processes Article Agronomic Characteristics of the Compost-Bedded Pack Made with Forest Biomass or Sawdust Lourdes Llonch 1 , Cecilia Gordo 2 , Marga López 2 , Lorena Castillejos 1, * , Alfred Ferret 1 and Teresa Balanyà 2 1 Animal Nutrition and Welfare Service (SNiBA), Department of Animal and Food Science, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain; Lourdes.Llonch@uab.cat (L.L.); Alfred.Ferret@uab.cat (A.F.) 2 Department of Agrofood Engineering and Biothecnology, School of Agro-Food and Biosystems Engineering, Universitat Politècnica de Catalunya, 08860 Castelldefels, Spain; ceci-2111@hotmail.com (C.G.); marga.lopez@upc.edu (M.L.); teresa.balanya@upc.edu (T.B.) * Correspondence: Lorena.Castillejos@uab.cat Abstract: To ascertain the agronomic value of the material resulting from the compost-bedded pack (CBP) in dairy barns, a cross-over experiment was designed with eight dry non-pregnant Holstein cows. The study was performed in two 11-week periods. Bedding materials used were: (1) CBP with sawdust (S) and (2) CBP with forest biomass (FB). Samples were taken from the raw bedding materials and from the CBP across the experiment. We conducted an additional study preparing two piles, one of each CBP material, to accomplish a composting process of 3 months, where samples were also taken. Granulometry and some chemical composition characteristics of FB made it a suitable bedding material to be used as CBP, but its high moisture content limited the ability to absorb liquid   manure. Both the degree of stability of the organic matter and the temperature evolution of CBP suggest that a real composting process did not occur. Finally, the composting process of the piles Citation: Llonch, L.; Gordo, C.; did not lead to any relevant change in CBP materials. From the agronomic point of view, S and FB López, M.; Castillejos, L.; Ferret, A.; present potentially valuable characteristics as regards organic amendment in the soil, thanks to their Balanyà, T. Agronomic Characteristics of the high organic matter content and low nutrient content. Compost-Bedded Pack Made with Forest Biomass or Sawdust. Processes Keywords: compost-bedded pack; dairy cows; forest biomass; organic amendment; sawdust 2021, 9, 546. https://doi.org/10.3390/ pr9030546 1. Introduction Academic Editor: Antoni Sánchez Fifty-five percent of the world’s population lives in urban areas, a proportion that is expected to increase to 68% by 2050 [1]. In this process, the number of both farms and Received: 2 March 2021 farmers has decreased, thus raising, at the same time, the urban population [2]. One of the Accepted: 16 March 2021 main consequences of this rural flight is an increase in forestland. Human activities have Published: 19 March 2021 directly caused approximately 60% of the new global tree growth [3]. Dry weather and damaged ecosystems with an accumulation of dead biomass due to rural abandonment Publisher’s Note: MDPI stays neutral increase the risk of forest fires. The accumulation of high fuel loads over large areas is the with regard to jurisdictional claims in main reason behind the occurrence of large fires [4,5]. In this context, it is necessary to find published maps and institutional affil- iations. new ways to reduce these high fuel loads to prevent large fires. The compost-bedded pack (CBP) is a loose housing system that is becoming more and more established on dairy farms in which cows are maintained for a long time on the compost. Improvements in the health, welfare and performance of cows, ease of farm chores and reduced building costs have been described in comparison with other housing Copyright: © 2021 by the authors. systems [6–8]. The composting process allows manure and urine to be stored, as long Licensee MDPI, Basel, Switzerland. as the pack is managed adequately, involving twice-daily tilling and periodic bedding This article is an open access article addition [7,9,10]. Tilling incorporates manure and air into the pack, thus promoting aerobic distributed under the terms and microbiological activity, heating the pack and drying the lying surface for cattle to lie conditions of the Creative Commons Attribution (CC BY) license (https:// on [11]. Bedding addition increases the water-holding capacity of the pack to control CBP creativecommons.org/licenses/by/ moisture. Compost-bedded pack barns in dairy cow farming mainly use sawdust (S) as 4.0/). a bedding material, but forest biomass (FB) could be an alternative material. When both Processes 2021, 9, 546. https://doi.org/10.3390/pr9030546 https://www.mdpi.com/journal/processes Processes 2021, 9, 546 2 of 12 materials were tested as CBP, some important microbial species affecting cow health were better controlled using FB, although in terms of cow comfort, FB did not appear to work as well as S [12]. However, the welfare assessment conducted did not detect health problems in the cows [12]. Once the CBP material is moved away from the barn, it is used as organic fertilizer on the farm. The lack of information about the agronomic value of this fertilizer source led us to study (a) the agronomic characteristics of the CBP obtained either with FB or S, after maintaining cows for three months on the pack, and (b) the agronomic characteristics of both composted materials after conducting an additional composting process of three months once the cows were moved away. 2. Materials and Methods Two different composting processes were established. The first involved maintaining cows on CBP, either FB or S, for 11 weeks. Once this one was completed, a second process began, based on composting piles from the two CBPs for 3 months. 2.1. Bedding and Compost Management Eight dry non-pregnant Holstein cows were individually allocated in roofed concrete floor pens, 5 m long and 2.5 m wide. Each pen was divided in 2 areas: a feeding area equipped with a feed bunk and a water trough, and a resting area. Pens were separated by a metal fence that allowed contact between animals. Cows were randomly assigned to 1 of 2 treatments in a cross-over design with 4 cows per treatment. The study was performed in 2 11-week periods with a 4-week washout period between them. Bedding materials used were: (1) CBP with S (CBP-S) and (2) CBP with FB (CBP-FB). The bulk density for these bedding materials was 182 ± 6.3 g/L and 240 ± 16.2 g/L for S and FB, respectively. During the washout period, cows laid on traditional wood shaving bedding. At the beginning of each period, pens were filled with 30 cm of the new bedding material. All CBPs were tilled twice daily (10 a.m. and 5 p.m.) at 30 cm depth with a rototiller, and an average of 0.8 kg/m2 /d of new bedding material was added on each pen CBP surface before tilling, when pen CBP moisture was greater than 60%. Compost-bedded pack moisture was measured weekly in both areas (feeding and resting) of each pen. Averages of 7.8 kg/pen/d in CBP-S and 7.9 kg/pen/d in CBP-FB of new bedding materials were added. Both CBPs were removed at the end of week 11 of each period. Daily ambient temperature and environmental humidity were obtained from 2 data loggers (UX100-003, Hobo, Algete- Madrid, Spain) located in the barn. At the end of the second experimental period, the composted bedding material of each pen was composited and used to prepare 2 composting piles, one of each bedding material, to obtain a composted material after 3 months of the composting process. Each pile, in the shape of a truncated pyramid and located in a roofed barn, was 4.8 m long, 2.5 m wide and 1.15 m high, with an approximate volume of 7.5 m3 . The piles were turned and rebuilt weekly to facilitate the aerobic process. It was necessary to water the piles on five occasions to maintain pile moisture between 45 and 65%. Daily ambient temperature and environmental humidity were obtained as described before. 2.2. Sampling Samples were taken from the raw bedding materials, the CBP and the composted piles. Raw bedding material was sampled throughout the experiment. Thus, in total, 4 samples of each one were collected and then stored at 4 ◦ C until analysis. Particle size of the raw bedding materials was measured using an electromagnetic sieve shaker (RP 200N, CISA Cedaceria Industrial S. L., Barcelona, Spain) to obtain their physical characterization. With regard to the CBP material, temperature was recorded daily and moisture weekly in the feeding and resting areas during each experimental period. Temperature was measured at 15 cm depth with a thermometer and a 15-cm sounding line (K/JR-200 + 800 ◦ C, Ventix, Sant Adrià de Besós, Spain). Sampling for chemical analysis was performed at week 