Microbial Metabolic Profile of Two Compost Teas and Their Biostimulant and Bioprotectant Effects on Chickpea and Pea Plants

Compost teas (CTs) can be considered natural microbial consortia, able to enhance biostimulation and defense in crops. This study focuses on two plant-derived CTs and their potential use as eco-friendly biofertilizers for chickpeas and peas, with the broader aim to protect soil fertility. Our experiments demonstrated that the two CTs have biostimulatory or inhibitory effects depending on dilution, target plant species, CT microbial load and metabolism, and age of CT preparation. Peas exhibited positive responses to treatments, while chickpeas could be negatively affected depending on CT concentration. The CT microbial load positively affected biostimulation for both plant species. The metabolic profiles of the CT-associated microbial communities were evaluated using the Biolog EcoPlate (TM) system. Spearman's correlation analysis allowed us to ascertain a positive interaction between root elongation and the microbial consumption of specific substrates, namely polymers, erythritol, and L-serine. On the contrary, phenolic compound consumption showed a negative correlation. In chickpeas, root and collar necrosis, estimated with the McKinney index, increased after treatment with CTs at the highest concentration, confirming a phytotoxic effect; but diagnostic analyses demonstrated that the necrosis was also partially attributed to pathogenic Fusarium spp. On the other hand, proper dilutions of treatments determined a decrease in necrosis severity, indicating putative CT biocontrol properties.