Metabolomics reveals the molecular mechanism of sewage sludge-derived nutrients and biostimulants stimulating resistance enhancement and the redistribution of carbon and nitrogen metabolism in pakchoi cabbage.

Alkaline thermal hydrolysis of sewage sludge to produce high-quality liquid fertilizer that contains protein, amino acid, organic acid and biostimulants is receiving wide attention, however, the impact on plants and environmental risks must be evaluated for sustainable use. In this study, the interactions between sewage sludge-derived nutrients, biostimulants (SS-NB), and pakchoi cabbage were investigated by a combination of phenotypic and metabolic approaches. Compared with SS-NB0 (single chemical fertilizer), SS-NB100, SS-NB50, and SS-NB25 had no effect on crop yield, but the net photosynthetic rate increased from 1.13 % to 9.82 %. In addition, the antioxidant enzyme activity (SOD) increased from 29.60 % to 71.42 %, with decreasing malondialdehyde (MDA) and HO levels by 84.62-92.93 % and 8.62-18.97 %, respectively, indicating positive effects on photosynthetic and antioxidant capacities. Leaf metabolomics revealed that SS-NB100, SS-NB50, and SS-NB25 resulted in up-regulation of amino acid and alkaloid metabolites, down-regulation of carbohydrate metabolites, and up-regulation and down-regulation of organic acid metabolites, which were involved in carbon and nitrogen redistribution. Galactose metabolism was inactivated by SS-NB100, SS-NB50, and SS-NB25, indicating the protective role of SS-NB in oxidative damage in cells. Furthermore, the application of SS-NB also resulted in a significant reduction in heavy metal contents (Cr, Ni, and Pb) and the target hazard quotient (THQ). The THQ values of Cd, Cr, Ni and Pb were <1.0 in SS-NB50, and may be considered as an optimal fertilization strategy. The result provided better understanding of the phenotypic and metabolic changes imposed by SS-NB-replaced chemical fertilizer nitrogen in pakchoi cabbage leaves.