Effect of Aloe Vera Residue Hydrochar on Two Successive Sowings of Cherry Tomato and on Soil Microbial Activity

Abstract To evaluate the effects of aloe vera-based hydrochar on tomato ( L. esculentum var. cerasiforme ) plants and on soil microbial functioning and diversity. Are the potential impacts of hydrochar on soil biota acceptable? Could hydrochar derived from aloe vera residues be valorized as a soil amendment? Two experiments were conducted with two soils amended at concentrations ranging from 0.5 to 4% (w/w). The first experiment assessed the effects of raw hydrochar on germination, growth, photosynthetic pigments, and oxidative stress biomarkers in tomato, along with several soil enzymatic activities, after 35 days and reassessed after six months with a second planting of tomato seeds. The second experiment focused on the impact of raw and water-washed, and hexane-washed hydrochar on soil respiration and microbial diversity. Most effects were dose-dependent, transient or significantly diminished over time, consistent with the degradation of potentially toxic labile components in aged hydrochar. The main concerns were reduced germination and increased soil salinity. Carotenoid plant levels and soil enzymes (dehydrogenase, phosphatase and ammonium oxidase) were the most sensitive parameters, while changes in antioxidant enzymes, proteins and malondialdehyde levels in plants occurred only at the highest dose. Raw and washed hydrochar increased soil respiration and altered microbial diversity in the short term, with functionality adapting after seven days as shown by Biolog-EcoPlate analysis. Six-month aged raw hydrochar was deemed safe for agricultural use and is a promising approach to valorizing aloe residues. Washing could shorten this period.