To determine the nutritive potential of the vermicomposts, macro- and micronutrient contents and the chemical characteristics were analyzed, as shown in Table 2. When compared with the control vermicompost (VRC—STD), the attributes of vermicomposts produced from tannery residues (VRC—S, VRC—C and VRC—SC) related to agricultural fertility, e.g., OM, TOC, and CEC, among others, were not significantly different (ANOVA bootstrap, p > 0.05).
For macronutrient contents, the VRC—STD had higher concentrations of P (0.70%), K (3.19%) and Mg (0.78%) than tannery-based vermicomposts, whereas VRC—S had higher values of Ca (2.11%) and S (0.49%). The lowest N value (1.32%) was in VRC—C, but the concentrations of other macronutrients were not affected. The VRC—SC had the lowest values of all vermicomposts for P (0.59%), K (0.88%), Ca (0.52%) and S (0.36%). Although the determined values were different, the concentrations did not exceed 1.00% (Table 2).
When micronutrient contents were compared, the differences were extreme (Table 2). Except for Fe, the concentrations of micronutrients in VRC—C always greatly exceeded those in the others: B (577.67 mg kg−1), Cu (602.28 mg kg−1), Mn (570.22 mg kg−1) and Zn (499.17 mg kg−1), i.e., values approximately fivefold higher than those determined in VRC—SC. However, the values observed in these two vermicompost types were up to 10,000-fold greater than concentrations determined in the VRC—STD and VRC—S. Therefore, micronutrient quantities were found from high to low as follows: VRC—C > VRC—SC > VRC—S > VRC—STD.
For the content of Cr, as expected, VRC—STD had values of Cr(III) and Cr(VI) below the detectable level (LOQ, 79.80 μg kg−1). In the vermicomposts containing tannery residues, the concentrations of Cr(III) were VRC—C at 7.47 mg kg−1, VRC—S at 4.33 mg kg−1 and VRC—SC at 1.09 mg kg−1, with all values significantly different (ANOVA bootstrap, p < 0.05). For Cr(VI), this form was not detected in any vermicompost (Table 2).
Sweet pepper cultivation
The attributes of the latosol were typical of a dYL soil (EMBRAPA 2018), i.e., acidic with a pH of 5.74, poor OM content of 1.52%, TOC of 0.25%, CEC of 64.91 cmolc kg−1, and small base saturation of BS 27.99%. For soil particle size distribution, the texture of the dYL was classified as sandy clay loam, with 31.00% clay, 11.00% silt and 58.00% sand (Table 3).
For nutrients, the dYL had low levels of essential elements for plants (Table 3). Therefore, the dYL did not satisfy the nutritional requirements for cultivation for agricultural production, with low levels of the exchangeable bases: K (0.33 cmolc kg−1), Ca (13.03 cmolc kg−1) and Mg (4.81 cmolc kg−1). For micronutrients, the concentrations from high to low were Mn (337.21 mg kg−1), Zn (191.26 mg kg−1), Cu (179.03 mg kg−1) and Fe (59.19 mg kg−1). Levels of Na and some other micronutrients were below the detectable level (LOQ) (Table 3).
Poor in OM and with low CEC and nutrients, the dYL was ideally suited to evaluate the nutritive potential of the vermicomposts. A soil rich in OM could confound interpretation of the results following vermicompost application because the effects of soil OM and OM added from the vermicompost on plant nutrition cannot be discerned individually. Additionally, incorporating OM into soil leads to improved CEC and BS, in addition to increasing TOC levels, which are fundamental attributes for soil quality in agricultural systems (Ndegwa et al. 2000; Canellas and Santos 2005; Tobiašová 2011).
Criteria were established for the transfer of seedlings from the trays to the plastic pots and then to the cultivation vessels: 5- and 10-cm height, respectively. Approximately, 7 and 30 days passed for each transplant, respectively.
