Biochars improve tomato and sweet pepper performance and shift bacterial composition in a peat-based growing medium

We previously observed that the biochars used in this study can replace 15% (v/v) of perlite in a peat-based growing medium (PBGM) without causing any harm to microbial biomass. However, little is known about their impacts on the plant-microbe-soil interactions and crop productivity. Thus, the aim of the present work was to evaluate in greenhouse trials, the effect of substituting perlite in PBGM by three types of biochar on tomato cv. Micro-Tom and sweet pepper cv. Redskin, growth and yield, on their water and fertilizer use efficiency and on the bacterial diversity found in PBGM. Biochars were derived from maple bark pyrolysed at 550 °C (M550) and 700 °C (M700) and from pine chips pyrolysed at 700 °C (P700). Tomato and sweet pepper plants were grown in a greenhouse and fertigated with mineral fertilizer at full or half of the recommended level. Compared to the control without biochar, adding 5, 10 or 15% (v/v) biochar improved plant water use efficiency and increased tomato and sweet pepper fruit dry weight yield respectively, by up to 32% and 54%. Biochar significantly increased N and P uptake efficiency, while reducing loss of NO3− and PO43− in leachates. In PBGM used to grow sweet pepper under full fertigation, the presence of 15% (v/v) M700 or P700 biochars significantly increased bacterial richness probably through the improvement of some ecological micro-niches such as C accessibility. This also allowed the establishment of potential plant growth promoting bacteria particularly Agrobacterium, Cellvibrio and Streptomyces, which showed a positive correlation with plant productivity and the chemical properties of PBGM. Our results suggest that the higher PBGM pH reached by adding 15% (v/v) biochar improved plant growth by increasing carbon, nitrogen and phosphorus availability and by favoring the establishment of plant beneficial bacteria. Therefore, substituting 15% (v/v) of perlite in PBGM with the three tested biochars appears to be a beneficial practice towards sustainable greenhouse production of tomato and sweet pepper, and this practice merit further investigation with other crops such as indeterminate cultivars. •Biochar can replace by up to 15% volume of perlite in a peat-based growing medium.•Biochar increased biomass and fruit yield by improving nutrient accessibility.•Biochar increased efficiency of N and P uptake and of water use.•Biochar can increase bacterial richness and induce shifts in bacterial composition.