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Organic farming, biol, biodigesters, nutritional composition, manure
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Rascón J, Juarez Contreras LdP, García Saldaña EW et al. Characterization of four digestates from different types of substrates used in biogas production in Northern Peru [version 1; peer review: 1 approved, 2 not approved]. F1000Research 2024, 13:24 (https://doi.org/10.12688/f1000research.142713.1)
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Research Article
Characterization of four digestates from different types of substrates used in biogas production in Northern Peru
[version 1; peer review: 1 approved, 2 not approved]
PUBLISHED 08 Jan 2024
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This article is included in the Agriculture, Food and Nutrition gateway.
Abstract
Corresponding author:
Jesús Rascón
Competing interests:
No competing interests were disclosed.
Grant information:
This study was conducted under the project entitled ‘Sistema de tratamiento de aguas mieles de café mediante la producción de biogás para el secado de café y biol para crear soluciones hidropónicas, como adaptación al cambio climático y el desarrollo de agricultura sostenible’. This research was funded by Programa Nacional de Investigación Científica y Estudios Avanzados (PROCIENCIA) del Consejo Nacional de Ciencia, Tecnología e Innovación Tecnológica (CONCYTEC ) of the Republic of Peru (grant no. 107-2021-FONDECYT).
The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript .
The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
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© 2024 Rascón J et al.
This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
How to cite: Rascón J, Juarez Contreras LdP, García Saldaña EW et al. Characterization of four digestates from different types of substrates used in biogas production in Northern Peru [version 1; peer review: 1 approved, 2 not approved]. F1000Research 2024, 13:24 (https://doi.org/10.12688/f1000research.142713.1)
First published: 08 Jan 2024, 13:24 (https://doi.org/10.12688/f1000research.142713.1)
Latest published: 08 Jan 2024, 13:24 (https://doi.org/10.12688/f1000research.142713.1)
The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript .
The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Introduction
Biol is a liquid organic fertilizer obtained by anaerobic digestion of organic substrates, such as animal manure and plant waste, without oxygen (Hernández-Sarabia et al., 2021). It is a product with high economic profitability, and above all, it has the necessary nutrients for the correct development of plants or to improve soil quality (Fahrurrozi et al., 2016; Kilic, 2023). Organic fertilizers, such as biols, are used in organic agriculture to improve the quantity and quality of crop yields; stimulate microbiological activity by increasing soil organic matter; or replenish nutrients present in the soil, such as nitrogen (N), phosphorus (P), potassium (K) or calcium (Ca), and which are used by plants for their development (Zandvakili et al., 2019). These effects have been proven in crops such as cattle pastures and forages (Soares Filho et al., 2018), legumes (Limam et al., 2018) or lettuce (Fedeli et al., 2023). Another beneficial effect of biols is the improvement in soil structure, both biologically and physicochemically (Thomas & Singh, 2019). Organic fertilizers favor the presence of air in the soil, helping water infiltration and improving nutrient adsorption by roots, helping to optimize water retention. They also increase the defense capacity against pests and diseases, and reinforce resilience against extreme climatic changes that crops may suffer compared to when agrochemicals are used. In addition, organic fertilizers have a longer duration since organic matter decomposes slowly (Chew et al., 2019; Mącik et al., 2020).
In agricultural production, animal excreta and slurry are common as fertilizers (Muktamar et al., 2016; Phibunwatthanawong & Riddech, 2019). These manures are rich in nitrogen, not metabolized by animals, which is released into the environment as ammonium during decomposition (Lu et al., 2017). Although it is a way to reduce costs for many agricultural producers, using fresh manures can cause problems. Some problems are pollution of surface and groundwater by nitrates; air pollution by gases such as methane, one of the causes of global warming; and, in some cases, loss of fertility by salinization (Li et al., 2016; Wongsaroj et al., 2021). On the other hand, agroindustrial production generates a large amount of organic waste, especially plant waste or wastewater (Pandit et al., 2021; Rosemarin et al., 2020). For example, coffee production generates waste from fermentation and washing, such as coffee pulp and water (Serna-Jiménez et al., 2022). These by-products of the coffee production process are considered waste and tend to cause environmental problems related to the greenhouse effect, soil deterioration and air and water pollution (Schmidt Rivera et al., 2020). However, these by-products are renewable resources rich in carbohydrates and bioactive compounds (Chala et al., 2018; Villa Montoya et al., 2020).
