Nano-biochar–nano-calcium oxide synergies enhance yellow-brown soil health and tea productivity via microbial, enzymatic, and genetic pathways

Tea (<i>Camellia sinensis</i> L. O. Kuntze) is an important economic crop widely cultivated in tropical and sub-tropical regions, where drought stress often limits its growth and productivity. Soil application of nano-biochar (nBC) and nano-calcium oxide (nCaO) offers a promising approach for enhancing soil health, tea quality, and yield. A pot experiment was executed to explore the synergistic effects of nBC and nCaO on soil enzymatic and microbial activities, N-P-C cycling genes, and the quality and yield of tea seedlings under drought stress. The results showed that, under drought stress, the combined application of nBC and nCaO significantly improved the soil physico-chemical and microbial properties viz. an increase in soil pH (23.29%), soil organic matter (53.18%), soil total carbon (30.56%), available N (63.12%), available P (140.85%), available K (32.92%), microbial biomass carbon (9.90%) and microbial biomass N (8.23%) compared with the control. This may have been due to manifold increase in the expression levels of N-C-P cycling genes such as <i>phoD</i> (5.2-fold), <i>phoC</i> (7.0-fold), <i>narG</i> (3.4-fold) and <i>GH31</i> (1.8-fold) and relatively higher abundance of archaeal and bacterial communities. Soil urease, acid-phosphatase, nitrate reductase, β-glucosidase, catalase, phosphomonoesterase, and <i>N</i>-acetyl-β-d-glucosaminidase enzyme values were 48.32, 13.34, 100.00, 43.37, 612.5, 61.30, and 43.65% higher, respectively, in soils amended with both nBC and nCaO than in the control under drought stress. Furthermore, co-application of nBC and nCaO significantly enhanced tea quality traits such as caffeine (5.89%), polyphenol (12.24%), total catechins (11.00%) and amino acid (16.17%), as well as yield parameters including plant height (10.43%), leaf area (97.55%) and 10-bud weight (42.53%) relative to the control. Overall, the combined application of nBC and nCaO substantially improved soil enzymatic and microbial activities, as well as tea quality and yield traits, under drought stress.