This study explores the production of bio-nutrients from bioactive compound-rich spent coffee grounds (SCG) and biochar (BC) through composting after inoculation with a biological agent and its impact on the growth performance of garden cress and spinach. The SCG was composted with six doses of BC (0, 5, 10, 15, 20, and 25%). The compost with 10% BC exhibited the best maturity, humification, and phytotoxicity index values of dissolved organic carbon (DOC), humification index (E4/E6), and germination index (GI). A metagenome analysis showed that compost starter enhanced the bacterial community’s relative abundance, richness, and diversity in SCG and BC treatments. This improvement included increased Patescibacteria, which can break down noxious phenolic compounds found in SCG and BC. The BC enriched the compost with phosphorus and potassium while preserving the nitrogen. In plant growth experiments, the total chlorophyll content in compost-treated garden cress and spinach was 2.47 and 4.88 mg g−1, respectively, which was significantly greater (p ≤ 0.05) than in unfertilized plants and similar to the plants treated with traditional fertilizer. Overall, the results show that the compost of SCG + BC was well-suited for promoting the growth of garden cress and spinach, providing adequate nutrients as a fertilizer for these leafy vegetables.

Purpose: This study examines the integration of biomass ash derived from the combustion of poultry litter at 700 °C into a nutrient-rich biowaste fertilizer. The objective is to recycle waste nutrients back into the soil system, with a particular focus on the pigmentation and bioactive compound profiles of black carrots (BC). Method: BCs were cultivated using compost formulations prepared by blending biomass ash with dairy cow manure at application rates of 0% (T0), 15% (T1), 25% (T2), and 50% (T3). After harvest, yield, color parameters, total phenolic content (TPC), total anthocyanin content (TAC), and antioxidant activity (AA) were evaluated. Results: The root fresh weights increased significantly (10.4%) at T1 but were negatively affected by increasing biomass ash in compost at T2 and T3. Specifically, the cultivation with 15% nutrient-enriched biomass ash compost produced the highest levels of TPC (2153 ± 121.94 mg100 g ^− 1 ), TAC (138.79 ± 8.00 mg100 g ^− 1 ), and antioxidant activity (32.23 ± 0.69 µM Trolox g ^− 1 ), indicating a beneficial effect at this supplementation level. Increasing the compost rate beyond 25% led to a decline in phytochemical content and antioxidant capacity, suggesting that excessive ash incorporation may negatively influence the accumulation of phenolic and anthocyanin compounds of BCs. These compositional changes were supported by both color measurements and image processing outcomes. Image analysis of BC powders clearly differentiated the 15% treatment from the control, mainly due to its darker appearance and higher anthocyanin content. Conclusion: This study suggests that nutrient-enriched biomass ash compost has the potential to improve the coloration, phytochemical profile, and antioxidant capacity of black carrots when applied at an ideal rate, highlighting its value as a sustainable soil amendment.