Zeolite-amended peat carriers improve long-term survival of Ensifer meliloti and maintain alfalfa nodulation efficiency.

Microbiological N fertilizers containing native highly effective and competitive rhizobial bacteria placed in a suitable carrier are the best choice for sustainable plant production. This study evaluated the ability of various peat-based formulations with different zeolite/CaCO₃ amendments to support the survival and preservation of symbiotic efficiency of alfalfa rhizobia. 15 different inoculants were tested, including five nitrogen-fixing efficient Ensifer meliloti strains (218, 225, 252, 4148ss and 4193cs) in three different carriers made of peat and perlite and supplemented with micro-zeolite (B1), nanozeolite (B2), and micro-zeolite + CaCO₃ (B3). The number of rhizobia after 90 days of storage at room temperature was above 10¹⁰ CFU/g of inoculant in most of the combinations, after 180 days of storage was 10⁹ CFU/g in all combinations, and in 330^th day stayed above 10⁸ CFU/g of inoculant for strains 218, 4148ss, and 252. Considering the whole period of storage, the most performant carriers were B3, with the addition of micro-zeolite + CaCO₃ and B2 with nanozeolite amendment. The efficiency of all inoculants, measured by the nodulation ability and shoot dry weight (SDW), was confirmed in inoculated alfalfa plants after six and eleven months of storage. The SDW of inoculated alfalfa plants increased from 2.5 up to 5 times compared to non-inoculated plants, while the nitrogen content (mg/plant) was also significantly increased. The SDW was influenced by strain, while the type of carrier and storage of inoculant did not have an effect. The inoculant formulations with zeolite and calcium carbonate showed good potential for rhizobia survival and effectiveness preservation during long-term storage, which should be further tested under field conditions.