11. Samples were collected at 15 cm depth, from the middle of the feeding and resting area Processes 2021, 9, 546 3 of 12 of each pen, and later composited by pen. The amount of sample collected by pen was 5 kg. Samples were stored at −18 ◦ C until analysis. Thus, we collected 4 samples per each material and period. Finally, the sampling of the piles was carried out every 10 days, collecting in total 9 samples of 6 kg each, and then samples were stored at −18 ◦ C until analysis. Temperature was measured at 40 cm and 1 m depths with a thermome- ter and a 1-m sounding line (K/JR-200 + 800 ◦ C, Ventix, Sant Adrià de Besós, Spain). Three samples were taken from each pile in the middle of 3 of the 4 sides of the truncated pyramid, at different depths to obtain a representative sample from each side, and were then composited by pile. The fourth side was next to a wall to facilitate the containment and prevention of landslide, making it inaccessible. In addition, during the composting process, the evolution of wet bulk density was monitored by weighing material of a known volume, according to [13]. Wet bulk density values were transformed to dry bulk density values by multiplying them by the corresponding dry matter (DM) content of samples. 2.3. Chemical Analysis At the moment of analysis, samples were defrosted at room temperature. A watery extract was obtained from each fresh sample using 40 g and 200 mL of distilled water. After 30 min of stirring, the extract was centrifuged at 3600 rpm for 15 min. In the supernatant, the following determinations were carried out: pH and electric conductivity (EC) using a pH meter (Crison GLP 21; Hach Lange Spain, S.L.U., L’Hospitalet de Llobregat, Spain) and a con- ductivity meter (Crison GLP 31; Hach Lange Spain, S.L.U., L’Hospitalet de Llobregat, Spain), and ammonia nitrogen by means of an ammonium selective electrode (Orion 9512, Termo Fisher Scientific, Barcelona, Spain). Moisture content was determined by drying samples for 24 h at 103 ◦ C. Ash content was determined in the dry and ground (1-mm screen) sample by loss on ignition at 600 ◦ C for 2 h based on the AOAC method 942.05 [14] to ascertain the organic matter (OM) content of samples. Chemical stability degree (SD), as a way to predict composting ability, was determined by Klason lignin determination in accordance with [15]. In dry and ground samples (1-mm), nitrogen content was determined by the Kjeldahl proce- dure based on the AOAC method 976.05 [14]. The C/N ratio was estimated from the OM and nitrogen content in accordance with [16]. Mineral nutrient content was determined by flame photometry in the case of K (Model 410, Corning, Halstead, UK) and by spectrophotometry in the case of P (Model Cary 60, Agilent Technologies, Singapore, Malaysia) after dissolution of ash obtained from the ignition of samples at 470 ◦ C in 3 N HNO3 . 2.4. Statistical Analysis Chemical composition of raw bedding materials was compared using the GLM pro- cedure of SAS (v. 9.3; SAS Institute Inc., Cary, NC, USA, 2011). Data related to the CBP material were analyzed by using the MIXED procedure of SAS (v. 9.3; SAS Institute Inc., Cary, NC, USA, 2011). The model contained the fixed effects of treatment, period and treat- ment × period interaction, and the random effect of pen was nested within the sequence, where the sequence is the order in which treatment is applied to the experimental unit. The Tukey multiple comparison test was applied to conduct mean separation across treat- ments and periods when the treatment × period interaction was significant. Regression analyses were performed to obtain the equations and the coefficients of determination between variables studied in the composted piles and the sampling days using the REG procedure of SAS (v. 9.3; SAS Institute Inc., Cary, NC, USA, 2011). The scopes of linear regression, obtained for each composted pile, were compared by means of a t-test after checking the homogeneity of variances. 