Epsilon Archive for Student Projects

Abstract

Soybean is a tropical legume that is widely used world-wide as a food product. Its advantage is its ability to fix nitrogen from the atmosphere to produce a high amount of protein, making soybean suitable as food and animal feed. Sweden imports large quantities of soybean, since domestic production is far below the demand. However, imports have negative impacts on the global environment, owing e.g. to deforestation in countries where soybeans are produced and long transportation routes. By increasing the production of soybean in Sweden, these negative effects could be avoided.
In order to carry out nitrogen fixation, soybeans need to co-exist with the nitrogen-fixing bacterial species Bradyrhizobium japonicum. This requires inoculation of soybean seed with different cultures of the bacterium before sowing, a practice which is currently performed every time soybean is grown in Sweden.
This study examined if annual re-inoculation is necessary, or if B. japonicum can survive in the soil after a soybean crop grown in field which is inoculated with this bacterium. Other questions examined were how long B. japonicum can survive in the soil, how different soil factors and cultivation measures affect B. japonicum, if there is a difference in survival rate of B. japonicum in different soil types and if the inoculation cultures Hi-Stick and E11 are suitable as inoculation material and result in similar nitrogen fixation.
These issues were studied in a greenhouse experiment with soybeans cultivated in different soils collected from fields in which soybeans had/had not been grown previously. It was found that B. japonicum was able to survive in soils in which soybean had been grown previously and that there was no need for re-inoculation of soybean seeds during a period of at least two years. The soil factors pH, nitrogen content and carbon content had varying and inconsistent effects on the survival rate of B. japonicum, making it difficult to estimate their overall influence. However, application of nitrogen (as manure) and high amounts of phosphorus in soil appeared to promote B. japonicum survival. The amount of potassium in soil had varying effects, but high amounts appeared to impair B. japonicum survival. As regards to soil type, B. japonicum survived better in light soils than in clayey soils. Both the inoculant cultures tested Hi-Stick and E11, proved to be suitable as inoculants of soybeans in this greenhouse study.