In general, plants from the SP—dYL and SP—NPK treatments had distinctly less development compared with those from SP—VRC. Plants grown on substrates containing vermicomposts showed more growth and more fruits were produced, with better appearance and greater weight and dimensions. Generally, as the amount of vermicompost increased, the yield (number of fruits) and plant health (visual analysis) increased.
The development of sweet pepper plants in SP—dYL and SP—NPK (control and reference treatments) was similar; however, fruits were only produced on plants in the SP—NPK treatment. Although vermicomposts with different compositions have been examined in previous studies, similarly satisfactory results have been reported (Bachman and Metzger 2008; Kaur et al. 2010; Vig et al. 2011; Kenyangi and Blok 2013; Nunes et al. 2016). Data related to the development of sweet pepper plants (e.g., growth, nutritive value, and fruit production, among others) obtained in this study are discussed below.
Plant growth in treatments SP—dYL and SP—NPK was not significantly different at 12.7 and 19.0 cm, respectively (ANOVA bootstrap, p < 0.05; Table 4). Therefore, the addition of NPK fertilizer did not significantly affect plant growth. Plant growth in the control and reference treatments was significantly different from all treatments in the SP—VRC group at all studied concentrations (ANOVA bootstrap, p < 0.05).
Plants cultivated with the control vermicompost (VRC—STD) added at 1.5%, 3.0% and 6.0% did not show significant differences in growth (ANOVA bootstrap, p < 0.05), after 120 days of the experiment. The difference in plant growth was ~ 6–28%, with mean growth between 32.0 and 41.0 cm (Table 4).
Analyzing plant growth under application of the vermicompost S, the mean growth of plants in treatments SP—1.5%—S and SP—3.0%—S was not significantly different at 24.7 and 30.3 cm, respectively. However, both levels of growth were significantly different compared with that of plants in the treatment SP—6.0%—S, with growth of 43.7 cm (ANOVA bootstrap, p < 0.05; Table 4).
When vermicompost C was applied, mean plant growth in treatments SP—3.0%—C and SP—6.0%—C was 45.0 and 49.7 cm, respectively, which were not significantly different. Plants in the treatment SP—1.5%—C grew 32.0 cm, which was significantly different from growth in the other levels of vermicompost C (ANOVA bootstrap, p < 0.05). At all concentrations, plant growth in the SP—SC treatments was not significantly different compared with that in the SP—C treatments (Table 4).
In general, the growth of all plants that received 1.5% vermicompost was not significantly different (ANOVA bootstrap, p > 0.05). Among all treatments, plants with the highest growth were in SP—6.0%—C and SP—6.0%—SC, both at ~ 49 cm. In samples from the group SP—VRC, plant growth was proportional to vermicompost concentration, with growth at 1.5% < 3.0% < 6.0% (Table 4).
Data related to the leaves (e.g., number, area, and nutritive value) were important to monitor and to qualify in the development of sweet pepper plants in different types of substrate. From the beginning of the experiment (~ 15 days after planting), the uniqueness in the dimensions of leaves could be observed. After 45 days, whereas the size of leaves from the SP—dYL and SP—NPK treatments remained with the same, leaves in the SP—6.0%—SC treatments had dimensions that were larger than a hand. For the leaf area (A), after 60 days, the area of leaves in SP—dYL was significantly smaller than that in the other treatments (8.33 cm2). In SP—NPK and SP—1.5%—VRC treatments, the areas of leaves were not significantly different, with values varying between 23.82 and 31.27 cm2 (ANOVA bootstrap, p < 0.05; Table 4).
In general, the nutrient contents in the leaves varied randomly, without tending to the expected. As the sweet pepper continued to develop, the concentrations of nutrients decreased as expected, primarily because of ion dilution in the tissues and organs of the plants (Tables 5 and 6) (Malavolta 1980; Malavolta et al. 1997); however, exceptions were observed.
Nutrients directly influenced plant development, as demonstrated by sweet pepper growth and production. Additionally, visual assessment of the plants, based primarily on the color, shape and texture of leaves, provided important information on the nutritional statuses of the plants (Malavolta 1980; Epstein 1997; Faquin 2005; Epstein and Bloom 2006; Fontes 2006).
