Impact of Salinity on Biological Nitrogen Fixation in Alfalfa (Medicago sativa) and Its Response to Applied Mineral Nitrogen.

As irrigation water supplies become scarce in California, more saline water will likely be used to irrigate field crops and forages such as alfalfa. A valid research question to ask is how salinity impacts biological nitrogen fixation (BNF) and its contribution toward fulfilling the nitrogen requirements of alfalfa. Our greenhouse study consisted of two experiments. The main experiment utilized inoculated seed (dusted with Sinorhizobium meliloti bacteria) of variety CUF101 sown into 40 cm deep pots containing washed sand and irrigated with water of either low salinity (LS = 1.5 dS/m EC_w) or high salinity (HS = 6.4 dS/m EC_w, emergence phase; 7.5 dS/m EC_w, mature plant phase) in combination with three nitrogen levels, 0 ppm, 18.7 ppm (=1/12 Hoagland’s N), and 112 ppm (=1/2 Hoagland’s N).  The non-inoculated experiment consisted of the same treatments, but the N1 level was excluded.  For Phase I (emergence), fifty seeds were sown in each pot, emergence percentages were determined and the plants were then thinned to 2 per pot. The adjusted emergence % at LS (1.5 dS/m) was 99%, while for HS (6.4 dS/m), only 49% of the plants emerged within 30 days. The difference between emergence at LS and HS salinity levels in both inoculated and non-inoculated treatments was statistically significant (P < 0.05). There were no significant differences in emergence between inoculated and non-inoculated treatments. For Phase II, the alfalfa will be harvested every three to four weeks to determine shoot dry weight and total N, Na⁺, K⁺ and Cl^- content. The method to assess biological nitrogen fixation will be the Total Nitrogen Difference method using non-inoculated plants for the reference.  Nodule counts and weights will be taken after the final harvest. The experimental results will be used to compare the alfalfa yield response to applied nitrogen under saline vs. non-saline conditions.