One of the solutions to these problems is the establishment of anaerobic digestion systems or biodigesters for biogas generation, mainly using animal excreta and slurry together with plant waste as substrates (H. Wang et al., 2018; Y. Wang et al., 2021). These systems are based on anaerobic fermentation of the substrates added to the biodigester to generate biogas. The excreta of animals such as cattle, horses, pigs, sheep, or goats are the most commonly used substrates in this system. Some of these animals have modifications in their digestive system, such as a rumen or a large caecum, which allow them to digest cellulose thanks to the biome found inside them (Fujimori, 2021). Within this biome are mainly cellulolytic bacteria (cellulose decomposers), amylolytic bacteria (break down starches), proteolytic bacteria (break down proteins), and methanogenic bacteria (produce methane) (Froidurot & Julliand, 2022; Hua et al., 2022; Palangi & Lackner, 2022). Excreta containing a biome rich in different types of bacteria makes them suitable as inoculums to generate biogas, in addition to being the main substrate for anaerobic fermentation (Sequeda Barros et al., 2023). One of the most essential bacterial groups in biodigesters is the methanogenic bacteria, which produce methane. This gas is the main component of biogas, with concentrations between 50% and 70% of the total. As a result, biogas is an environmentally friendly fuel by closing the carbon cycle (Nwachukwu et al., 2022).
Another useful digestate, biol, is also produced in biodigesters for biogas production. Biol is a liquid organic fertilizer rich in nutrients (Barrena et al., 2019). One of the great benefits that biol has is its low concentration of bacteria, thus releasing fewer pathogens into the environment than fresh manures. Depending on the type of substrate used, they differ in their nutrient composition or concentrations of pathogens or bacteria (Risberg et al., 2017). However, one of their biggest problems is their concentration of nitrogenous compounds, as plants usually use only as much as they need. Excess nitrogenous compounds can affect soil fertility and impair crop production (Nkoa, 2014; Samoraj et al., 2022).
Establishing the nutritional composition and other parameters is an excellent way to know that using a biol will not harm a crop or soil. By establishing this composition, we can know two things. The first is to know if the biol, depending on the substrate used, meets the crop’s nutritional requirements, and the second is to know the amount of possible contaminants it may have (Chatzistathis et al., 2020). Some of the most commonly used substrates in biogas and biol production are excreta from cattle (Jafari-Sejahrood et al., 2019; Khayum et al., 2018), pig (Shen et al., 2019), alpaca (Fernandez-Lizama et al., 2019), goats (Hanafiah et al., 2017) or chickens (Matheri et al., 2017). These substrates can be mixed with other substrates, such as plant biomass (Martí-Herrero et al., 2019), organic waste from factories or households (Čater et al., 2015; Pavi et al., 2017), or even sewage (Guilera et al., 2020; Mohammed et al., 2017). Excreta, organic waste, and wastewater also have the function of inoculum, having a good content of fermenting bacteria (Wi et al., 2023). However, in some cases, formulated inoculums are added to enhance the anaerobic process, mainly when part of the substrate contains plant biomass rich in lignocellulose, which is difficult to decompose (Kainthola et al., 2019). All biols generated from the above substrates possess many nutrients and are generally safe for crops (Islam et al., 2019).
In northeastern Peru, agriculture is the Amazon department’s most important economic activity. The production of cattle, pigs, and horses stands out, as well as rice, cocoa, and coffee. With the growing demand for sustainable and environmentally friendly production, many small and large farms are adapting technologies to achieve this goal, with biodigesters for biogas and biol being the most viable for this purpose. However, more knowledge of the composition of the biols generated must be gained. Although there are several studies on the characterization of liquid organic fertilizers, there are no studies on the biols generated in biodigesters for biogas production. Determining the amount of nutrients present and their physical, chemical, or microbiological characterization is important. Mainly because nutritional requirements and specific conditions are needed depending on the crop type. To this must be added the type of substrate used. Therefore, the objective of the present work was to determine the microbiological, physicochemical, organic matter, and nutrient composition in four types of biol generated in different biodigesters for biogas production fed with four types of substrates. At the same time, it was determined which were the most important parameters at a general level, how they behaved, and which were the most influential ones in each biol.
Methods
Biodigester sampling
Four different types of biols obtained from biodigesters for biogas production implemented in different districts of the Amazonas Department in northern Peru were studied. These biodigesters were fed with different substrates: pig manure (T1), cattle manure (T2), horse manure (T3), and cattle manure with coffee processing water (T4). The main design and operating parameters of the biodigesters are reported in Table 1.
Table 1. Main design and operation parameters of the biodigesters implemented in Northern Peru.