Concerning the appearance of leaves, a yellowing was observed in plants of the SP—dYL treatment after 5 days of cultivation that persisted for the entire experiment, with the same occurring after 45 days for plants of the group SP—1.5%—VRC. At concentrations of 3.0% and 6.0% vermicompost, the yellowing occurred at 75 and 90 days of cultivation, respectively. In the SP—NPK treatment, no change was observed in leaf color throughout the experiment. Although the application of mineral fertilizer did not contribute significantly to plant development, nutrition was guaranteed at all plant developmental stages.
For plants that received the vermicompost control, leaf area in the SP—1.5%—STD treatment (27.24 cm2) was significantly different from that of the others (43.12 cm2 in SP—3.0%—STD and 60.73 cm2 in SP—6.0%—STD), with the difference between the two higher levels not significant. Leaf area of plants from SP—C and SP—SC treatments was not significantly different and their leaves had the largest dimensions and therefore areas (ANOVA bootstrap, p < 0.05).
For macronutrients (Table 5), in a comparison between the control and reference treatments (SP—dYL and SP—NPK, respectively), the concentrations in plants from SP—NPK were higher than those in the control, as expected. However, concentrations of K (5.71%) and S (0.79%) were the exceptions and were below those of plants in SP—dYL. Furthermore, the addition of N, P and K from the mineral fertilizer did not guarantee the highest concentrations of these nutrients among treatments, except for N (2.29%). The highest value of P was determined in SP—6.0%—S (0.67%) and that of K in SP—1.5% —STD (6.63%). The lowest concentrations of macronutrients were in SP—C and SP—SC treatments, with the lowest values of P (0.18% in SP—3.0%—SC), K (2.16% in SP—6.0%—SC) and S (0.71% in SP—1.5%—SC), most likely due to ion dilution in the plant tissues (Malavolta 1980; Malavolta et al. 1997).
Concerning the micronutrients (Table 6), the highest concentrations were in plants from the SP—6.0%—STD treatment, with high levels of B (164.12 mg kg−1), Zn (145.84 mg kg−1) and Fe (1316.88 mg kg−1); and in the plants from SP—SC treatments, with high levels of Cu (29.64 mg kg−1 in SP—3.0%—SC) and Mn (492.57 mg kg−1 in SP—3.0%—SC). With the exceptions of Zn and Fe, the lowest concentrations of micronutrients were in plants from the SP—C treatments: B (50.64 mg kg−1) and Cu (0.04 mg kg−1) in SP—6.0%—C; and Mn (100.14 mg kg−1) in SP—3.0%—C.
Consistent with our findings, many studies have demonstrated the effects of nutrients on growth, development and fruit production in sweet pepper (Malavolta 1980; Marti and Mills 1991; Aguilera-Gomez et al. 1999; Riga and Anza 2003; Faquin 2005; Albuquerque et al. 2011).
Marti and Mills (1991) studied the effects of Ca uptake by roots on plant growth and development, in addition to its correlation with the N concentration in the substrate. Albuquerque et al. (2011) studied the effect of the soil K concentration on the production of red sweet pepper, in addition to that on biometric fruit data. Riga and Anza (2003) evaluated the effects of Mg deficiency on the physiology of pepper plants and found that it affected plant growth and development. Moreover, Aguilera-Gomez et al. (1999), among other studies, examined the effects of P uptake on the development of sweet pepper plants. Overall, vermicomposts clearly sufficiently nourished the sweet pepper plants to promote their growth, development and fruit production.
Fruit production (red sweet pepper, at the mature stage), in addition to fruit characteristics (dimensions, weight and appearance), are important attributes for evaluating the effects of vermicompost addition on sweet pepper cultivation. In general, the addition of vermicompost was a biostimulant to the plants in this study, increasing the number and quality of cropped fruits (Table 7).
