Effects of Lactobacilli inoculum on tomato plants, soil nitrogen transformations and greenhouse gass emissions.

In the framework of the EU Green Deal, the reduction of chemical fertilization is promoted to enhance sustainable agriculture. An ecological option is to utilize the potential of soil microorganisms to improve plant growth and secure crop production. Past studies focused mainly on Rhizobia and their specific plant-growth promoting effects on target plant hosts (N-fixing). Recently, Lactobacilli (Firmicutes) gained increasing appreciation as soil nutrient regulators, besides their known plant protecting properties, for a broad range of plant hosts.

In this pot experiment, we studied the short-term effects of a Lactobacillus soil inoculant (LB) on tomato plants (Solanum lycopersicum L. var. extasi) and nitrogen kinetics, in two different soils (sandy (S), loamy (L)), with chemical (U; urea; 460 N mg/kg) and organic (DC; digestate compost; 22 g/kg) treatment. An unfertilized control (C) treatment for each soil was also included. The experiment lasted 115 days and two experimental setups run in parallel; soil containers a) without and b) with plants. During the experiment soil concentrations of NH₄+, NO₃-, NO₂-, CO₂ and N₂O were measured. At the end of the experiment, additional agronomic traits (total biomass, total N) and soil N mineralization potential (AMN) were measured. 

The addition of LB in C and DC treatments increased soil NH₄+ that ranged 1.5 – 28% relative to the treatments without LB, for both soils. When LB was added in the U treatment we observed a negative effect, -3% and -54% for L and S soil, respectively. A contradicting pattern was observed for soil NO₃-, when LB was added in all treatments, soil NO3- increased and decreased in L and S soil, respectively. Soil NO₂-, CO₂ and N₂O emissions increased in DC and U treatments for both soils, relative to C. Interestingly, when LB was added we observed a consistent decrease in soil NO₂- and N₂O emissions but a consistent increase in CO₂ emissions for both soils was observed, relative to those treatments without LB. The addition of LB had a positive effect on plant biomass and total plant N for all treatments and for both soils, except U+LB treatment in S soil. As for AMN rates, there was not a consistent pattern.

In conclusion, our preliminary results indicate a positive effect of non-N-fixing lactobacilli inoculant on plant attributes for soils amended with compost. Furthermore, we documented a microbial approach to mitigate potential N₂O emissions from organic amendments for the first time.

The BSc research work by Katerina Bouzaki was partly supported by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the 2nd Call for H.F.R.I. Postdoctoral Research Projects (#1053) awarded to Principal Investigator Dr. Georgios Giannopoulos.

This project was co-implemented with industrial partner Corteva Agriscience Hellas SA.