In this study, we assessed the potential employ of diverse peat/biochar-based mixtures as growing substrates for curly endive grown in a protected environment.
Results on pH confirmed those by Dispenza et al. [
15] who evidenced lower pH values in a peat-based substrate compared to the peat/biochar mixtures or biochar-based growing substrates. Our findings are also similar to those by Tian et al. [
24], who evidenced lower pH levels in a peat based growing medium than in green waste biochar and in their combination. Our outcomes on pH values are, furthermore, in accord with those reported by Choi et al. [
11], who by inquiring the influence of biochar combinations with pine-bark based growing media on growth and development of horticultural crops, found that 100% biochar addition increased the substrate pH. In the present study, the pH detected in those growing media, which included up to 70% of biochar were in the 6.1–8.2 range. This range is convenient for curly endive [
25]. Results on pH revealed that pH values increased with the increase of the pyrolysis temperature. This is in accord with the results of Conz et al. [
26] who, studying the influence of pyrolysis temperature and feedstock type on agricultural properties and stability of biochars, found that, overall, pH increased with the increase of the pyrolysis temperature (ranging from 350 to 650 °C). Results concerning EC demonstrated that both increased the biochar percentage in the growing substrate and pyrolysis temperature increase EC values. Similarly Dispenza et al. [
15] found that substrate EC increased with the increase of biochar content (from 12 to 36 mS m
−1 for 100% peat and 100% biochar, respectively) when biochar substituted peat in the growing medium used for
Euphorbia ×
lomi. Moreover our findings on EC are in accord with those obtained by Kloss et al. [
27], Oh et al. [
28] and Song and Guo [
29], who found increased EC with the pyrolysis temperature increase. Higher EC values feature higher soluble salt availability, which in turn, presumably, accounted for the higher plant performance evidenced in this study. Nevertheless, our study is in contrast with the report of Conz et al. [
26], who found that increasing pyrolysis temperature did not increase EC. In this regard, we could hypothesize that this different result is due to a different type of raw materials used for biochar production. Our outcomes are partially conform to those by Dispenza et al. [
15] who found higher values of N in the medium with 100% peat and affirmed that the N content had no significant variation among the growing media amended with (from BC20 to BC100). In the present study, P concentrations dropped as biochar percentage increased. However, pyrolysis temperature did not influence substrate P content. Our outcomes are in line with those experienced by Karami et al. [
30], who testing green waste compost and biochar soil amendments in order to reduce lead and copper mobility and uptake to ryegrass, report that P availability was lower in a soil amended with biochar than in green waste compost-amended soil. Furthermore, results are in accord with those of Altland and Locke [
31] who, inquiring how biochar type might affect macronutrient retention and release from soilless media, evidenced that increasing levels of biochar will result in a substantial increase of K in the substrate and should be considered for fertility programs although being a modest source of P for ornamental plant production. Our outcomes on P content are, also, in accord with those by Conz et al. [
26] who evidenced equal P concentration with the increase pyrolysis temperature for sawdust and rice husk biochars. Results on K content confirmed that, both increased biochar percentage in the growing substrate and pyrolysis temperature increase K concentrations. Others have reported that K content increases with higher levels of biochar (Dispenza et al.) [
15]. Moreover, our data are in agreement with those by Bedussi et al. [
32] who found that the addition of spruce wood biochar to peat allowed the preservation of stable and high levels of K in the pore water, both in the root free substrate and in the rhizosphere in planted and non-planted soilless growing media. Nonetheless, our results on K concentration are in contrast to Conz et al. finding [
26] who report no influence of the pyrolysis temperatures on substrate K concentrations for sugarcane straw, rice husk, poultry litter and sawdust biochars. Outcomes displayed that Ca, Mg and Na contents decreased with the biochar percentage increase in the growing media. Furthermore, pyrolysis temperature did not influence Ca, Mg and Na concentrations. This is confirmed by Dispenza et al. [
15]. Additionally, our findings agree with Conz et al. [
