The Effects of Salinity and Nitrogen Fertilizer on Growth and Nitrogen Uptake of Alfalfa.

The response of alfalfa to three levels of salinity and four rates of nitrogen (N) was measured in a pot study at California State University, Chico. Seeds were inoculated with a commercial rhizobia inoculant and planted in plug trays in the greenhouse. Seedlings were transplanted into 5-gallon buckets filled with a mixture of soil, sand and peat moss, and placed in an open field. All plants received macro- and micro-nutrients, plus equivalent to 30 kg ha^-1 starter N at the planting. After the first cut, plants received four levels of urea fertilizer (0, 30, 60, and 120 kg N ha-1) with the first irrigation. At the onset of salinity treatments, all plants were cut to just above the crown, and subsequently, the plants were watered with tap water, low salinity (EC=~5 ds/m) or high salinity (EC= ~10ds/m) water with the saline solutions prepared using NaCl, Na₂SO₄, MgSO₄ and CaSO₄·2H₂O. Daily watering was based on the total plant evapotranspiration and the pot field capacity to eliminate leaching. Preliminary results show that plants irrigated with tap water (non-saline) produced 10 and 27% more biomass than plants under the low EC and high EC, respectively. Plant biomass and total plant N (g/pot) were similar across the three N fertilizer treatments; however, on the average, N-treated plants produced 18% more biomass and accumulated 45% more N compared to the control (0 N fertilizer). Results will be presented in more detail including δ¹⁵N comparisons to compare N acquisition from biological N fixation vs. applied mineral N.