Soil sampling and chemical analysis

Agricultural soil from Ingenio Quesería S.A. de C.V. (sugarcane area) was used to transplant the plants in polyethylene bags. Completely random sampling was carried out by taking six of the bags of soil in order to obtain three homogeneous samples of 1 kg each. This procedure was followed at the beginning (unamended soil) and at the end of the trial (soil amended with biochar after 70 days).

The physical and chemical properties were determined according to the norm NOM.021-RECNAT-2002 (^{Semarnat, 2002}) with the corresponding methodologies: pH (AS-02); moisture content (AS-05); organic matter content (AS-07); total nitrogen (AS-08); texture (AS-09); phosphorus (AS-11); cation exchange capacity and exchangeable bases (AS-12); micronutrients (AS-14); electrical conductivity (AS-18); Ca Mg, Na and P (AS-19); total carbonates (AS-20).

Bamboo biochar and performed analyses

We used 25-year-old overripe (dead) bamboo culms of Bambusa vulgaris var. vulgaris Schrad. ex Wendl. and Bambusa vulgaris var. vittata Schrad. ex Wendl. from the "La máquina" plot of Ingenio Quesería S.A. de C.V., selected through random sampling, with a field moisture range of 20-25 %. The extracted culms were sawn into 20 cm logs and pyrolized in a NOVATECH^® vertical kiln for 4.5 h at an average temperature of 550 ºC. Once carbonized, the particle size was reduced using a KRETOR^® extractor mill, and the percentage of particles retained on the 6.3, 4.0, 2.0, 0.84, 0.5, 0.4, 0.074, 0.044 mm sieves was determined using a MOMBOLDY^® vibratory sieve shaker.

The physical and chemical analyses of the biochar were performed according to the following standards: pH ASTM D 1293-18 (^ASTM,1999), organic matter UNE-EN-14774 (^{UNE-EN, 2010}); nitrogen, phosphorus and potassium, according to NOM-021-RECNAT-2000 established in methodologies AS-08; AS-10 and AS-13, respectively (^{Semarnat, 2002}).

Experimental design

A 2² factorial arrangement was applied, whose factors were BBV (Bambusa vulgaris), BBVVV (Bambusa vulgaris vittata) and the levels of 10 and 20 t ha^-1, plus a control. The evaluated treatments were: T0 (S-AB) without biochar application; T1= 10 t ha^-1 of BBV; T2= 20 t ha^-1 of BBV; T3= 10 t ha^-1 of BBVVV and T4= 20 t ha^-1 of BBVVV; T3= 10 t ha^-1 of BBVV and T4= 20 t ha^-1 of BBVV. The experimental unit was each sugarcane plant. A randomized block experimental design with six replications was used, with a total of 30 experimental units of Saccharum spp. var. CP 72-2086. The blocking factor was the particle size.

Application of biochar

^{Hernández-Hernández (2015)} used charcoal residue doses corresponding to 0, 10, 15, 15, 20, 25, 30, 35 and 40 t ha^-1 with favorable results in the application of 10 and 20 t ha^-1, which is why these were used in the experiment documented herein. Prior to application, both biochars were weighed individually for each experimental unit in order to obtain doses equivalent to 10 and 20 t ha^-1, which were added to the top of each bag; subsequently, they were watered with a watering system. The duration of the experiment was 70 days after the application of biochar.

Response variables evaluated in the crop

The variables studied were root volume, height, diameter of each of the primary and secondary shoots of the sugarcane plants, as well as the number of total shoots. Evaluations were performed every 15 days for 70 days after transplanting; at each measurement, six individuals per treatment were randomly selected for evaluation.

Statistical analysis

The response variables were analyzed by analysis of variance (ANOVA) based on a generalized linear model and Tukey's comparison of means, with a 5 % significance level, using the Minitab statistical package, version 17. The statistical model used was the one proposed by ^{Sitún (2005)} :

Where:

μ= Overall mean of the response variable to be evaluated

Ti= The effect of the i^th level of the different treatments to be used

Bj= The effect of the j^th block or repetition

εij= Experimental error associated with the i-j^th experimental unit

Bamboo biochar characterization

Table 1 shows the results of the analyses. The biochars of both species exhibited a moderately acid pH, in contrast to other studies in which they have been characterized as alkaline materials and have been used to improve the pH balance of the soil (^{Amonette and Joseph, 2009}). The alkaline pH of biochar is due to the relationship of the production temperature and the type of feedstock; i.e., wood-based biochar tends to have a higher pH than biochar made from crop residues and manure (^{Gul et al., 2015}; ^{Abujabhah et al., 2016}).

