Effect of bacterial root symbiosis and urea as source of nitrogen on performance of soybean plants grown hydroponically for bioregenerative life support systems (BLSSs)

摘要

Soybean is traditionally grown in soil, where root symbiosis with Bradyrhizobium\udjaponicum can supply nitrogen (N), by means of bacterial fixation of atmospheric N2.\udNitrogen fertilizers inhibit N-fixing bacteria. However, urea is profitably used in soybean\udcultivation in soil, where urease enzymes of telluric microbes catalyze the hydrolysis\udto ammonium, which has a lighter inhibitory effect compared to nitrate. Previous\udresearches demonstrated that soybean can be grown hydroponically with recirculating\udcomplete nitrate-based nutrient solutions. In Space, urea derived from crew urine could\udbe used as N source, with positive effects in resource procurement and waste recycling.\udHowever, whether the plants are able to use urea as the sole source of N and its effect\udon root symbiosis with B. japonicum is still unclear in hydroponics. We compared the\udeffect of two N sources, nitrate and urea, on plant growth and physiology, and seed\udyield and quality of soybean grown in closed-loop Nutrient Film Technique (NFT) in\udgrowth chamber, with or without inoculation with B. japonicum. Urea limited plant growth\udand seed yield compared to nitrate by determining nutrient deficiency, due to its low\udutilization efficiency in the early developmental stages, and reduced nutrients uptake (K,\udCa, and Mg) throughout the whole growing cycle. Root inoculation with B. japonicum\uddid not improve plant performance, regardless of the N source. Specifically, nodulation\udincreased under fertigation with urea compared to nitrate, but this effect did not result in\udhigher leaf N content and better biomass and seed production. Urea was not suitable as\udsole N source for soybean in closed-loop NFT. However, the ability to use urea increased\udfrom young to adult plants, suggesting the possibility to apply it during reproductive\udphase or in combination with nitrate in earlier developmental stages. Root symbiosis\uddid not contribute significantly to N nutrition and did not enhance the plant ability to\uduse urea, possibly because of ineffective infection process and nodule functioning in\udhydroponics.