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

Abstract

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

Curly lettuce production using cassava wastewater as a potassium source

“…The nutritional content of cassava flour processing (wastewater) is significant but varies significantly among the sources (Costa et al, 2021b). As previously discussed, soils with low clay content may benefit most from CWW application; thus, soil conditions (physical and chemical) and nutritional nutrient concentration of the applied CWW will likely influence lettuce plant characteristics.…”

“…High volumes of CWW are generated by the cassava flour industries in Brazil and around the world; however, this byproduct is generally discarded in unproductive areas of the soil or waterways (Souza et al, 2019;Malik et al, 2020;Oghenejoboh et al, 2021). Conversely, some studies have already been conducted showing the beneficial effects of CWW application on corn (Barreto et al, 2014;Ramos et al, 2018), Capsicum chinense (Costa et al, 2020), and lettuce (Costa et al, 2021b).…”

“…Duarte et al (2012) observed increased leaf area, plant height, and fresh and dry mass with CWW as a fertilizer for a lettuce crop if an adequate dose is used. Costa et al (2021b) also studied the microbial activity of lettuce-cropping soil and plant responses. They observed increased plant variables, soil nutritional status, and soil microbial activities depending on CWW dose.…”