Origin of coffee pulp and cow manure

Coffee pulp (CP) from the coffee zone of Huatusco de Chicuellar, Veracruz, Mexico, located at 19° 13' N latitude and 96° 58' West longitude was used. It was a proportional mixture of different varieties of coffee plants: Colombia, Costa Rica, Oro Azteca, Catimor, Caturra, Garnica, Borbón and San Román. This coffee pulp was the result of the wet process system to extract the coffee bean. Cow manure (CM) came from cattle of Colegio de Postgraduados, Campus Montecillo, Texcoco, State of Mexico. The cattle were fed on a mixture of oats, alfalfa and 17 % Clayton Malta^® commercial feed. The parameters measured in the raw wastes were pH, total N, % moisture, C/N ratio and organic matter content (Table 1).

Treatments

The established treatments were: 100 % coffee pulp (control), 50 % coffee pulp with 50 % cow manure (50 CP:50 CM) and 25 % coffee pulp with 75 % cow manure (25 CP:75 CM), for pre-composting and vermicomposting processes. To obtain these proportions, 3 kg of coffee pulp were weighed for the control; 1.5 kg of coffee pulp with 1.5 kg of cow manure for the 50 CP:50 CM treatment, and 0.75 kg of coffee pulp with 2.25 kg of cow manure for the 25 CP:75 CM treatment. In all cases the weighing was carried out on a wet weight basis.

Pre-composting was carried out for 26 days in a greenhouse using four replicates per treatment, placing 3 kg of mixture in 30 × 50 × 20 cm plastic containers used as composters. Vermicomposting was carried out in a controlled environment chamber (Sherer^® Marshall, Michigan, USA) at 30 °C. Forty grams of Californian red worm (Eisenia fetida) were used for each treatment after the pre-composting phase.

A randomized complete block experimental design with four replicates per treatment was used. To determine significant differences between the treatments, analysis of variance (ANOVA) and Tukey test were applied to the data using the SAS statistical package Version 9.1 ^{(SAS Institute, 2004)}.

Sampling

Compost samples were taken for analysis at the end of the pre-composting period. Vermicompost samples were analyzed 60 days after inoculation of the earthworms. The samples were dried at room temperature, then ground and sieved on 2 mm mesh as indicated in the NMX-FF-109-SCFI-2008 standard for earthworm humus test methods ^{(SEC, 2008)}.

Evaluated variables

The pH was determined by the potentiometric method, using a digital potentiometer (Conductronic PC45^®, Mexico) at the 1:5 ratio. Organic matter (OM) and total organic carbon (C_TO) were determined using the loss on ignition method at 400 °C ^{(Dabadie et al., 2018)}. The cation exchange capacity (CEC) was determined through the ammonium distillation method ^{(Rodríguez and Rodríguez, 2002)}. Total nitrogen was determined by the micro-Kjeldhal method according to NOM-021-RECNAT-2000 ^{(SEMARNAT, 2002)}.

Microbial respiration (CO₂ emission) of the coffee pulp and its different mixtures with cow manure was evaluated according to ^{Anderson (1982)}. Fifty grams of mixture were weighed per treatment and placed into a closed glass flask. A vial with NaOH was placed inside it for all treatments, including the control. Every 24 h the contents of the vial were precipitated with BaCl₂ and titration was carried out with H₂SO₄. The procedure was carried out for 31 days, and the CO₂ emission was expressed in mg of C-CO₂ per gram of compost. To obtain the emission rate, the accumulated CO₂ was divided by the total time (31 days).

The extractable carbon (AH + AF) was obtained through the Kononova-Belchikova method ^{(Cabrera et al., 2002)} and measured with ^{Walkley and Black (1934)} procedure. The extractant solution was Na₂P₂O₇ • 10H₂O, and 5 g of compost or vermicompost were used. To separate the humic substances, the extract was acidified to pH 2.0 with H₂SO₄ for 24 h. Subsequently, the supernatant in the form of fulvic acids was decanted and the precipitate (humic acids) was measured by chemical titration with NaOH.

The humification indices used to evaluate the humification process in the vermicompost were: humification ratio (HR): (C_EX/C_TO) × 100, humification index (HI): (C_HA/C_TO) × 100, humic acids percentage (HAP): (C_HA/C_EX) × 100 and polymerization index (PI): C_HA/C_FA, where C_EX corresponds to the extractable humic carbon (HA + FA), C_TO is the total organic carbon, C_HA is the carbon of humic acids and C_FA the carbon of fulvic acids ^{(Roletto et al., 1985}; ^{Senesi, 1989)}.

Physicochemical characteristics and maturity parameters

The NMX-FF-109-SCFI-2008 standard establishes an optimum range of 20 to 50 % for the content of organic matter; however, values outside this range were obtained, which was also observed by ^{Bernal et al. (1998)} and ^{Raviv et al. (2004)} in a stable and mature compost. ^{Bernal et al. (1998)} confirmed a decrease in all parameters (C/N ratio, total N and pH) using a mixture of poultry manure, cotton waste, and olive-mill wastewater, from 21.1 to 9.4 C/N ratio and from 81.5 to 64.8 % OM. ^{Raviv et al. (2004)} used cow manure with 81 % OM and 23 initial C/N, and obtained 70 % OM and a final C/N ratio of 15.6; both confirmed pH lower than 8. The OM percentage depends on the characteristics and operational conditions ^{(Richard and Zimmerman, 1995)}, the manure sources showed greater organic matter content.

