Cassava wastewater as ecofriendly and low-cost alternative to produce lettuce: impacts on soil organic carbon, microbial biomass, and enzymatic activities

The processing of cassava roots for starch extraction in factories generates a large amount of cassava wastewater (CW), which is rich in nutrients that are beneficial to plants. The reuse of this agro-industrial by-product is important for farmers and the environment as an alternative means to support soil fertility and plant production. Lettuce is the most important salad vegetable grown in the world. The objective of this study was to evaluate the impact of CW doses on soil organic carbon, basal respiration, microbial biomass, enzymatic activities, and lettuce growth in an Entisol. The experiment was conducted in pots in a greenhouse at room temperature in a completely randomized design with ten replications. The treatments consisted of the application of CW in increasing volumes that corresponded to fractions of the field capacity (FC) of the soil (100% FC = 206.6 ml dm-3): 0% (negative control), 10%, 20%, 30%, 40%, and 50% of field capacity (FC) per pot. An additional treatment without CW was also included, applying only 1.0 g of mineral fertilizer per pot (20% N, 10% P, and 20% K). After 28 days of emergence, the 10% FC treatment responded similarly to mineral fertilizer, increasing the length of the branches (+42%), number of leaves (+45%), aerial fresh matter (+202%) and the concentrations of Chlorophyll A (+ 33%), and Chlorophyll B (+40%), in addition to soil organic carbon (+15%), all compared to the control. The enzymatic activities in the soil were shown to be sensitive to CW dosage, especially for urease, which grew linearly as a function of the increased pH and K+ ions in the soil with the application of CW. The dose 20.7 ml CW dm-3 (10% FC) was that most increased plant variables, but the nutritional status of the soil and microbial activities benefited more from higher doses, starting from 30% FC, a scenario that could benefit plants more in future phenological stages, when there will be greater nutritional demand