Specifically, the biol obtained from pig manure (T1) was collected from a biodigester implemented at the military base in the district of Jazan (5°56′16.85″S 77°58′48.10″W, 1349 m.a.s.l.). The biol obtained from cattle manure (T2) was collected from a biodigester implemented in the experimental fields of the Universidad Nacional Toribio Rodríguez de Mendoza (UNTRM) in the district of Chachapoyas (6°14′0.33″S 77°51′5.54″W, 2305 m.a.s.l.). The biol obtained from horse manure (T3) was collected from a biodigester implemented in an experimental station of the UNTRM in the district of Florida-Pomacochas (5°49′19.02″S 77°57′40.23″W, 2253 m.a.s.l.). Finally, the biol obtained from the co-digestion of cattle manure and coffee processing water (T4) was collected from a biodigester implemented in a coffee farm in the district of Santa Rosa (6°26′48.53″S 77°28′29.45″W, 1845 m.a.s.l.).
All the biodigester samples were collected from the storage pond of each biodigester. Four samples were collected from the same biodigester for two weeks. Subsequently, the four samples from each biodigester were carefully mixed and stored at 4 °C before being analyzed in the laboratory.
Biol characterization
Four groups of parameters were analyzed in the laboratory: microbiological, physicochemical, organic matter, and nutrients. All analyses were performed twelve times for each biol and at the Soil and Water Research Laboratory of the UNTRM (Rascón, 2023).
The microbiological parameters analyzed were total coliforms (TC), fecal coliforms (FC), Escherichia coli (EC), Salmonella (SA), and Methanococcus (ME). All were determined by the Most Probable Number method, following the recommendations of APHA et al. (2017) for TC, CF, and EC; EPA (2006) for SA; and Acuña et al. (2008) for ME.
The physicochemical parameters analyzed were pH, electrical conductivity (EC) with a multiparametric equipment brand SI Analytics, model HandyLab 680; total solids (TS), by drying at 105°C in an oven brand MMMgruop, model Venticell; and sulfates (SULF) using a Thermo Scientific atomic absorption spectrophotometer, model Genesys 10S UV-Vis. All this follows the methodologies established by the APHA et al. (2017) and EPA (1978).
The chemical oxygen demand (COD) parameter was analyzed regarding organic matter. For COD, use was made of a HACH brand digester block, model DRB220, and a Thermo Scientific brand atomic absorption spectrophotometer, model Genesys 10S UV-Vis, following the methodology established by APHA et al. (2017).
The nutrients analyzed were total nitrogen (TN) using a semi-automatic Kjeldahl distiller Selcta brand, model PRO - NITRO S, following the methodology established by APHA et al. (2017); phosphorus (P) using a Thermo Scientific brand atomic absorption spectrophotometer, model Genesys 10S UV-Vis, using the modified Olsen method (Carter & Gregorich, 2007); Potassium (K), Calcium (Ca), Magnesium (Mg), Sodium (Na), Iron (Fe), Manganese (Mg), Zinc (Zn), Copper (Cu), Aluminum (Al) and Boron (B), by acid digestion, using an Agilent atomic emission spectrophotometer, model MP-AES 4100, following the methodology established by APHA et al. (2017).
Statistical analysis
A descriptive statistics analysis (mean, standard deviation) of the composition of each of the four biols evaluated was performed. Subsequently, a principal component analysis (PCA) was applied, an ideal statistical method to reduce the dimensionality of a large data set (Van Der Maaten et al., 2009). PCA identifies the variance in correlated variables to generate a reduced set of uncorrelated variables known as principal components (PC). These PCs are weighted linear combinations of the original variables (Thioulouse et al., 2018). In this research, the PCA was determined using a correlation matrix. Eigenvalues were calculated to measure the importance of the components. Once the PCA was calculated, the number of components to be used was determined using the criterion of considering a sufficient number of components able to explain between 70% and 90% of the total variation of the original variables (Rencher, 2012). Finally, a biplot was used to interpret better the first two principal components (Jolliffe, 2002). The most important parameters were established through correlation by matrix multiplication between the matrix with the loading vectors of nutrients and PCs and the diagonal matrix constructed from the standard deviation of the principal components. After that, the most important parameters of the evaluated biols were those with a strong correlation, which bore greater than ±0.70 (Akoglu, 2018). Finally, to know the behavior of the parameters and determine which parameters were the most influential in each biol, a biplot with ellipses complemented with a Permutation-based Multivariate Non-Parametric Analysis of Variance (PERMANOVA) was used to confirm the dissimilarity between the biols found (Anderson & Walsh, 2013). All statistical analyses were performed at a significance level of P<0.05 with R statistical software version 4.3.0 (R Development Core Team, 2023).
Results and Discussion
Characterization of the obtained biols
After analyzing all the parameters of the biols for their characterization, it was found that about the microbiological parameters, the equine manure biol (T3) had shallow values of TC, FC, ECO, SA, and ME for the rest of the biols, indicating its innocuousness. However, the highest values of microbiological parameters were reported for the pig manure biol (T1) (Table 2).
Table 2. Concentrations of the parameters analyzed in the biols obtained from the biodigesters implemented in northern Peru.
