Nutrient recycling of source-separated human faeces using biochar immobilized indigenous psychrotrophic bacteria for sustaining the agroecosystems of north-western Himalaya

The Himalayan composting toilets (CTs) offer a sustainable solution for converting human faeces (HF) into compost, supplementing the low-fertile land of the region. However, CTs face challenges such as delayed composting processes (6–8 months), increased heavy metal content, and foul odour. Therefore, the current study evaluated biochar-amended psychrotrophic bacteria for HF degradation under low-temperature conditions (10 ± 2 °C). Out of 153 psychrotrophic bacteria isolated from HF compost, 17 bacterial strains were selected based on highest and two or more hydrolytic activities. Furthermore, considering the isolation source, bacterial strains were examined for haemolytic activity, biofilm formation, cytotoxicity and seed germination assay. In total, 14 potential strains belonging to Pseudomonas, Microbacterium, Arthrobacter, Streptomyces, Glutamicibacter, Rhodococcus, Serratia, Exiguobacterium, and Jeotgalicoccus genera were considered safe for both human handling and plants. The composting process was conducted in modified plastic drums at 10 ± 2 °C for 90 days through two treatments: Treatment 1 (T1) involving HF, non-immobilized biochar and cocopeat, and Treatment 2 (T2) involving HF, consortium-immobilized biochar and cocopeat. The consortium-immobilized biochar (T2) degraded HF within 90 days with hemicellulose and cellulose degradation ratios of 73.9% and 62.4%, respectively (p ≤ 0.05). The compost maturation indices like C/N ratio (16.5 ± 1.85), total nitrogen (2.66 ± 0.07), total phosphate (0.4 ± 0.005), total potassium (1.8 ± 0.05) also improved in T2 treatment (p ≤ 0.05). Additionally, T2 was more effective in achieving safe levels of faecal coliforms (< 1000 MPN g⁻¹) and reducing heavy metal content compared to T1. 16S rRNA amplicon-based analysis demonstrated an enhancement of bacterial community diversity in T2, with the presence of Rhodococcus, Pseudomonas, Arthrobacter, and Streptomyces at the end of the composting period promoting HF degradation. Furthermore, T2-fertilized soil showed a germination index (121 ± 0.4, p ≤ 0.05) and stimulated root, shoot and yield by 110%, 45.2%, and 288%, respectively, in pea (Pisum sativum var. AS-10) compared to T1 (49.6%, 19%, and 5.8%, respectively) (p ≤ 0.05). In conclusion, the developed biochar-based formulation proved effective in degrading HF at low temperatures, mitigating foul odours, reducing heavy metals, and enhancing the agronomic value of the final compost. This study presents a promising approach for the sustainable management of HF that can supplement the non-nutritive soil of high-altitude regions.