Comparison of the Effect of Solid and Liquid Digestate on the Growth of Lettuce (Lactuca sativa L.) Plants

As a consequence of the Russian invasion of Ukraine, Europe is facing a shortage of chemical fertilizers for agriculture. Therefore, the use of byproducts of biomass anaerobic digestion, e.g., solid (SD) and liquid (LD) digestate, could be a key solution to cope with this problem. In this framework, the present study aimed to evaluate the effect of both SD and LD, derived from the same feedstock, on the biometric, physiological, and biochemical parameters of lettuce (Lactuca sativa L.) plants. Compared to the controls, the plants treated with 3% (w/w) SD showed a reduction in leaf fresh biomass, chlorophyll content, performance index, fractal dimension, and antiradical activity, while vitamin C increased by 18.8%. An opposite response was observed for the lettuce plants treated with 3% (v/w) LD, which showed an increase in all the above-mentioned parameters, except for vitamin C, which decreased by 39.8%. Interestingly, the content of malondialdehyde, which is correlated with cell membrane lipid peroxidation, increased in the SD-treated plants (+39.7%) and decreased (−42.1%) in the LD-treated plants. These results strongly support the use of LD in agriculture as a valuable product to improve the productivity and nutritional quality of crop plants.