Biochar characterization research indicates that the pH range of most biochars varies between 6.2 and 13.0 (^{Chan and Xu, 2009}; ^{Srinivasarao et al., 2013}). In the present test, the pH of the different bamboo biochars was similar to that of oak wood (6.38) and Acai palm (6.3) (^{Lehmann et al., 2011}; ^{Nunes et al., 2019}). Both species have components such as cellulose and hemicellulose, which are common in all woody plants and in some bamboo species, such is the case of the genus Bambusa. Therefore, the utilization of biochar from Bambusa vulgaris and Bambusa vulgaris vittata are alternatives that alkalinize the pH of the soil.

According to ^{Jia et al. (2013)}, the pH of biochar is related to the amount of organic and phenolic acids derived from the thermal decomposition of cellulose and hemicellulose. Similarly, ^{Gul et al. (2015)} point out that one reason for the increase in the pH of soil with biochar is the presence on its surface of negatively charged carboxyl and hydroxyl groups that bind H+ ions from the soil solution, which reduces the concentration of H+ ions in the soil solution and increases its pH.

The electrical conductivity of the biochars was 2.35 dS m^-1 in BBV and 1.290 dS m^-1 in BBVV; i.e. they were moderately saline (^{Brady and Weil, 2008}). Therefore, according to ^{Hernández-Hernández (2015)}, there is no risk of salinity issues for the cultivation sugar cane when biochar is used as an amendment agent. The results of the present investigation coincide with those cited by ^{Concilco et al. (2018)}, who reported that the EC of biochar was 2.55 dS m^-1 in Bambusa vulgaris var. vulgaris and Bambusa oldhammi Munro.

The total carbon content of BBVVV was 53.55 %, which was higher than that of BBV (50.54 %); however, it was higher than that of gliricidia biochar (33 %) (^{Hernández-Godínez, 2015}), but lower than the total carbon (84.2 %) of a biochar of B. vulgaris vittata and that of B. vulgaris (86.3 %), both at a temperature of 650 °C (^{Brito and Filho, 1987}). It should be noted that the carbon content is closely related to the pyrolization temperature (^{Demirbas, 2004}; ^{Uchimiya et al., 2010}).

Phosphorus content was 8 532.8 mg kg^-1 for BBV, which was higher than that of BBVVV (6 836.1 mg kg^-1) ―an excellent result for bamboo biochar. Both values exceed the 743 mg kg^-1 of a biochar from sugarcane bagasse (^{Hernández-Godínez, 2015}). ^{Castellanos et al. (2000)}; ^{Jiaet al. (2013)} and ^{Yao et al. (2013)} propose that biochar with high phosphorus concentration can be used as slow-release fertilizers, but highlight the need to correctly establish the dosage, frequency and time of application, since phosphorus is a slow-release fertilizer (^{Hernández-Hernández, 2015}).

The phosphorus content of the soil increased considerably, due to the release by the utilized biochar. ^{Yao et al. (2013)} suggests applying biochar directly to the soil, as a fertilizer, for sustainable crop production, and as a carbon sequestrator.

The organic matter content was higher in BBVV, with 92.28 %, compared to 86.44 % in BBV. Both results are superior to those indicated by ^{Domínguez (2013)} for a biochar used as organic substrate for the growth of sugarcane seedlings, in which 60.50 % and 63.50 % organic matter was obtained, and to those reported by ^{Concilco et al. (2018)} in a commercial biochar of B. vulgaris and B. oldhammi, which had a mere 0.25 %.

Table 1 shows the total concentration of Mg: 1 350 mg kg^-1, 1 201 mg kg^-1; Ca: 2 122.5 mg kg^-1, 631.1 mg kg^-1; Na: 33, 34.48 mg kg^-1; Cu: 12.93 mg kg^-kg^-1, 42.6 mg kg^-1; Zn: 16.96 mg kg^-1, 27.39 mg kg^-1, and B: 17.11 mg kg^-1, 50.84 mg kg^-1 of BBVV and BBVVV. The results for both biochars in these elements were higher than that cited by ^{Rojas (2017)} for a coffee husk biochar, and by ^{Concilco et al. (2018)} for a commercial biochar from B. vulgaris and B. oldhamii.

