Impact of composts from decentralized composting models on R. officinalis physiology and nutrient contents under abiotic stress

The loss of organic matter in soils, in conjunction with water scarcity and salinization constitutes a grave problem in the Mediterranean region. The utilization of composts derived from novel decentralized models, such as agrocomposting, community composting and decentralized urban composting, is hypothesized as an effective strategy with the potential to enhance the physical, chemical and biological characteristics of the soil. The application of compost has been demonstrated to enhance the resilience of plants to abiotic stresses, including drought and salinity. In this context, eight decentralized model composts were employed as organic amendments in a degraded soil to assess whether they could mitigate the stress experienced by the Mediterranean shrub R. officinalis under conditions of drought or salinity for one month. Therefore, measurements of gas exchange, emission of volatile organic compounds and reflectance during stress were performed in order to determine the degree of adaptation of the plants. Composts from community composting were found to be more effective in alleviating the effects of abiotic stress, while the ineffectiveness of composts from decentralized urban composting depended on their characteristics (e.g. humic substances and electrical conductivity). An initial increase in monoterpene emission rates was observed, which decreased at the end of the experimental time in a positive correlation with leaf RWC. Stress also led to a reduction in photosynthesis (A) and stomatal conductance (gs). Reflectance was more affected under drought conditions and by the application of compost with a high electrical conductivity, which likely hindered chlorophyll synthesis. Thus, the present results have enabled the selection of appropriate composts for utilization in agriculture under stress conditions considering plant biomass values and compost characteristics (EC, humic substances content, and the K/Na ratio).