different rates of inorganic nitrogen before and after the formation of N(2)-fixing nodules could be considered an effective strategy to reduce nitrogen fertilizer supply in hydroponic cowpea crops without compromising yield.