3. Results and Discussion 3.1. Comparison between Bedding Materials Sawdust obtained from sawmills is a bedding material commonly used in CBP barns for dairy cows [17,18], while FB is an alternative material that, in the present study, was composed of tree bark and vegetal fibers from a Mediterranean forest. Both ma- Processes 2021, 9, 546 4 of 12 terials showed an acidic pH, a low EC value, high OM content and low nutrient content, resulting in a high C/N ratio (Table 1). These characteristics are common of bedding materials used in CBP, together with a low moisture to assure the absorption capacity of animal urine and a particle size < 25 mm to promote the microbial activity due to the increased growing surface [11,18]. However, certain differences between both materi- als can be highlighted regarding their physical and chemical properties. Sawdust was slightly acidic with a higher EC value and OM content and lower in moisture and nutrients, resulting in a higher C/N ratio in S than in FB (Table 1; p < 0.001). In addition, S showed a lower moisture than FB and a finer granulometry because the proportion of particles < 2 mm was 49.2% and 22.0% for S and FB, respectively. Table 1. Chemical composition and granulometry of raw bedding materials. Material Item SEM p-Value S 1 FB 2 pH 5.01 6.01 0.112 0.001 EC, mS/cm 0.43 0.23 0.011 0.001 Moisture, % 10.2 32.1 1.97 0.001 OM, % DM 99.1 88.6 0.43 0.001 Stability, % 28.6 52.2 0.22 0.001 Organic N, % DM 0.20 0.35 0.026 0.001 C/N ratio 286 131 20.0 0.001 P, % DM 0.006 0.024 0.0020 0.001 K, % DM 0.054 0.131 0.0088 0.001 Particle size, % >25 mm 0.0 0.0 0.02 0.434 25–12.5 mm 0.6 8.6 0.76 0.001 12.5–10 mm 0.7 6.2 0.50 0.001 10–6.3 mm 8.3 13.2 0.74 0.001 6.3–5 mm 6.8 10.3 1.26 0.077 5–2 mm 34.3 39.7 2.99 0.232 <2 mm 49.2 22.0 4.62 0.002 1 S = sawdust. 2 FB = forest biomass. Stability was greater in FB than in S (Table 1; p < 0.001). The authors of [19] pointed out that the lignin content of hardwoods (angiosperms) is usually in the range of 18–25%, whereas this content ranges between 25 and 35% in the case of softwoods (gymnosperms). These higher values can be attributed to the great lignin content of pine bark, between 38 and 58%, an ingredient visually present in FB. The content of C and N and the C/N ratio of the sawdust used in the present exper- iment was similar to that reported by [11], but with a P and K content ten times lower. In the case of FB, there is a lack of information with regard to this material when used in CBP barns. A similar bedding material obtained from the forest (conifer forest litter) to be used in CBP was described by [17], with similar C and macronutrient contents, and with a C/N ratio more suitable than sawdust for a composting process and greater than 50 to minimize ammonia losses [20]. However, the material described by [17] and the FB used in the present experiment differ in their granulometry. In our case, 61.7% of the material was less than 5 mm, while this percentage was lower for the conifer forest litter. This, together with the remaining particle fractions, suggests that the forest material described by [17] was more heterogenous and coarse than FB, probably due to the grinding machinery used. Differences in granulometry explain the differences in bulk density, which were 123 and 240 kg/m3 for conifer forest litter and FB, respectively. Processes 2021, 9, 546 5 of 12 3.2. Comparison between Compost-Bedded Pack (CBP) 3.2.1. Temperature and Moisture Temperature and moisture affect the biological transformation of organic materials, particularly in aerobic conditions. Moisture and oxygen availability allow the micro- bial activity which leads to a temperature increase due to exogenous chemical reactions of molecule degradation. In a process such as composting, this temperature increase is observed as an indicator of an adequate evolution. However, in the case of CBP, the objective was not the biological transformation of the material as in the compost- ing but the availability of safe housing for animal breeding. The presence of a C-rich material and the availability of a N source from feces and urine, moisture and oxygen provide the conditions for microbial growth and, consequently, a temperature increase. Climatic conditions during the CBP process are shown in Table 2. The average ambient temperature was 14.7 ± 3.63 ◦ C in Period 1 and decreased from 19.5 to 9.1 ◦ C. In Period 2, the average ambient temperature was 11.2 ± 2.49 ◦ C and increased from 5.9 to 14.1 ◦ C Thus, Period 2 was colder than Period 1, as reflected in the lower temperatures recorded in CBP in Period 2. In Period 1, the mean CBP temperature was 