26], who affirm that pyrolysis temperature has no effect on Mg concentration. Our work highlighted that increasing poplar biochar percentage in the growing substrates corresponds with an increase of particle density and bulk density. These results are supported by several authors [
11,
15,
24,
33]. Outcomes on yield traits pointed out that an increase of poplar biochar percentage (until 70% of the total volume) and pyrolysis temperature (700 °C) can improve curly endive crop performance. This is probably because biochar can retain high amounts of exchange cations due to its high porosity and surface/volume ratio and, consequently, can enhance plant nutrients uptake and availability [
9,
34,
35]. Furthermore, we found that increasing biochar percentage results in an increase of biomass. Similarly, Tian et al. [
24] refer that mixing green waste biochar with peat provides a better physical environment than biochar alone or peat alone in growing media for
Calathea rotundifola cv. Fasciata. However, our data remarked that the lowest plant yield performance was recorded from plants grown on 100% biochar or 100% peat. This is probably due to the decline of growing medium physicochemical traits such as high pH and EC levels. Our results agree with those of Dispenza et al. [
15] and Rondon et al. [
36] who, by studying the biological nitrogen fixation in common beans (
Phaseolus vulgaris L.) with bio-char additions, reported that adding a high rate of Eucalyptus wood biochar to a poor soil in a soilless study crop performance possibly due to a micronutrient shortage caused by an augmentation of soil pH.
Outcomes on total phenolic revealed that the highest values were found in plants from biochar 70%_450 and biochar 100%_450 treatments. These findings are supported by those of Zulfiqar et al. [
37] who, by inquiring the effects of the amendment of a biochar (BC) or a biochar-compost mixture (BioComp) to a peat-based substrate at 20% by volume on the growth of
Syngonium podophyllum, found that total phenolic contents were higher in plants grown in BC- or BioComp-amended media. In our study, the lowest curly endive crop performance was observed in plants cultivated in a growing substrate with 100% biochar. This remarks that the unfavorable plant growing conditions of the media and the stressful conditions induce an accumulation of phenolics [
38,
39,
40,
41]. Therefore, we could hypothesize that the higher total phenolic content is due to a stress caused by a non-optimal curly endive-cultivation environment (pH and EC stresses). Our study revealed, also, that the higher SSC and ascorbic acid contents were recorded in curly endive plants grown on biochar at 70% and a pyrolysis temperature of 450 °C and that TA was not affected by the growing substrates tested. Our results highlighted a decrease of N content and an increase of K in leaves as the biochar content in the growing mediums increased. The highest leaf Ca concentrations were observed in plants from biochar at 20% and pyrolysis temperature of 700 °C. Whereas, other minerals such as P and Mg were not influenced by the growing media. The foliar mineral contents were in the range that permits a satisfactory plant growth and development in agreement with the average concentration of mineral nutrients in plant dry matter. As reported by Awad et al. [
42], the release of nutrients from biochar-based material may be one of the likely mechanisms for improving K, Mg, Mn and Zn uptake by plant root systems in perlite + rice husk biochar substrate. In particular, Awad et al. [
42] found that rice husk biochar induced the growth of beneficial microorganisms on its surfaces. This may enhance the uptake of nutrients by plant. Furthermore, comparable mechanism for improving the uptake of macronutrients by maize plants in soil treated with biochar were described by Kim et al. [
43], Lee et al. [
44] and Rehman et al. [
45]. In our study, even though a higher amount of leaves K content, regularly implicated in the photosynthetic process, is positively correlated to percentage of biochar in the growing media, the highest leaves K levels were collected in plants grown on substrates with biochar at 70%. The explanation for this might be due to the fact that 100% biochar substrates caused a plant stress due to non-optimal pH and EC. Our results on Ca are similar to those observed by Zhang et al. [
46] who, investigating the effects of the biochar and humic acid amendments on the quality of composted green waste found that leaves nutrients content significantly increased when
Calathea insignis plants were grown in media consisted of composted green waste and 20% coir biochar. However, as reported by Conz et al. [
26], a large variability on plants nutrient uptake depends on the origin of biochar-based materials used for biochar production [
26].