Effect of biochar on soils

According to the results of the soil analysis before and after the application of biochar, the pH increased from 6.93 to 7.69, so it is classified as moderately alkaline. According to other studies, biochar with alkaline pH can be used in acid soils as liming material for pH improvement (^{Rojas, 2017}), as pH is related to nutrient availability for plant growth and development (^{Gul et al., 2015}).

The electrical conductivity (EC) increased from 0.76 to 1.56 dS m^-1 (Table 1), which is considered moderately low, indicating that the soil is free of the presence of salts, an ideal condition for sugarcane growth. In addition, the cation exchange capacity (CEC) increased from 12.85 to 24.69 mEq 100 g^-1. Studies have shown that biochar can alter soil fertility, either by direct nutrient supply or by increasing the CEC (^{Liang et al., 2006}), which favors nutrient retention and avoids leaching losses (^{Liang et al., 2006}).

The organic matter (OM) content decreased from 6.09 to 5.91 %. According to the parameters of NOM-021-RECNAT-2000 (^{Semarnat, 2002}) volcanic soils are of medium OM content, and non-volcanic soils are very high in OM; therefore, the soil used in the experiment documented herein is of medium content. OM has an important effect on the CEC, because it potentially retains and exchanges nutrients, in addition to promoting soil microbiological activity (Lehman and Joseph, 2009; ^{Abujabhah et al., 2016}). With the application of bamboo biochar, there was an increase in phosphorus, potassium, calcium, magnesium and sodium, while some micronutrients such as copper, iron, zinc, and boron decreased (Table 1).

Table 2 summarizes the physical properties of the soil before and after biochar application. The bulk density (Da) was 1.60 g cm^-3; based on the texture, it corresponds to a sandy loam soil (^{Semarnat, 2002}). Likewise, there was a change in the physical properties of the soil, since it exhibited a greater moisture retention in those treatments where biochar was applied, which was enhanced by the field capacity and the permanent wilting point (Table 2).

Effect of biochar on the sugarcane plant height

The analysis of variance carried out on the results of the height variable (Table 4), establishes that the addition of biochar to the soil with the different treatments had a significant effect on the height of the plants. According to the comparison of means between treatments with the Tukey test (Table 3), it indicates that when treatment 4 (20 t ha^-1 BBVV) was applied, the height of the sugarcane plants was significantly higher than with treatment 2 (BBV 20 t ha^-1). There was no significant statistical difference between the control (S-AB), treatment 1 (BBVV with 10 t ha^-1), and treatment 3 (BBVV with 10 t ha^-1). This indicates that the plant height variable alone does not represent a solid criterion for determining a particular superior treatment, but that the rest of the agronomic variables must be taken into consideration.

Effect of biochar on the diameter

The analysis of variance carried out on the variable stem diameter (Table 4) and the Tukey's mean comparison between treatments, establishes that the addition of biochar to the soil with the doses used at 70 days did not have a significant effect on this variable, since the effect attributed to all treatments is equal. The variable stem diameter during the development or growth of the sugarcane crop shows few pronounced increases, i.e., it is a particular characteristic of this hybrid.

Effect of biochar on the stem number population

The variable number of milling stalks per hectare is one of the most relevant variables, since it directly impacts the yield potential in the field. The analysis of variance carried out on the results of the variable number of stems (Table 4) establishes that the addition of biochar to the soil with each of the various treatments had a significant effect on this variable. According to Tukey’s test mean comparison of means between treatments (Table 3), when treatment 2 BBV-20 t ha^-1 (a) and treatment 4 BBVV-20 t ha^-1 (a) are applied, the stem population is statistically superior to that of the control treatment (b). In treatments 1 BBVV-10 t ha^-1 (ab), 3 BBVV-10 t ha^-1 (ab) the means were the same.

Effect of biochar on the root volume

The comparison of treatment means with Tukey's test (Table 3) indicates that when treatment 4 (BBVV-20 t ha^-1) was applied, there was a statistically significant increase in the root volume with respect to the control treatment (S-AB).

The analysis of variance carried out on the root volume variable (Table 4) establishes that the addition of biochar to the soil in the different treatments had a significant effect on this variable.