Effect of the Combination of Biochar and ZnSO4 on Soil Properties and Lettuce Zinc Uptake

Micronutrient addition to soil is crucial for improving crop yield. Within the framework of the circular economy, it is necessary to seek more efficient fertilizers. This would reduce fertilizer consumption while serving as a strategy to mitigate the negative effects of climate change. This study proposes the combined use of a traditional source of a Zn fertilizer (ZnSO4) together with wood biochar to improve lettuce (Lactuca sativa L.) crop yield. An experiment was designed in which a dose of 8 mg Zn kg−1 as ZnSO4·7H2O was added to Cambisol soil, mixed with or without biochar (5%), for lettuce growth. Among other soil properties, Zn bioavailability, microbial biomass, and available water were monitored in the soil, while photosynthetic pigments, Zn content, and biomass production were determined in plants. All treatments increased plant biomass production. Biochar treatments (biochar and biochar/ZnSO4) increased fresh biomass by 324%, while ZnSO4 addition resulted in a 158% increase in lettuce yield. This can be due to several factors, such as biochar being a C source, the improvement of soil water content after biochar addition, and the increase in Zn leaf content in all treatments with respect to the control soil. All of these likely had a positive effect on photosynthesis. This is corroborated by the increase in total chlorophyll, chlorophyll, and carotenoids in the treatments with ZnSO4, biochar/ZnSO4, and biochar. The application of biochar alone increased this property by more than 168%, with a positive impact on soil quality. Our research demonstrates that it is possible, in some cases, to prepare fertilizers combining ZnSO4 and biochar, leading to increased plant Zn uptake and improved crop yield.