34.7 ± 5.64 ◦ C in CBP-S and 28.3 ± 6.87 ◦ C in CBP-FB, and in Period 2, 31.2 ± 5.88 ◦ C and 25.9 ± 3.08 ◦ C in CBP-S and CBP-FB, respectively. The lower CBP temperatures recorded in CBP-FB would indicate a lower microbial activity due to the greater OM stability of this material and bigger particle size. However, in both cases, these temperatures were below the recommended range values for an effective composting [10,21,22], although this frequently occurs in the context of CBP management [7,11,23]. Average environmental humidity was 81.5 ± 4.21 in Period 1 and ranged between 74.1 and 90.4%. In Period 2, average environmental humidity was 74.5 ± 7.18% and ranged between 61.6 and 87.3%. In CBP-S, the initial moisture was 10% on average and the mean value was 54.1 ± 16.97% in Period 1, and 55.0 ± 16.43% in Period 2. In CBP-FB, the initial moisture was 32% on average and the mean moisture was 62.0 ± 14.00% in Period 1, and 59.4 ± 7.21% in Period 2. A similar moisture content was expected in both CBPs because we decided to add new bedding material when the moisture was greater than 60%, applying an average amount of 7.8 kg/d and 7.9 kg/d in CBP-S and CBP-FB, respectively. However, the moisture was slightly higher in CBP-FB than in CBP-S. The moisture and granulometry of FB could also have contributed to this final result. Table 2. Climatic conditions, temperature and moisture content of compost-bedded pack (CBP) made with sawdust (CBP-S) and forest biomass (CBP-FB) during the time periods studied. Period 1 Period 2 Climatic conditions Temperature, ◦ C 14.7 ± 3.63 11.2 ± 2.49 Humidity, % 81.5 ± 4.21 74.5 ± 7.18 CBP temperature, ◦ C CBP-S 34.7 ± 5.64 31.2 ± 5.88 CBP-FB 28.3 ± 6.87 25.9 ± 3.08 CBP moisture, % CBP-S 54.1 ± 16.97 55.0 ± 16.43 CBP-FB 62.0 ± 14.00 59.4 ± 7.21 3.2.2. Chemical Characteristics The chemical composition of CBP samples at week 11 is shown in Table 3. The pH values recorded were basic and there were increases in the pH and EC values from the raw bedding materials to CBP samples. This increase can be attributed to feces and urine provided by the cows. The pH and EC values were affected by the treatment × period interaction (p = 0.001 and p = 0.003, respectively). In Period 1, CBP-FB pH was higher than CBP-S pH, whereas CBP-FB EC was lower than CBP-S EC. In contrast, CBP-FB pH was lower than CBP-S pH in Period 2, whereas CBP-FB EC was not different from CBP-S Processes 2021, 9, 546 6 of 12 EC. The low EC value recorded in CBP-FB in Period 1 could be due to its higher moisture resulting in a higher dilution rate because this determination was made with a wet sample. Table 3. Chemical composition of compost-bedded packs (CBP) after 11 weeks of use. Period 1 Period 2 p-Value Item SEM CBP-S 1 CBP-FB 2 CBP-S CBP-FB T P T×P pH 8.00 c b 8.96 a 7.81 c 0.078 0.001 0.021 0.001 8.43 EC, mS/cm 3.78 a 2.39 b 3.84 a 3.79 a 0.163 0.001 0.001 0.003 Moisture, % 61.3 b 69.7 a 63.7 b 63.2 b 1.09 0.002 0.037 0.001 OM, % DM 91.1 a 83.4 c 89.9 a 85.7 b 0.54 0.001 0.115 0.008 Stability, % ND ND 31.7 49.2 1.25 0.001 ND ND N, % DM Organic 0.89 1.12 1.07 1.21 0.036 0.001 0.002 0.101 Ammonia 0.07 0.08 0.05 0.04 0.014 0.999 0.034 0.444 Total 0.96 1.19 1.12 1.24 0.039 0.001 0.005 0.109 C/N ratio 48:1 a 35:1 c 40:1 b 35:1 c 1.1 0.001 0.002 0.005 P, % DM 0.24 b 0.28 a 0.28 a 0.25 a,b 0.009 0.879 0.458 0.003 K, % DM 1.22 b 1.64 a 1.78 a 1.62 a 0.098 0.038 0.001 0.015 1 CBP-S = compost-bedded pack made with sawdust. 2 CBP-FB = compost-bedded pack made with forest biomass. a–c Means within a row with different superscripts differ (p < 0.05). The treatment × period interaction also affected the OM content (p = 0.008). The OM content was higher in CBP-S than in CBP-FB in both periods, but this difference was greater in Period 1 than in Period 2. The content of organic N was affected by treatment (p = 0.001), being higher in CBP-FB than in CBP-S. These results could be due to the differences detected in the raw bedding materials where OM was higher in S than in FB, and organic N was higher in FB than in S (Table 1). The period also affected organic N (p = 0.002), being higher in Period 2 than in Period 1. With regard to ammonia N, this content was affected by period (p = 0.034), being greater in Period 1 than in Period 2. However, these differences are not relevant from the