Assessing the effects of compost-derived beneficial bacteria on tomato plants using a Rhizobox system

Over the past decades, the widespread use of chemical fertilizers and intensive agricultural practices has contributed to soil degradation and the progressive loss of soil biodiversity, a key factor in the interactions between plants and soil organisms. In particular, soil microbial communities play a fundamental role in plant growth and health by enhancing nutrient acquisition, immunity, and increasing tolerance to environmental stresses. In this context, the application of biofertilizers such as compost represents a sustainable strategy to improve soil fertility and restore these biological interactions. This study evaluated the effects of compost-derived plant growth-promoting (PGP) bacteria on tomato plants (Solanum lycopersicum) using a Rhizobox system that enables detailed observation of root architecture development. In the first phase, four previously isolated bacterial strains were subjected to in vitro antagonism assays to identify compatible inocula. The results led to the selection of Kocuria rhizophila and Glutamicibacter arilaitensis. Subsequently, plants were grown in three different substrates (soil alone, soil amended with compost, and soil amended with sterile compost) and subjected to four treatments (control, single inoculation with K. rhizophila, single inoculation with G. arilaitensis, and inoculation with a bacterial consortium). Despite the high mortality observed in the Rhizobox system, the surviving plants provided preliminary insights into the combined effects of soil conditions and bacterial inoculation on plant physiology, assessed through biomass and photosynthetic efficiency measurements. In parallel, soil water retention capacity and available water content (AWC) were analysed to determine whether plant responses could also be attributed to compost-induced changes in soil hydraulic properties. The results showed that compost-amended soils exhibited higher AWC than non-amended soil, indicating an improved capacity to retain and supply water to plants. Further studies will be necessary to optimize the experimental system and to better understand the potential of compost-derived bacteria as biofertilizers for sustainable agriculture.