As for the physicochemical parameters, the biols of horse manure (T3) and cattle manure with coffee processing water (T4) had an acid pH, while the biols of pig manure (T1) and cattle manure (T2) reported alkaline pH values. At the same time, the EC of the cattle manure biol (T2) and cattle manure biol with coffee processing water (T4) was low, indicating low concentrations of dissolved salts in the liquid medium, contrary to the horse manure biol (T3). TS and SULF, parameters of great importance in biols (Somers et al., 2020), were low in the swine manure biol but high in the horse manure biol (T3) (Table 2).
Organic matter, a parameter of great interest for improving soil structure and water holding capacity, and which is expressed in terms of DOC (Manasa et al., 2020), was very high in the cattle manure biol with coffee processing waters (T4), being approximately 5 to 10 times higher than in the other biols (Table 2).
The main nutrients analyzed, N-P-K, had very different concentrations depending on the biol. P concentrations were very high in the biol of cattle manure with coffee processing water (T4). In contrast, the concentration of TN was higher in the biols of horse manure (T3) and cattle manure with coffee processing water (T4). The concentration of K was high in the cattle manure biol (T3) but very low in the cattle manure biol with coffee processing water (T4). On the other hand, nutrients such as Ca, Mg, and Al had deficient concentrations in the swine manure biol (T1) but were very high in the horse manure biol (T3). As for Fe and Mn concentrations were high in the cattle manure biol with coffee processing water (T4) and low in the cattle manure biol (T2). Likewise, Zn and Cu concentrations were high in the cattle manure (T2) but low in the swine manure biol. Na concentrations were very high in the horse manure biol (T3), but very low in the cattle manure biol with coffee processing water (T4). Finally, it should be noted that B concentrations were only detected in the pig manure biol (T1) (Table 2).
The processing of biols through biogas production is considered a form of clean energy production, which contributes to the environment and nourishes plants (Holm-Nielsen et al., 2009). Therefore, it is important to know the physicochemical characteristics of a biol since it is possible to know its effectiveness for agricultural production (N. Wang et al., 2021). Before applying any biol, it is necessary to know its microbial load, especially pathogenic bacteria such as Escherichia coli or Salmonella, in order not to put at risk neither the development of plants nor food safety (Carraturo et al., 2022; Cathcart et al., 2022). Pig manure biol was the only one that presented values that put food safety at risk. This evidence shows that anaerobic digestion systems do not always eliminate the pathogens present in manure, so pasteurization would be necessary before use to reduce the levels of these pathogens (Ntinas et al., 2021; Proskynitopoulou et al., 2022).
Biols from pig manure (T1) and cattle manure (T2) were slightly alkaline, with mean values of 7.58 and 7.76, respectively, while biols from horse manure (T3) and cattle manure with coffee processing water (T4) with mean values of 4.01 and 4.27 respectively. These results are ambiguous in finding acidic biols because it is normal to obtain biols with a pH range between 6 and 8 (Fagbohungbe et al., 2019; Samoraj et al., 2022). The reason for finding acidic pH is due to acidification within the biodigester (Sanchez- Beltrán et al., 2021), partly due to the higher amount of plant debris that may be in the horse manure or coffee processing waters, which are usually very acidic, as substrate (Getachew et al., 2023). However, biols with acidic pH are usually free of pathogenic microorganisms whose use would not imply risks to soil quality and, thus to crops and food safety (Parra-Orobio et al., 2021).
The concentrations of organic matter in swine, bovine, and equine manure biols are practically low, something to be expected, as it happens in most organic manures (Bhatt et al., 2019). The organic and nutritional composition of the characterized biols shows evident differences between them. However, it should be noted that the contractions of P, K, Ca, and Mg, as well as the concentrations of the rest of the micronutrients, are also low, being in organic form and needing a decomposition and mineralization process to be available to plants (Geisseler et al., 2021; Meena, 2019). It is understandable given that the substrates used are diverse and present a unique composition due to the particularities of the digestive systems of the animals from which they come, as well as the type of plant waste (Risberg et al., 2017). That said, it is considered that biols could be applied to all crops for agricultural production, as it contains essential nutrients that promote plant growth and yield (Shaji et al., 2021). Biols are frequently used for horticulture, in crops such as lettuce (Faran et al., 2023) or broccoli (Weimers et al., 2022), but it is also advisable to use it for pasture and forage production (Moreno Sandoval et al., 2022).
