Alfalfa yield under elevated CO2 and temperature depends on the Sinorhizobium strain and growth season

The aim of the present study was to analyse the effect of elevated CO2 at different temperature, inoculation with three Sinorhizobium meliloti strains and growth in two seasons (summer and autumn) on plant growth, photosynthesis and N2 fixation (estimated as apparent nitrogenase activity, ANA). One of the objectives was to demonstrate that most efficient N2-fixing strains could improve alfalfa forage production. The second objective was to test if S. meliloti strains alter their efficiency under climate change conditions. The third objective was to show if seasonal changes may modify alfalfa productivity. Interaction of CO2 and temperature enhanced alfalfa dry matter in both seasons; however, plants produced more dry matter in autumn than in summer, due to the negative effect of elevated summer temperature on N2-fixation. Higher yield in summer corresponded to plants in symbiosis with 102F78 strain being not related to enhanced nodule dry matter or apparent nitrogenase activity but to putative lower carbon consumption for N2 fixing process. Contrariwise, in autumn the highest yield was obtained by 102F34 as a consequence of increased nodule dry matter induced under elevated CO2 and temperature.

► CO2 and T enhanced plant DM in summer and autumn regardless of the Sinorhizobium meliloti strain
► Alfalfa was less productive in summer due to the negative effect of high T on N2 fixation.
► In summer, growth of nodulated alfalfa under elevated CO2 was limited by N availability.
► Higher yield of alfalfa with 102F78 was related to lower C cost for N2 fixation in summer.
► Higher yield with 102F34 was related to increased nodule DM induced by CO2 in autumn.