Hydroponic cultivation of soybean for Bioregenerative Life Support Systems (BLSS): the effect of nitrogen source and bacterial root symbiosis

Soybean [Glycine max (L.) Merr.] is one of the plant species selected as a candidate for hydroponic cultivation in biological life support systems (BLSS) for human space exploration, because of the high nutritional value of seeds, rich in proteins and lipids. Nitrate based recipes are the most common to feed crops in hydroponics, however, they require a high amount of acid to buffer pH fluctuations in the recycling nutrient solution. In this respect, urea deriving from crew urine, which represents about 85% of recycled nitrogen, could be considered as an ammonium source for plant growth, to partially replace nitrate, while improving the root zone ionic balance. Root symbiosis with Rhizobium could also contribute to nitrogen nutrition, supplying ammonium by means of bacteria fixation of atmospheric nitrogen. In addition, the efficiency of plant-bacterial symbiosis is negatively affected by nitrate, while it seems to be enhanced by ammonium. The aim of this experiment was to evaluate the effects of nitrogen source in the nutrient solution and bacterial root symbiosis, in terms of plant growth, seed production and quality and resources use efficiency. Plants of soybean cultivar ‘OT8914’ were cultivated in a Nutrient Film Technique system, under controlled environmental conditions. The following treatments were factorially combined: two nitrogen sources in the nutrient solution: urea vs nitrate; root symbiosis with Bradirhizobium japonicum: inoculation vs not inoculation. Results showed that urea as the sole nitrogen source did not fulfil the needs of soybean plants, particularly during the early growth stages, resulting in significant reduction in plant growth and seed yield compared to nitrate. Root inoculation with B. japonicum did not improve plant performances, regardless of the nitrogen source. The positive effect of urea expected on root nodulation was not sufficient to improve the performance of inoculated plants compared to those not inoculated