Most important general parameters of the biols obtained
After applying the principal component analysis (PCA), two principal components (PC) were selected that jointly explained 80% of the data variance. Fourteen parameters of great importance in the evaluated biols were identified (Table 3). The pH and B showed a strong negative correlation with PC1, while TS, SULF, K, Ca, Mg, Na, and Al had a strong positive correlation with PC1. On the other hand, Cu showed a strong negative correlation with PC2, while DOC, P, and Mn showed a strong positive correlation with PC2. It is important to highlight the microbiological parameters; although they did not show a strong correlation, they were close to a strong negative correlation, especially TC, FC, ECO, and ME.
Table 3. Results of the principal component analysis of the parameters evaluated in the biols obtained.
The use of biol is highly recommended as it is a product that increases and stimulates growth and development in a large number of crops, such as potato, wheat, tomato or bell pepper, among others (De Corato et al., 2023; Garg et al., 2005). It is essential to know which are the most important parameters of the biol, since this will allow knowing how efficient the product is according to the type of crop and how it should be applied for agronomic purposes (Fernandez-Bayo et al., 2020). Nutrients in a biol must have balanced levels for proper crop development and good yields. The ideal levels will depend on each crop’s nutritional requirements and the type of soil where the biol is applied (Seelam et al., 2022).
The pH and B are found to be two critical parameters in biols, given their influence on plant nutrient availability and soil quality in general (Tadesse et al., 2022). The pH, which indicates acidity or alkalinity, can limit nutrient uptake and affect the soil microbiome if it is not in a suitable range, depending on the type of crop and soil in which the biol is applied (Ferrarezi et al., 2022). On the other hand, B is an essential micronutrient necessary for plant growth by participating in cell wall formation and nutrient transport. An adequate level of B can prevent soil deficiencies and improve crop quality and yield (Das & Purkait, 2020).
TS parameters, SULF, K, Ca, Mg, Na and Al, are also critical for biols (Lee et al., 2023; Rizzo et al., 2020). STs provide organic matter and nutrients to the soil, improving its structure and water-holding capacity, so it is ideal for biols to have high concentrations of STs (Pandey et al., 2023). SULF, in a proper balance, provides sulfur, an essential nutrient, in a form that plants can absorb and utilize for growth and development (Afzal et al., 2020). Likewise, K, one of the essential macroelements for crops, plays a crucial role in crop growth and development, being essential for the regulation of water balance, fruit development, disease and stress resistance, and the transport of other essential nutrients, such as TN and P, within the plant (Johnson et al., 2022). Ca, Mg, and Na are also essential micronutrients for crops; the first two are necessary for forming cell walls and for the correct photosynthetic activity of plants (Martinez et al., 2020). Likewise, Na, necessary in small amounts, greatly influences the ionic balance by being part of the exchangeable cations in the soil (Cairo-Cairo & Diaz-Martin, 2019). However, excess Na can lead to soil salinization, reducing water availability and causing ionic toxicity, subjecting plants to osmotic stress (Ristorini et al., 2020). As for Al, although in itself it cannot be considered a nutrient, it has a significant influence on soil and plant health, especially in acid soils. Excess Al can be toxic to plants, altering nutrient uptake and damaging roots, reducing crop productivity (Shetty et al., 2021).
Other important parameters for the evaluated biols are Cu, DOC, P, and Mn. Both Cu and Mn are essential micronutrients, which in adequate amounts control diseases in the case of Cu, which has fungicidal and bactericidal capabilities (Miller et al., 2022). In the case of Mn, it is involved in photosynthesis and several plant metabolic processes (Alejandro et al., 2020). P is another essential macronutrient, with N and K, which plays a fundamental role in plant growth and development, involved in photosynthesis, energy transfer, and DNA formation (Saravana Kumar et al., 2020). DOC is a way to measure the organic matter load of a biol, but not its quality, for which it is necessary to determine the contractions of micronutrients and macronutrients (Fernández-Domínguez et al., 2021). However, knowing the organic matter levels is essential when fertilizing a crop with a biol (García-López et al., 2023). As it decomposes, organic matter provides essential plant nutrients such as N, P, and K. At the same time, it improves soil quality by improving soil structure, promoting microbial activity, increasing the amount of organic carbon and enhancing soil biodiversity (Ji et al., 2023; Mahmood et al., 2020).
Behavior of the obtained biol parameters
The PCA biplot allows us to evaluate how the parameters of the biols correlate with each other. For its elaboration, the first two PCs were used, which retain 79% of the variance of the data. The first thing we can highlight in the Biplot graph is the strong negative correlation between pH and Al. On the other hand, it can be seen that SULF, Mg, ST, Ca, K, Na, and EC have a strong positive correlation. P and Mn also have a strong positive correlation, as well as between all microbiological parameters and B (Figure 1).
Figure 1. Biplot graph of the parameters analyzed in the biols obtained.
The red circles highlight the variables and the area where the coincident vectors overlap in the biplot.
