Methylobacterium symbioticum Applied as a Foliar Inoculant Was Little Effective in Enhancing Nitrogen Fixation and Lettuce Dry Matter Yield

Nitrogen (N) is a limiting ecological factor for plant growth in most agroecosystems. Biological N fixation, especially from nodulated legumes, has been promoted in recent decades as an alternative or complement to industrially synthesized N fertilizers. The possibility of utilizing N-fixing organisms from the phyllosphere that demonstrate effectiveness across a wide range of crops is particularly exciting. In this study, we examined the N-fixing capacity and the impact on lettuce growth of an inoculant recently introduced to the market, which contains the microorganism Methylobacterium symbioticum and is recommended for various cultivated species. A pot experiment was conducted using a factorial design, which included the inoculant (No and Yes) and four N rates (0 (N0), 25 (N25), 50 (N50), and 100 (N100) kg ha −1 of N), with four replicates, over four lettuce growing cycles. The inoculant had a significant effect on dry matter yield (DMY) only during the second of the four growing cycles. The mean values of the four growing cycles ranged from 9.9 to 13.7 g pot −1 and 9.9 to 12.6 g kg −1 in pots that received and did not receive the inoculant, respectively. On the other hand, plants exhibited a robust response to N applied to the soil, showing significant increases in both DMY and tissue N concentration across all growing cycles. Mean values of DMY in the treatments N0 and N100 ranged from 5.6 to 8.9 g pot −1 and 12.5 to 16.1 g pot −1 , respectively. N concentration in tissues varied inversely with DMY, indicating a concentration/dilution effect. The difference in N concentration between treated and untreated plants, used as an estimate of fixed N, was very low for each of the soils’ applied N rates, assuming average values for the four growing cycles of −1.5, −0.9, 2.4, and 6.3 kg ha −1 for N0, N25, N50, and N100, respectively. This study emphasized the low amount of N supplied to lettuce by the inoculant and its limited effect on DMY. Generally, in biological systems with N-fixing microorganisms, achieving high fixation rates requires a high level of specificity between the microorganism and host plant, a condition that seems not to have been met with lettuce. Considering the importance of the subject, is imperative that further studies be conducted to determine more precisely in which crops and under what growing conditions the inoculant proves to be a valuable input for farmers and an effective method for reducing N mineral fertilization.