Regarding production, plants subjected to the control treatment (SP—dYL) did not produce any fruit, whereas those exposed to the reference treatment (SP—NPK) produced only one harvested fruit per plant. The optimum results were obtained for the plants exposed to the SP—3.0%—SC and SP—6.0%—SC treatments, which produced three fruits per plant. In addition, the plants subjected to the SP—6.0%—S, SP—3.0%—C, and SP—6.0%—C treatments produced two fruits. For the other treatments, the plants only produced one fruit (Table 7).
Some plants also produced red sweet peppers that did not meet esthetic standards, with irregular shape and coloration, resulting in no commercial value. The worst results were observed for plants in the treatments of group SP—STD because for each two well-formed fruits, one fruit was an anomaly (50% of production). Some treatments showed a significantly higher proportion of well-formed fruits, including in SP—6.0%—SC (5:1 ratio), with only two of ten total fruits showing anomalies, and in SP—3.0%—SC (ratio 8:0), SP—6.0%—C (ratio 6:0), and SP—3.0%—C (ratio 4:1) (ANOVA bootstrap, p < 0.05). Plants exposed to some of the treatments showed only well-formed fruits, i.e., SP—6.0%—C, SP—3.0%—SC and the reference treatment, SP—NPK (Table 7).
For fruit formation, less time was required for formation and maturation of red sweet peppers as the vermicompost concentration increased, except for the plants in the SP—C treatments, which all had maturation times between 92 and 110 days that were not significantly different (ANOVA bootstrap, p < 0.05). Additionally, plants in all the treatments with 6.0% vermicompost, some treatments with 3.0% (i.e., SP—3.0%—STD, SP—3.0%—C and SP—3.0%—SC) and the SP—NPK reference treatment showed maturation times that were not significantly different at 87–105 days (ANOVA bootstrap, p < 0.05, Table 7).
In general, fruits with shorter maturation times had higher weights, varying between 100.8 and 134.5 g, with no significant differences (ANOVA bootstrap, p < 0.05). Fruits obtained on plants from treatments with 1.5% vermicompost had fruits that weighed between 23.1 and 46.7 g, with values that were significantly different (ANOVA bootstrap, p < 0.05). The difference between fruit weights in the treatments SP—STD and SP—VRC was approximately 480%, indicating the biostimulant effect of vermicompost application on fruit production and attributes.
Concerning the dimensions of fruits, a relation between size and circumference could not be established because larger fruits could have a smaller circumference and vice versa. In general, plants that received 3.0% and 6.0% vermicompost produced fruits with the largest dimensions, varying between 10.5 and 12.5 cm in size and 17.0 and 23.0 cm in circumference, with no significant differences (ANOVA bootstrap, p < 0.05). In the treatments that received 1.5% vermicompost, the fruit size and circumference ranged between 6.0 and 8.0 cm and 11.2 and 13.3 cm, respectively (Table 7).
Ribeiro et al. (2000) studied the use of vermicomposts as an alternative for the organic production of sweet pepper in a protected environment. Additionally, the authors compared their results with experiments using a eutrophic red argisol (eRA) and under the application of NPK, imitating a conventional plantation. Although the structure and design of their study were similar to those of this study, the soil selected was different. An eRA (4.26% OM) could mask the results and influence of vermicompost on sweet pepper development because this soil had a certain nutritional potential, different from that of the dYL (EMBRAPA 2018). These findings are similar to those observed in our study. However, in that study, plants that received vermicompost had better results than those planted with NPK and in the control soil based on the following: (1) in organic cultivation (with vermicompost), the production was 16.0 t ha−1, whereas in the conventional planting, production was below 13.1 t ha−1; (2) the production in the control treatment (eRA) was below that obtained in organic cultivation but above that in conventional planting at 13.60 t ha−1; and (3) organic sweet peppers had an average weight of 72.9 g, compared with 67.8 g in the conventional and 65.5 g in eRA treatments.
