Wastewater sludge as a substrate for growth and carrier for rhizobia: the effect of storage conditions on survival of Sinorhizobium meliloti

The inoculation of legumes with rhizobia is used to maximise nitrogen fixation and enhance the plant yield without using N fertilisers. For this reason many inoculant types were developed and optimised. In our study, the effects of the growth medium, the carrier, the temperature and the storage period were determined on the survival of Sinorhizobium meloliti. Secondary sludge from Communauté Urbaine de Québec wastewater treatment plant and standard medium (YMB) were used for rhizobial growth. Dehydrated sludge from Jonquière wastewater treatment plant, peat and a mixture of peat and sludge were used as carrier materials. Results showed that the wastewater sludge offered better protection for rhizobia survival during freezing and thawing at −20 °C than the standard medium. In general, results also showed the suitability of using sludge as a carrier because it had the same or a higher potential than peat to support survival of S. meliloti. In the case of YMB-grown rhizobia, peat- and sludge-based carriers appeared to be similar in terms of survival rate during the storage at 4 and 25 °C. For secondary sludge-grown rhizobia, the survival was better in sludge than in peat based carrier. Generally, the cell count remained higher than 108 cells/g for up to 80 days at 4 and 25 °C in both carriers (sludge and peat). However, for the secondary sludge-grown cells stored in peat-based carrier at 4 °C, the viable cells decreased under 108 cells/g at the 81st day of storage but remained acceptable compared to the standard (107 cells/g of carrier).