agronomic point of view. In addition, there was a treatment × period interaction in the C/N ratio (p = 0.005). Although this ratio was greater in CBP-S than in CBP-FB in both periods (p = 0.001), in agreement with the differences detected in the raw bedding materials, and it was greater in Period 1 than in Period 2 (p = 0.002), the difference between treatments was greater in Period 1 than in Period 2. The contents of the other macronutrients (P, K) were affected by the treatment × period interaction (p = 0.003 and p = 0.015, respectively). For P and K, the content of CBP-FB was greater than in CBP-S in Period 1, but the content between treatments was not different in Period 2. However, considering the higher differences found between raw bedding materials, where the P and K content was higher in FB than in S, the differences between CBPs were lower and in accordance with the raw materials in Period 1, or were not observed in Period 2. In any case, these differences are not relevant from the agronomic point of view. The pH value, EC, moisture and the contents of N, P and K increased in CBP samples with regard to the raw bedding materials with high OM content, due to the high macronu- trient content supplied by feces and urine. This would explain the C/N ratio decrease in CBP. Thus, the final characteristics of CBP will be linked with the bedding material added throughout the process. Taking into account the amount added (7.8 kg/d and 7.9 kg/d in CBP-S and CBP-FB, respectively) and the bulk density of the bedding material, the amounts added were 15.6 m3 /cow/year and 12.0 m3 /cow/year, for CBP-S and CBP-FB, respectively. These values are inside the range reviewed by [18]. In addition, the chemical characteristics found in the present experiment for CBP-S are in the range found by [11] and [24] for wood shavings and wood chips. The SD of the OM in CBP, measured in Period 2, was greater in CBP-FB than in CBP-S, in agreement with the differences recorded in the raw bedding materials. The SD measured in CBP-S remained far away from 50, the threshold used to consider a compost mature. Taken together with the temperature evolution of CBP, this suggests that there was not a real composting process. and [24] for wood shavings and wood chips. The SD of the OM in CBP, measured in Period 2, was greater in CBP-FB than in CBP- S, in agreement with the differences recorded in the raw bedding materials. The SD meas- ured in CBP-S remained far away from 50, the threshold used to consider a compost ma- Processes 2021, 9, 546 ture. Taken together with the temperature evolution of CBP, this suggests that there was 7 of 12 not a real composting process. 3.3. Comparison between the Composted Piles 3.3. Comparison 3.3.1. Temperature, Moisturebetween and Bulk theDensity Composted Piles 3.3.1. Temperature, Moisture and Bulk Density From the beginning to the end of the composting period, ambient temperature in- creased from 22 toFrom 26 °Ctheandbeginning environmentalto thehumidity end of the composting from 47 to 72%.period, ambient The initial temperature in- temper- creased from 22 to 26 ◦ C and environmental humidity from 47 to 72%. The initial tempera- atures of the composting piles were 35.0 °C and 30.4 °C for S and FB, respectively, the final ◦ and 30.4 ◦ C for S and FB, respectively, the final temperaturestures beingof 46.0 the composting °C and 44.5piles were 35.0 °C (Figure 1a). CThese temperatures were obtained temperatures beingat46.0 ◦ C and 44.5 ◦ C (Figure 1a). These temperatures were obtained after after averaging data recorded depths of 40 cm and 1 m. In both piles, a temperature increase wasaveraging observed data duringrecorded at depths process, the composting of 40 cm achieving and 1 m. In the both piles, a temperature thermophile phase increase was observed during the composting process, achieving the thermophile phase (>40 ◦ C), (> 40 °C), and reaching sanitation temperature for S (≥ 55 °C◦more than 14 d according to and reaching European Regulation sanitation 2019/1009), temperature while FB remained (≥55 C for S below more this than limit. 