Enzyme-specific responses of the tomato rhizosphere in salt-affected soil under different agronomic treatments

Soil salinization is a global problem that compromises soil fertility and crop production. This study aimed to investigate the rhizosphere soil of tomato plants under different agronomic treatments in salt-affected soil during two stages of crop growth (flowering and harvest phases). Specifically, the research focused on evaluating changes in the properties of i) control soil without any amendments, ii) soil amended with 25 Mg ha-1 of composted digestate, and iii) soil inoculated with 10 kg ha-1 of plant growth-promoting microorganisms. Salt ion accumulation in the investigated soils had a negative impact on microbial biomass and enzyme activity, with chloride content exhibiting the highest toxicity. The activity of hydrolases involved in the C, N, and S cycles was higher in the rhizosphere soil compared to the bulk soil, regardless of the stages of crop growth. Conversely, enzymes involved in the P cycle showed a different behaviour between bulk soil and rhizosphere soil depending on time. The rhizosphere soil exhibited higher phosphatase activity compared to the bulk soil during the flowering phase, with a significant reduction of activity at the harvest stage. Overall, the different treatments had a scarce impact on plant performance, as demonstrated by both very similar productivity and agronomic parameters. Therefore, the response of tomato to salt accumulation during the monitored crop cycle is mainly based on the ability of plants to affect rhizosphere soil properties according to the development stage.