Two-season effects of grape pomace biochar on soil properties, nutrient availability, plant growth and soil microbial communities

Grape pomace biochar represents a promising pathway for valorizing winery residues into a nutrient-enriched soil amendment. However, its agronomic performance remains uncharacterized. This study evaluated its effects at application rates of 5, 10, and 15% on soil properties, nutrient dynamics, microbial characteristics, and lettuce ( Lactuca sativa L. ) growth over two growing seasons in a pot experiment. Biochar significantly modified soil chemistry, increasing pH, TOC, TN, WHC, and CEC in a dose-dependent trend. The total and Mehlich 3-extractable K and P increased significantly, with bioavailable K increasing 8 times in the first and 10 times in second season, and bioavailable P increasing 2 times in both seasons. Soil microbial biomass remained stable, while basal respiration and structure of the microbial community shifted consistently with dose, indicating an increasingly active and G-bacterial and fungal-oriented soil microenvironment. Lettuce biomass increased by 17% at the lowest biochar dose and by up to 27% at the highest dose during the first season, demonstrating its agronomic benefits. No increase in biomass was observed in the second season due to unfavorable conditions primarily attributed to the late planting date, although soil improvements and microbial stimulation persisted, suggesting that the soil response to biochar is more stable than the short-term response of the plant. In general, grape pomace has been proven to be a safe and effective amendment that provides sustained nutrient release and improves soil function. The 5% dose appears to offer the most practical balance between agronomic benefit and resource input.