TC: Total coliforms; FC: Fecal coliforms; ECO: Escherichia coli; SA: Salmonella; ME: Methanococcus; EC: Electrical conductivity; TS: Total solids; SULF: Sulfates, COD: Chemical oxygen demand; TN: Total nitrogen; P: Phosphorus; K: Potassium; Ca: Calcium; Mg: Magnesium; Na: Sodium; Fe: Iron; Mn: Manganese; Zn: Zinc; Cu: Cupper; Al: Aluminum; B: Boron.
Agronomically, soil pH is a deterministic parameter that affects the availability and uptake of nutrients for plants (Liu et al., 2022). Having a pH less than 5 results in a higher concentration of Al, which is toxic to plants as it solubilizes into ionic forms (Al3+). Al3+ ions tend to displace essential cations such as Ca2+ and Mg2+ in the soil matrix, which react with water and release H+ ions, which increase soil acidity (Ahmed et al., 2022; Kar et al., 2021). This situation is unfavorable for plant development since the Al3+ ion inhibits root elongation and reduces nutrient absorption capacity (Vera-Villalobos et al., 2020). This behavior can be seen especially in the biols of equine manure (T3) bovine manure with coffee processing water (T4), which presented an acid pH and higher Al concentration.
The correlation observed between SULF, Mg, ST, Ca, K, Na, and EC reflects the diversity of sources and characteristics of these elements in the manures and plant material used as substrates in the different biodigesters. These nutrients and EC are related to soil salinity and biol quality (Jin et al., 2022; Manasa et al., 2020). An increase in any of these parameters in the biols to be applied can lead to an increase in soil salinity, adversely affecting plant health. This increase interferes with the absorption of water and nutrients, which reduces crop growth and yield, so the appropriate biol must be selected according to the type of crop and soil (Corwin, 2021). Among the biols evaluated, the one with the highest values for all these parameters was the equine manure biol, so its direct application can be a risk for soil salinity. One way to apply biols with high salt concentrations is to manage this salinity through different strategies, contributing to sustainable agriculture and reducing the risk of soil degradation. Some of the most common strategies are to favor soil leaching or to apply the biol in salinity-resistant crops (An et al., 2022).
The behavior between P and Mn is interesting from an agronomic point of view, as both nutrients are involved in plants’ photosynthetic, metabolic, and energetic processes (Taliman et al., 2019). P is a crucial component of adenosine triphosphate (ATP) energy molecules, which play a central role in cell energy transfer. On the other hand, Mn is an essential enzyme cofactor for oxygen release during photosynthesis and ATP generation (Chandra & Roychoudhury, 2020).
The last correlation observed between microbiological parameters (TC, FC, ECO, SA, and ME) and boron is intriguing from an agronomic perspective, given its impact on soil health, nutrient availability, and possible effects on soil fertility. The behavior found suggests that these bacteria’s activity influences B availability to plants. The bacteria evaluated decomposed organic matter in the biols, releasing organic compounds and nutrients, including B, previously bound to organic matter (Cuartero et al., 2021). At the same time, bacteria can mobilize nutrients in soils by mineralization and solubilization, thus being able to transform inorganic B compounds into soluble forms and facilitating their availability to plants (Diao et al., 2023; Ponomarev et al., 2022). We can verify this in the pig manure biol (T1), which presented a higher microbiological load in addition to being the only one with the presence of B.
The most influential parameters in each type of biol obtained
A biplot graph was made again, but in this case, ellipses were used to group the observations according to the type of biol. In the biplot with ellipses, it can be seen that the biols are dissimilar, so they are differentiated from each other. The PERMANOVA analysis confirms that the groups are significantly dissimilar (F = 516.22, P-Value = 0.001***). Regarding the parameters that most influence each biol, it was observed that nutrients did not influence the cattle manure biol (T2). The pig manure biol (T1) was significantly influenced by pH and bacterial parameters, emphasizing these in their nutritional composition. On the other hand, the horse manure biol (T3) was greatly influenced by SULF, ST, Mg, and Ca, emphasizing these in its nutritional composition. Finally, the biol of cattle manure with coffee processing water (T4) was strongly influenced by DOC, P, and Mn, highlighting these in their nutritional composition (Figure 2).
Figure 2. Biplot plot of the parameters analyzed together with the biols grouped with ellipses.
TC: Total coliforms; FC: Fecal coliforms; ECO: Escherichia coli; SA: Salmonella; ME: Methanococcus; EC: Electrical conductivity; TS: Total solids; SULF: Sulfates, COD: Chemical oxygen demand; TN: Total nitrogen; P: Phosphorus; K: Potassium; Ca: Calcium; Mg: Magnesium; Na: Sodium; Fe: Iron; Mn: Manganese; Zn: Zinc; Cu: Cupper; Al: Aluminum; B: Boron.
