Bell Pepper Plant Growth Responses and Soil Property Changes to Humic Substances and Deficit Irrigation in Controlled Environments.

Organic matter-derived soil amendments containing humic substances (HS) have a functional role to mitigate soil quality degradation, especially organic matter loss and biological unbalance, but their response to water deficit levels is unknown. In this study, we mixed lignite-derived humic substances (HS) with pot soil and applied severe (20% water holding capacity, WHC), mild (40% WHC), moderate (60% WHC) deficit and sufficient (80% WHC) irrigation levels to bell pepper (Capsicum annuum L.) plants grown in growth chamber and greenhouse conditions. Plant morphology and physiology were assessed biweekly in growth chamber and monthly in greenhouse, while plant root traits, soil chemical properties, and soil microbial properties were analyzed at the end of the study. HS significantly increased plant height and stem diameter during seedling development, and decreased plant transpiration rates during early growth while maintaining photosynthesis to the same level as control (increased water use efficiency) especially under severe and mild deficit levels. HS significantly increased plant biomass accumulation in the growth chamber but less in the greenhouse. HS increased root length, surface area and dry weight which might due to the higher nitrogen content in HS treated soil. HS also significantly increased total bacterial in soil. Severe and mild deficit stress significantly decreased plant growth performance, but was similar in moderate and no stress levels. The results indicated that HS may reduce water loss in plants exposed transiently to water deficit levels. This suggests that HS can ameliorate mild to severe stress short-term, with the potential long-term agricultural use under moderate stress or no stress conditions by increasing soil nutrient cycling capacity and microbial enrichment and activity.