Study zone

This research was carried out in the facilities of Grupo Xonacatzy, tomato producers in the municipality of Tepetitla de Lardizábal in Tlaxcala.

Analytical methods

The pH was measured with a potentiometer (conductronic pH120). Biogas production was measured according to ^{Boe et al. (2010)}. The methane content was quantified by gas chromatography, using a thermal conductivity detector (TCD) and a HP-Plot Q capillary column.

Materials

The biofertilizer used was a strain of Azospirillum brasilense Cd collection donated by the Soil microbiology laboratory of the Benemérita Universidad Autónoma de Puebla (BUAP). Agromil plus (http://www.tacsa.mx/deaq/src/productos/76_28. htm) and the one used by the producer where the experiment was performed: Regena MIN HL 15 (http://regena.mx/productos.html) and Nubiotek-RFZ (http://www.bioteksa.com/productos/index.php).

Anaerobic digestion process

120 L plastic digesters were used, with 7% dry basis solids and 70% capacity. One digester was fed with cow manure and another with pig manure. The air was extracted with a vacuum pump and then sealed, kept in the greenhouse for 78 days. The pH, biogas production and its methane content were measured every week. When the process ended it was filtered to obtain the biol and germination rate was determined according to ^{Zucconi et al. (1981)}.

Cultivation of tomato

Beds of 15 m long * 0.2 m high * 0.3 m wide were used, plants cuttings of 3 months old were placed at 30 cm, the following treatments were applied every 10 days on a foliar way: a) biol of cow manure 500 ml at 50%; b) biol of pig manure 500 ml at 50%; c) Regena at a single dose of 2 kg m^-3 was applied, subsequently 500 ml of Nubiotek at a concentration of 15 ml in 13 L of water were applied every 10 days. This is the fertilization system used by the tomato producer where this research was carried out; d) 1% Agromil, 500 ml dose; and e) Azospirillum brasilense at a concentration of 4*10⁷ UFC ml^-1. As control, a bed without adding any substance was used. Irrigation was performed by dripping in a pithy manner. From day 49 until 140 days the tomato was harvested and weighed as it was ripe, that is, completely red.

pH variation, biogas and methane production

Figure 1 shows the pH values, accumulated biogas and methane percentage obtained during the anaerobic digestion of cow manure. The initial pH was 6.82, it decreased during the first two weeks due to the hydrolytic and acidogenic stages, then it increased and ended with a value of 7.95; during the whole process the pH was higher than 6.5, which is within the limits indicated by ^{Varnero (2011)} for the proper operation of a biodigester, which are of 6.5 to 8. 244.805 L of biogas were obtained during the 78 days, with a percentage of recorded minimum methane of 14.8% and a maximum of 55.5% (week 5). In the first 3 weeks, the percentage of methane registered was low (14 to 20%), from week 4 the methane content in the biogas increased, coinciding with the increase in the pH value, indicating the consumption of acids for the production of acetate and later of methane. pH values are similar to those reported by ^{Rico et al. (2011)} who obtained initial pH of 6.9 and final of 7.8, submitting cow manure to DA, they obtained biogas with a methane percentage of approximately 65%.

Figure 1 Biogas production, methane and pH values during the DA process of cow manure. 

Figure 2 shows the variation of these same parameters but for the case of the anaerobic digestion of pig manure. The initial pH was 6.36, during the first two weeks it decreased due to the acids formation, subsequently it increased when these were consumed for the following stages of anaerobic digestion; however, the pH generally remained below 6.5, which affected the methanogenic bacteria and thus the formation of methane, since methanogenic bacteria at pH less than 6.5 are inhibited (^{Marchaim, 1992}). A total biogas production of 188.675 L was obtained during the 78 days, with a minimum recorded methane percentage of 10.7% and a maximum of 17.4%. ^{Menard et al. (2011)} used batch digesters fed with pig manure, obtaining a methane percentage of 13.88%, a percentage that resembles that obtained in this research.

Figure 2 Biogas and methane production and pH values during the DA process of pig manure. 

The germination index (IG) indicates whether the material evaluated has phytotoxic substances or not, according to ^{Zucconi (1981)}, a material will not be phytotoxic if its IG is greater than 80% and will be a promoter of plant growth if this value is greater than 120%. Table 1 shows the IG obtained with the biol from both manures.

Table 1 Germination index (IG) obtained with both biols. 

Concentrations of 5 and 10% were non-phytotoxic and in the case of biol of cow manure, it was a promoter of root growth, so it can be used for irrigation. ^{Young et al. (2012)} evaluated the phytotoxicity of a digestate based on cow manure and reported stimulation in root growth at concentrations of 25%, it was non-phytotoxic at 50% concentrations and inhibitory at 100% concentrations.

Tomato production

The fruit was harvested and weighed as it reached maturity, the treatments where the bioles were applied were the first where the fruits reached maturity, which is an indication of the presence of substances that stimulate fruit maturation, the highest yield with both bioles was recorded between 108 and 129 with a maximum at 118 days. Azospirillum showed high yields between 108 and 140 days, with maximum at 118. Regena and Nubiotek treatment showed maximum yield between 108 and 140 days, with the maximum at 129 days. With the addition of Agromil the maximum was reached at 140 days. In all cases, more product was obtained compared to the control. Figure 3 shows the specific production of tomato.

Figure 3 Specific production of tomato for each treatment. 

Table 2 summarizes the total tomato production obtained with each treatment, as well as the cost of production. With the biol obtained from the anaerobic digestion of cow manure, the lowest cost and the highest tomato production were obtained, which was 8% higher than using the fertilizer system used by the producer. The second highest yield was obtained with the use of Azospirillum; as well as, applying Regena plus Nubiotek, but the application cost of the biofertilizer was higher, followed by the yield obtained with the application of the pig biol and Agromil. Being greater in all the cases compared to the obtained without the addition of fertilizer.

Table 2 Yield and tomato production cost.