PM: Pig manure; CM: Cattle manure; HM: Horse manure; CM+CPW: Cattle manure with coffee processing water
The PCA analysis of the evaluated parameters reveals significant variability in the nutritional composition of the four biols. These results emphasize the importance of understanding the different and complex interactions between the origin of the substrates used to make the biols and the anaerobic digestion process (Ran et al., 2023). These nutritional differences imply that each biol is best suited to each crop type according to its nutritional requirements (Kovačić et al., 2022). It should be noted that biols may show intrinsic variability due to individual differences in animals, management practices, storage, or type of plant waste used for mixtures (Samoraj et al., 2022). This variability may result in biols with different nutrient profiles.
Firstly, it was observed that no nutrient stood out in the cattle manure biol (T2). One reason for this was the initial composition of cattle manure. The diet or the health status of the animals influences their composition. It could be that the animals did not receive a nutritious diet or they were given some medication or vitamin supplement that unbalanced the nutrient composition of the manure, which was used for the elaboration of biol composition (Chozhavendhan et al., 2023; Manyi-Loh et al., 2019). Another reason would be that there was a natural decomposition process before being used for biol manufacture, degrading or volatilizing some nutrients (Bareha et al., 2021). This type of biol could be used in crops or soils that do not require a specific nutrient input.
On the other hand, the significant influence of pH and microbiological parameters in pig manure biol (T1) may be due to several factors, such as the type of diet pigs have and their digestive system (Shi et al., 2019). Pigs are monogastric animals, having only one stomach, unlike ruminant animals, which makes their manure rich in bacteria given the nature of their digestion (Li et al., 2020; Marchwińska & Gwiazdowska, 2022). At the same time, the health of these animals also plays a role. Healthy pigs tend to have a very diverse gut microbiome as they are omnivores, which can be reflected in the manure and, thus in the final biol (Froidurot & Julliand, 2022). Farmer management is also vital in the presence of bacteria such as Escherichia coli and Salmonella, which may indicate poor hygiene in the facilities where pigs are kept (L. Wang et al., 2021).
In contrast, horse manure biol (T3) strongly influenced SULF, ST, Mg, and Ca. Horses have a herbivorous diet, and their digestive system is adapted to process high-fiber plant material, possessing a large caecum and a functionally developed colon, which allows them to break down cellulose and other plant components (Altangerel et al., 2021). At the same time, these nutrients may not be absorbed by the intestinal flora of horses as they do not require large amounts (Joch et al., 2022). The mineral composition of forages and feeds fed to horses, which may vary according to soil type and related agricultural practices, should also be considered (Silva et al., 2022). This type of biol, rich in salts, may be ideal in soils deficient in this type of nutrients or crops requiring high concentrations of these nutrients (Symanczik et al., 2023).
Finally, the cattle manure biol with coffee processing waters (T4), highlighted DOC, P, and Mn. As with the other biols, the initial composition of the manure used to make the biol is something to consider. Cows, ruminant animals, usually have excreta rich in organic matter, with grass and forage remains (Romero et al., 2022). However, it should be noted that the excreta was mixed with water from coffee processing, which gives an extra contribution to organic matter (Erazo & Agudelo-Escobar, 2023). At the same time, coffee, apart from being rich in organic compounds, is rich in minerals such as K, Mg, and P, contributing to their presence in the biol (Alemayehu et al., 2021; Shin et al., 2020). The high content of organic matter, phosphorus, and manganese in this biol could make it valuable for application in poorly fertile soils and low availability of these nutrients for plant growth.
Conclusions
Characterizing the microbiological, physicochemical, organic matter, and nutrient composition of any type of biol, including those that, besides manure, are added to agricultural wastes such as coffee processing waters, provides a valuable perspective on how and when they should be applied according to the type of crop or soil. The significant variability found in the composition of biols, depending on the type of substrate used for their preparation, is worth noting. The pig manure biol was rich in bacteria; the cattle manure biol did not stand out in any nutrient. At the same time, a large amount of salts characterized the horse manure biol, and finally, the cattle manure with coffee processing water was rich in organic matter and P. Therefore, the selection of each type of biol should be made carefully according to the crop’s nutritional requirements and the soil conditions. In addition, critical parameters such as pH, B, or P, among other nutrients, which play a fundamental role in the availability of nutrients for plants and soil quality, were evidenced. These findings support the usefulness of biols, particularly those incorporating agricultural wastes such as coffee processing waters, as versatile organic fertilizers that can improve soil fertility and stimulate crop growth, thus contributing significantly to sustainable agricultural practices and food security by producing high-quality food.