A decrease in the percentage of organic matter was observed when adding cow manure to the coffee pulp (Table 2), as reported by ^{Pierre et al. (2009)} when applying goat manure in a mixture with coffee waste. The decrease in OM is associated to the evolution of CO₂ ^{(Benito et al., 2009)}. At the end of 31 days, the control and the 50 CP:50 CM and 25 PC:75 CM treatments accumulated 383.82, 610.79 and 587.47 mg C-CO₂ g^-1 of compost, respectively (Figure 1). According to ^{Defrieri et al. (2005)}, the high production of CO₂ is related to an active development of the microbial metabolism, this suggests that the addition of cow manure promoted the microbial activity in the compost and improved the mineralization of the coffee pulp.

Figure 1 Accumulative production of CO₂ in the composts. Treatments with the same letter are not statistically different (Tukey, *P ≤ 0.05, **P ≤ 0.01). Control (100 % coffee pulp), 50 CP:50 CM (50 % coffee pulp with 50 % cow manure) and 25 CP:75 CM (25 % coffee pulp with 75 % cow manure).  

The CEC values obtained at the end of the pre-composting and vermicomposting processes (Table 2) were higher than the CEC value of the compost made only with coffee pulp. The high CEC values are explained by the increase of humic substances in the process ^{(Mayhew, 2004)} and the concentration of minerals as there is less organic matter.

Humic substances

Contrary to what was reported in literature, the extractable humic C (HA + FA) decreased after vermicomposting; however, extractable C increased as cow manure was added (Table 3), which agrees with the microbial respiration data (Figure 1), as the highest CO₂ values ​​were present in the treatments with cow manure. The treatments in which cow manure was incorporated had a faster mineralization that led to the transformation of organic matter in a more compact time. The CO₂ emission rates of the treatments with cow manure (50 CP:50 CM: 18.95 ± 2.13, 25 CP:75CM: 19.70 ± 2.23) were significantly higher than the control (12.38 ± 1.71) with only coffee pulp.

In the composting process, the amount of extractable C could depend on the type of substrate or manure used. The results of the present experiment differ from those obtained by ^{Pierre et al. (2009)}, who did not find significant differences in terms of extractable C when using different proportions of goat manure with coffee pulp; also, ^{Paredes et al. (2000)} reported that the extractable C remained constant throughout the process when analyzing mixtures of residues of different origin, including manures; thus, it is not appropriate to consider it as a maturity parameter of organic matter.

^{Moreno and Moral (2008)} and ^{Chefetz et al. (1996)} indicated that in the humification process there is a gradual increase in the fraction of humic acids and a parallel decrease in fulvic acids. In the present study, although an increase in humic acids was observed, a decrease in fulvic acids was only observed in the 50 CP:50 CM treatment (Table 3); probably the extraction method influenced the results.

Humification Indices

The humification indices obtained after pre-composting and vermicomposting (Table 4) were compared with the parameters established by ^{Roletto et al. (1985)} to determine the composts maturity. The humification ratio (HR) indicates the percentage of extractable humic carbon contained in the total organic carbon. All the treatments surpassed the HR parameter. ^{Roig et al. (1988)}, among others, indicated that the percentage of extractable humic carbon (C_EX/C_TO) cannot be considered as an adequate index of humification of organic matter, since it remains practically constant throughout the process in mixtures of waste of different origin ^{(Paredes et al., 2000)}. The humification index is the total of humic acids contained in the organic carbon, which must be ≥ 3.5 for a stable compost. Besides, humic acids are used because they have a higher molecular weight and degree of polymerization ^{(Steelink, 1985)}.

The humic acids percentage (HAP) represents the amount of humic acid contained in the extractable carbon. This parameter, like the others, will increase as the maturation time increases; iIn addition, this parameter allows to see the difference between the already humified material and the one that is still being transformed. As in compost there are no humins, the rest consists of fulvic acids. The 50 CP:50 CM treatment after precomposting reached more than 50 % of HAP (Table 4) as mentioned by ^{Roletto et al. (1985)}. After vermicomposting, the treatments with manure exceeded this percentage. The best treatment was the one containing equal parts of coffee pulp and cow manure (P ≤ 0.05), the manure contributes positively to the humification of the coffee pulp vermicompost.

The polymerization index (PI) is the ratio between humic and fulvic acids. The best treatment was 50 CP:50 CM. The control treatment showed no humification after pre-composting. The treatment with the highest percentage of manure after pre-composting was the least humified; however, it exceeded the established value for the polymerization index (≥ 1) after vermicomposting. Table 5 shows the parameters with significant difference (P ≤ 0.01) between pre-composting and vermicomposting. These were compared with the parameters established in literature.

Manure treatments comply with the parameters proposed by ^{Roletto et al. (1985)}; thus, these products attained maturity. The best treatment was 50 CP:50 CM; it was statistically different from the control and the 25 CP:75 CM was an appropriate treatment in terms of polymerization index and humic acids percentage. Likewise, the latter treatment presented a high degree of stability compared to the control and the 25 CP:75 CM treatment.