Journal of Crop Production and Processing

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Bahmanyar M A, Emadi S M, Mousavi Elyerdi A. The Effects of Applying Sewage Sludge Compost Amended with Different Organic Matter and Its Biochar, on Yield and Minerals Concentration in Lettuce (Lactuca sativa L.). Journal of Crop Production and Processing 2025; 15 (1) :107-126
URL: http://jcpp.iut.ac.ir/article-1-3342-en.html

Extended Abstract
Introduction: In many parts of the world, vegetables form a significant part of the family's daily food basket due to their great nutritional value and the important role they play in human health. Lettuce (Lactuca sativa L.) is an annual plant from the Asteraceae (Compositae) family that is rich in vitamins and essential minerals for human’s healthy nutrition. Due to the lack of organic matter and the deficiency of nutrients in most soils of the arid-semiarid regions, it is necessary to supplement these soils to enhance organic matter and nutrients content of the soils. The main sources of organic material for soils include sewage sludge, compost and their biochar. Adding sewage sludge to the soil, due to the presence of some pollutants, heavy metals, and pathogenic agents may cause contamination in the product and ultimately threaten human's health. Therefore, the use of sewage sludge as compost and its biochar in vegetable cultivation to solving the problem of sewage sludge disposal has been quite affordable from both economic and environmental point of view. Biochar is known as an organic fertilizer for agricultural production systems, due to its high durability and its richness in nutrients. Also, during the process of biochar formation, pollutants and heavy elements are reduced in sewage sludge and hence biochar is used as a modifier and improver of agricultural soil properties. As a result, the use of biochar has the potential to be hired to tackle productivity limitations through increased soil fertility. Therefore, biochar is capable to reduce the concentration of heavy elements in the plant, leading consequently to increase in the concentration of nutrients and crop yield. Thus, this research was conducted with the aim of investigating the effect of applying sewage sludge compost amended with different organic matter (rice straw and citrus shoots) and its biochar, on yield and minerals concentration in lettuce (Lactuca sativa L.).

Materials and Methods: This experiment was carried out in pots in the form of a randomized complete block design with 13 treatments and three replicates in the greenhouse of Sari Agricultural Sciences and Natural Resources University, Sari, north of Iran, in 2023. The treatments included: raw sewage sludge, sewage sludge biochar, sludge compost containing rice straw, sludge compost containing rice straw-biochar, sludge compost containing citrus shoots, sludge compost containing citrus shoots-biochar (all at two levels of half and one percent) and the control treatment. Sewage sludge, sewage sludge compost and their biochar were mixed with poting soil following passing through a 10 mesh or 2 mm sieve for each treatment before transplanting (half percent and one percent of poting soil weight, i.e. equaling to 40 and 80 grams, respectively). After implementing the treatments, two lettuce seedlings (Roman cultivar) were planted in each pot. In the stage of maximum vegetative growth of lettuce (about two and a half months after planting), the dry weight of the shoot and root of lettuce was determined with the help of a balance after drying in an oven at a temperature of 70 ºC for 3 days. Using plant samples taken, the concentration of N, P, K, Fe, Mn, Cu, Zn, Cd, Ni, and Pb was measured in the shoots and roots of lettuce plants.
Results and Discussion: The highest dry weight of the shoots (30.1 g/pot) and roots (12.2 g/pot) of lettuce were obtained in the presence of 1% sewage sludge biochar (BSS1) and the lowest amounts (17.9 and 4.71 g/pot, respectively) were observed in the absence of sewage sludge biochar, i.e. in the control treatment. Being rich in nutrients and improving soil properties, sewage sludge biochar led to an increase in plant yield. The maximum concentration of nitrogen in the shoots (5.02%) and roots (3.04%) of lettuce was obtained in 1% sewage sludge biochar treatment (BSS1), indicating 63% and 1.2 times increases, respectively, compared to the the control treatment (i.e. absence of sewage sludge biochar). With the application of 1% sewage sludge biochar (BSS1) and 1% sludge compost containing citrus shoots-biochar (BCc1), the phosphorus concentration of the shoots and roots increased. Also, the maximum concentration of potassium in the shoots (5.52%) and roots (1.80%) of lettuce was observed in the presence of 1% sludge compost containing rice straw-biochar (BCr1) and 1% sludge compost containing citrus branch-biochar (BCc1) treatments, respectively. The highest concentration of copper in shoots and roots of lettuce was obtained in the presence of 1% sludge compost containing citrus branch-biochar (BCc1) treatment. The 1% sludge compost containing rice straw-biochar (BCr1) treatment led to the highest concentration of manganese in the shoots and roots of lettuce. The 1% sewage sludge biochar (BSS1) treatment resulted in the highest concentration of iron and zinc in the shoots and roots of lettuce. Due to the negative surface elecritcal charge, biochar adsorbs cationic nutrients and with the availability of these elements as well as the richness of biochar itself in essential nutrients for plant growth, it leads to an increase in the absorption of nutrients by the plant. The highest concentration of lead, nickel and cadmium in the shoots and roots of lettuce was obtained upon exposure to the 1% sewage sludge treatment (SS1); they indicated 1.35 times, 89% and 3.65 times increases, respectively, in the shoot of letteucecompared to the control treatment. The 1% sewage sludge treatment (SS1) resulted in 100%, 72% and 2.8 times increases, respectively, in the concentration of these metals in the roots of lettuce. By converting the sewage sludge into compost and particularly into biochar, the heavy elements of sewage sludge became immobile and stabilized, resulting in decreases in the concentration of the mentioned heavy elements in the plant shoot and root.
Conclusions: Application of sewage sludge biochar, sludge compost containing rice straw-biochar and sludge compost containing citrus shoot-biochar, particularly at the level of 1%, showed the highest amount of dry weight in the shoots and roots of lettuce. The latter treatment led, also, to increases in the concentration of nitrogen, phosphorus, potassium, copper, manganese, iron and zinc in the shoots and roots of lettuce. These beneficial effects can be attributed to the fact that biochars are richer in nutrients, improve soil physical conditions, and thus enhance plant growth, than sewage sludge treatment. Application of sewage sludge resulted in the maximum concentration of lead, nickel and cadmium in the shoots and roots of lettuce; though, converting the sewage sludge into biochar, led to stabilization of these heavy metals and hence less avialability to the plant roots and consequently decreases in the heavy metals concentration in lettuce plants. Therefore, application of 1% sewage sludge biochar, sludge compost containing rice straw-biochar and sludge compost containing citrus shoot-biochar is recommended to achieve maximum yield and dry weight and maximum concentration of macro and micro elements and reduce the absorption of heavy elements by lettuce.