14 temperature The d according to European achieved at d 90 in both piles would indicate that the composting process hadtemperature Regulation 2019/1009), while FB remained below this limit. The not ended, achieved at d 90 in both piles would indicate that the composting process probably due to the low C/N ratio and the lignocellulosic content of both materials, had not par- ended, probably due to the low C/N ratio and the lignocellulosic content of both materials, particularly in ticularly in FB. The initial moisture of the S pile and the FB pile was 61.5% and 62.2%, FB. The initial moisture of the S pile and the FB pile was 61.5% and 62.2%, respectively, respectively, whereas the final moisture was 47.9% and 55.4% (Figure 1b). The values over whereas the final moisture was 47.9% and 55.4% (Figure 1b). The values over the period the period were adequate for a composting process, being over 50% in order to maintain were adequate for a composting process, being over 50% in order to maintain microbial microbial activity [25]. The piles were watered periodically, which helped to conserve the activity [25]. The piles were watered periodically, which helped to conserve the moisture. moisture. The bulk density evolution is shown in Figure 2. Dry bulk density increased The bulk density evolution is shown in Figure 2. Dry bulk density increased slightly, and slightly, and changes were due to mineralization and particle size reduction. Bulk density changes were due to mineralization and particle size reduction. Bulk density was higher in was higher in the FB pile than in the S pile. the FB pile than in the S pile. Figure 1. Effect of treatment (sawdust = S and, forest biomass = FB) on temperature (a) and moisture (b) of the composted piles. Figure 1. Effect3.3.2. Chemical of treatment Characteristics (sawdust = S and, forest biomass = FB) on temperature (a) and mois- ture (b) of the composted piles. Initial and final values of the variables measured in the piles are shown in Table 4. In a composting process, an initial acidification followed by a gradual alkalinization is a common evolution of the pH medium. However, in the present experiment, the changes in the pH values of the piles were small and did not respond to this pattern (Figure 3). The pH values were more basic in S than in FB, which implies greater risk of N losses in S. With regard to EC, an increase in its value was expected during the composting process due to OM mineralization. This happened in S but not in FB, proving that composting was less strong in the FB pile. Processes 2021, 9, Processes x FOR 2021, PEER REVIEW 9, 546 8 of 128 of 12 220 200 Bulk density, kg MS/m3 180 160 S 140 FB 120 100 0 12 19 26 47 62 90 Time, d Figure 2. Bulk density evolution of samples taken from the composted piles (sawdust = S and forest Figure 2. Bulk=density biomass FB). evolution of samples taken from the composted piles (sawdust = S and for- est biomass = FB). 3.3.2.Table 4. Initial Chemical and final values (mean ± SD) of the variables measured in the composted piles made Characteristics with sawdust (S) and forest biomass (FB). Initial and final values of the variables measured in the piles are shown in Table 4. In a composting process, an initial acidification S followed by a gradual alkalinizationFB is a com- Item mon evolution of the pH medium. However, inDay the90present experiment, the changes in the Day 0 Day 0 Day 90 pH values of the piles were small and did not respond to this pattern (Figure 3). The pH ± 0.017 8.39 ± 0.035 valuespHwere more basic in8.72S than in FB, which risk±of implies greater7.45 0.015 7.72 ± 0.100 N losses in S. With EC, mS/cm 3.59 ± 0.145 5.58 ± 0.106 3.39 ± 0.025 3.10 ± 0.061 regardMoisture, to EC, an%increase in61.5 its ± value 0.24 was expected during 47.9 ± 0.15 the composting 62.2 ± 0.29 process 55.4due to ± 0.18 OM mineralization. OM, % DM This happened 91.7 ± 0.18 in S but not in FB, 87.4 ± 0.13 proving that composting 86.3 ± 0.30 was less 82.9 ± 0.03 strongStability, in the FB% pile. 31.7 34.9 49.2 53.9 N, % DM Organic Table 4. Initial 0.88 and final values (mean ± SD) of the ± 0.039 1.05 ± 0.058measured variables in± 1.16 0.013 the composted1.27piles± 0.054 Ammonia 0.049 ± 0.0004 made with sawdust (S) and forest biomass (FB). 