In a comparison of studies, the fruit weight (84.6–134.5 g), for example, when 3.0% and 6.0% vermicompost were applied in our study, was notably higher than the 72.9 g found by Ribeiro et al. (2000). For fruit size, our organic sweet peppers also had larger dimensions, 9.3 cm in circumference, than the 9.0- and 8.9-cm circumferences found in the conventional cultivation and in eRA, respectively. Although our results were apparently superior, both studies indicated the efficacy and viability of the use of vermicompost in systems of organic agriculture and sweet pepper cultivation.
In general, because of the variety of soils in Brazil and the wide availability of agricultural wastes, scientific works are produced with different aims and goals, making comparisons among studies difficult. However, in all studies reported in the literature, specifically in the areas of environmental chemistry or agronomy and focused on organic agriculture, results and conclusions are consistent with those of this study; organic wastes prepared as inputs for organic agriculture can be applied in the planting of sweet pepper or other crops of economic and social interest.
For the dynamics of Cr, the absorption of Cr(III) begins at the root, followed by transport through the tissues of the stem and stalks, reaching the leaves and fruit (Gropper et al. 2009; Thor et al. 2011; Hua et al. 2012). In this study, in some of these stages, Cr accumulation occurred (Table 8).
Initially, the root is expected to act as a type of filter that prevents assimilation of an excessive concentration of Cr by the plant, with accumulation in root tissues (Gropper et al. 2009; Thor et al. 2011; Hua et al. 2012). In the rhizosphere, the Cr(III) levels varied among treatments, although no trend was observed, with values ranging from 23.86 to 65.12 μg kg−1. However, for the treatments SP—dYL, SP—NPK, SP—STD, and SP—1.5%—S, Cr(III) was below the detectable level in roots, indicating metal absorption followed by transport, without accumulation in the root system.
In general, the Cr(III) concentrations varied as follows: fruits > stem and stalks > leaves = root, with exceptions (Table 8). The Cr(III) levels in fruits, stems and stalks were higher than those in the other tissues, most likely because of their abilities to accumulate metals in their cells and tissues. In the leaves, the levels of Cr(III) were not significantly different among the treatments, ~ 41 μg kg−1. In the stems and stalks, the levels of Cr(III) in the treatments SP—dYL, SP—NPK, SP—STD, and SP—1.5%—S were statistically similar, ranging between 60.71 and 64.50 μg kg−1; in the other treatments, levels of Cr(III) were also not significantly different, ranging from 82.19 to 92.44 μg kg−1 (ANOVA bootstrap, p < 0.05; Table 8).
Finally, the fruits are the organs that are of greatest concern because the food security of the consumer is ultimately affected (Gropper et al. 2009; Thor et al. 2011; Hua et al. 2012). In the sweet peppers, Cr(III) concentration varied between 145.06 and 165.20 μg kg−1. Fruits cropped in the treatments SP—NPK and SP—STD, at all concentrations, had levels of Cr(III) that were statistically similar. The Cr(III) levels determined in the treatments involving vermicomposted tannery residues were also statistically similar (SP—S, SP—C and SP—SC; ANOVA bootstrap, p < 0.05; Table 8). Additionally, tannery residues added in vermicomposting did not increase the Cr contents in fruit because the fruit concentrations of Cr(III) were statistically similar to those quantified in the fruits that received the vermicompost control (VRC—STD) at the same concentration (1.5%, 3.0% or 6.0% V/V).
When comparing the Cr(III) determined in the fruits in this study with other foods from different studies that are commonly reported as ‘Cr-rich’ or a ‘Cr-source’, at the level of chromium enrichment in our cropped red sweet peppers, the fruits would be available for human consumption (Table 9). For example, broccoli has 22.0 μg Cr 100 g−1, and brown bread and potatoes have 4.4 and 1.5 μg Cr 100 g−1, respectively. In red and green grapes, the Cr concentrations are 6.5 and 2.1 μg Cr 100 g−1, respectively. In our study, red sweet pepper showed, in general, concentrations ranging from 3.8 to 21.4 μg Cr 100 g−1, which are close to the concentrations present in other foods (Vique et al. 1997; Oliveira and Marchini 2008; NHI 2012).