Nodulation and biomass development of yellow and blue lucerne under pH stress

Abstract: Approximately 77% of global arable land use is devoted to forage cultivation, which is the basis for animal production and global food security. Therefore a sustainable and biologically diverse forage cultivation is needed. Feed for cattle needs to contain a good balance between fibre, energy and protein. Legumes are important for forage quality and fix their own nitrogen, reducing the need for N-fertilization. Medicago sativa L. (lucerne) is an agronomically important legume species that is cultivated on over 30 million hectares worldwide. Lucerne fixates N² in symbiosis with bacterial species E.meliloti, which attaches to the root and forms nodules. E. meliloti is acid sensitive and at pH<6 nodulation is reduced. This leads to decreased biomass production and quality. The aim of this study was to evaluate the pH tolerance of two lucerne subspecies; Blue lucerne (BL) and yellow lucerne (YL). The hypotheses were 1) YL will be more tolerant than BL against low pH, but BL will produce more biomass than YL in high pH conditions; 2) A low pH will decrease the nodulation formation and quality for BL and YL; and 3) A low pH will decrease biomass quantity for BL and YL. A mineral soil with different rates of lime was used to create six treatments with pH 5.7-6.5. 60 pots containing YL and BL cultivars inoculated with Nitragin Gold, a commercial E. meliloti mix were placed in a randomized block design in a greenhouse chamber. After 75-81 days the pots were harvested. Non-destructive measurements were made during growth and destructive measurements were made at harvest. The significance of four factors; cultivar, pH, cultivar'pH and time'cultivar, were statistically tested for 11 response variables connected to biomass and nodulation. The effect of liming on biomass and nodulation was also evaluated in a regression analysis using lime rate as an explanatory variable. The soil pH kept rising after sowing, even for the non-limed soil, and ended up higher than intended at 6.5-6.8. This meant it was not possible to properly evaluate the effect of pH stress on BL and YL. The statistical analyses partly confirmed and partly contradicted hypotheses 2) and 3). In the end, however, none of the hypotheses could be confidently confirmed since the right conditions for the study were lacking. One practical finding of this study is that Nitragin gold is compatible with both SW Nexus and Karlu. The pH sensitivity of E. meliloti is also highlighted. Future directions for research would be to continue researching YL response to pH stress, as well as E. meliloti strain compatibility, perhaps in a Northern Swedish climate over several seasons, using several cultivars.