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Rascón J, Juarez Contreras LdP, García Saldaña EW et al. Characterization of four digestates from different types of substrates used in biogas production in Northern Peru [version 1; peer review: 1 approved, 2 not approved]. F1000Research 2024, 13:24 (https://doi.org/10.12688/f1000research.142713.1)
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Version 1
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PUBLISHED 08 Jan 2024
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How to cite this report:
Samuel Dahunsi O. Reviewer Report For: Characterization of four digestates from different types of substrates used in biogas production in Northern Peru [version 1; peer review: 1 approved, 2 not approved]. F1000Research 2024, 13:24 (https://doi.org/10.5256/f1000research.156291.r243505)
The direct URL for this report is:
https://f1000research.com/articles/13-24/v1#referee-response-243505
https://f1000research.com/articles/13-24/v1#referee-response-243505
NOTE: it is important to ensure the information in square brackets after the title is included in this citation.
Reviewer Report 29 Aug 2024
Olatunde Samuel Dahunsi,
Bowen University, Iwo, Osun, Nigeria
Approved
VIEWS 9
I have no issue evaluating this manuscript as it is a clearly designed and well-implemented research effort. The findings supports the use of biols from different animal wastes. Authors only need to ensure the following:
1. Correct English must ... Continue reading
1. Correct English must ... Continue reading
CITE
HOW TO CITE THIS REPORT
Samuel Dahunsi O. Reviewer Report For: Characterization of four digestates from different types of substrates used in biogas production in Northern Peru [version 1; peer review: 1 approved, 2 not approved]. F1000Research 2024, 13:24 (https://doi.org/10.5256/f1000research.156291.r243505)
The direct URL for this report is:
https://f1000research.com/articles/13-24/v1#referee-response-243505
https://f1000research.com/articles/13-24/v1#referee-response-243505
NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article.
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12
How to cite this report:
Onyango Nyang'au J. Reviewer Report For: Characterization of four digestates from different types of substrates used in biogas production in Northern Peru [version 1; peer review: 1 approved, 2 not approved]. F1000Research 2024, 13:24 (https://doi.org/10.5256/f1000research.156291.r311801)
The direct URL for this report is:
https://f1000research.com/articles/13-24/v1#referee-response-311801
https://f1000research.com/articles/13-24/v1#referee-response-311801
NOTE: it is important to ensure the information in square brackets after the title is included in this citation.
Reviewer Report 10 Aug 2024
Not Approved
VIEWS 12
The study evaluated the properties of 4 digestates from digestates in Northern Peru. It evaluated microbiological, physiological,organic and nutrient composition.
It could have been appropriate if number lines were used for easy reference during review.
General ... Continue reading
It could have been appropriate if number lines were used for easy reference during review.
General ... Continue reading
CITE
HOW TO CITE THIS REPORT
Onyango Nyang'au J. Reviewer Report For: Characterization of four digestates from different types of substrates used in biogas production in Northern Peru [version 1; peer review: 1 approved, 2 not approved]. F1000Research 2024, 13:24 (https://doi.org/10.5256/f1000research.156291.r311801)
The direct URL for this report is:
https://f1000research.com/articles/13-24/v1#referee-response-311801
https://f1000research.com/articles/13-24/v1#referee-response-311801
NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article.
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18
How to cite this report:
Rietra R. Reviewer Report For: Characterization of four digestates from different types of substrates used in biogas production in Northern Peru [version 1; peer review: 1 approved, 2 not approved]. F1000Research 2024, 13:24 (https://doi.org/10.5256/f1000research.156291.r271747)
The direct URL for this report is:
https://f1000research.com/articles/13-24/v1#referee-response-271747
https://f1000research.com/articles/13-24/v1#referee-response-271747
NOTE: it is important to ensure the information in square brackets after the title is included in this citation.
Reviewer Report 14 Jun 2024
Rene Rietra,
Wageningen University & Research, Wageningen, Gelderland, The Netherlands
Not Approved
VIEWS 18
The manuscript is about the analyses of 4 repeated sampling moments digestate samples from 4 digesters, resulting in 12 analyses per digester. The numbers are unclear. The authors have determined 22 parameters. Subsequently the authors do statistical analysis to find
... Continue reading
CITE
HOW TO CITE THIS REPORT
Rietra R. Reviewer Report For: Characterization of four digestates from different types of substrates used in biogas production in Northern Peru [version 1; peer review: 1 approved, 2 not approved]. F1000Research 2024, 13:24 (https://doi.org/10.5256/f1000research.156291.r271747)
The direct URL for this report is:
https://f1000research.com/articles/13-24/v1#referee-response-271747
https://f1000research.com/articles/13-24/v1#referee-response-271747
NOTE: it is important to ensure the information in square brackets after the title is included in all citations of this article.
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