0.004 ± 0.0002 0.013 ± 0.0006 0.001 ± 0.0005 C/N ratio 52.0 41.8 37.2 32.6 Processes 2021, 9, x FOR PEER REVIEW P, % DM S 0.24 ± 0.017 0.31 ± 0.040 FB 0.27 ± 0.021 0.24 ± 0.011 9 of 12 Item K, % DM Day 0 ± 0.071 1.74 90 ± 0.134 Day2.02 0 ± 0.269 Day1.94 90 ± 0.066 Day1.87 pH 8.72 ± 0.017 8.39 ± 0.035 7.45 ± 0.015 7.72 ± 0.100 EC, mS/cm 3.59 ± 0.145 5.58 ± 0.106 3.39 ± 0.025 3.10 ± 0.061 Moisture,10 % 61.5 ± 0.24 47.9 ± 0.15 62.2 ± 0.29 55.4 ± 0.18 OM, % DM 91.7 ± 0.18 87.4 ± 0.13 86.3 ± 0.30 82.9 ± 0.03 Stability, % 31.7 34.9 49.2 53.9 9 N, % DM Organic 0.88 ± 0.039 1.05 ± 0.058 1.16 ± 0.013 1.27 ± 0.054 0.049 ± 0.0004 0.004 ± 0.0002 0.013 ± 0.0006 0.001 ± 0.0005 pH Ammonia 8 C/N ratio 52.0 41.8 37.2 S 32.6 P, % DM 0.24 ± 0.017 0.31 ± 0.040 0.27 ± 0.021 0.24 ± 0.011 FB K, % DM 7 1.74 ± 0.071 2.02 ± 0.134 1.94 ± 0.269 1.87 ± 0.066 6 0 12 19 26 33 47 62 75 90 Time, d Figure 3. Evolution of pH of samples taken from the composted piles (sawdust = S and forest biomass = FB). Figure 3. Evolution of pH of samples taken from the composted piles (sawdust = S and forest bio- mass = FB). Although a small decrease in the OM in both piles was observed, the content was high in both cases at d 90, which would indicate only a partial transformation of this OM during composting. This could be explained by the high C/N ratio recorded at d 0, which 6 0 12 19 26 33 47 62 75 90 Time, d Processes 2021, 9, 546 9 of 12 Figure 3. Evolution of pH of samples taken from the composted piles (sawdust = S and forest bio- mass = FB). Although Althougha asmallsmalldecrease decreaseininthe theOM OMininbothbothpiles pileswaswasobserved, observed,the thecontent contentwas was high in both cases at d 90, which would indicate only a partial high in both cases at d 90, which would indicate only a partial transformation transformation of thisofOM this during composting. OM during This could composting. This be explained could by the high be explained by the C/N highratio C/N recorded at d 0, which ratio recorded at d 0, reflected a N lack,aand which reflected by the N lack, andhigh presence by the of lignin of high presence in wood lignin waste, in wood which waste,would which explain would the difficulty explain the in the OMin difficulty degradation. Thus, at theThus, the OM degradation. end ofatthe theprocess, end of the a large quantity process, of a large OM remained non-degraded. Nevertheless, a slight increase in quantity of OM remained non-degraded. Nevertheless, a slight increase in stability was stability was observed, from 31.7 tofrom observed, 34.9%31.7and 49.2 to to 34.9% and53.9%, 49.2 to respectively, for S and 53.9%, respectively, forFB. S and According to [15], FB. According tothe [15], value over 50% recorded in the FB pile at d 90 would indicate that the value over 50% recorded in the FB pile at d 90 would indicate that the material was the material was chem- ically stable and chemically would stable and produce a progressive would produce degradation a progressive of OM inofanOM degradation eventual soil ap- in an eventual plication and a consequent soil application gradualgradual and a consequent release of nutrients. release There was of nutrients. Therea slight was aincrease in the slight increase content of N, P and in the content of N,K Pfrom anddK0fromto d 90d 0intothe S pile, d 90 in thewhich S pile,can be explained which by a concen- can be explained by a tration process process concentration due to the dueOM decrease. to the OM decrease.In the FB pile,FBthis In the slight pile, increase this slight was only increase was ob- only served in the N content. The initial ammonia N content was low in observed in the N content. The initial ammonia N content was low in both piles, in agreement both piles, in agree- ment withwith their their low N low total total N content. content. AlthoughAlthough it wasin it was lower lower in FB FB than in than in S, its evolution S, its evolution over time wastime over similarwastosimilar that normally to thatobserved normallyinobserved a composting process because in a composting afterbecause process a slight increase after a during slight the firstduring increase days, itthe then decreased first till d 90 days, it then (Figure 4). decreased tillThis d 90decrease (Figure could 4). Thisbe decrease explained by utilization could by microorganisms, be explained by utilization by themicroorganisms, nitrification process the or the loss caused nitrification processby or turning [26]. the loss All these caused by values turning were [26].close All to or below these values thewere upper limittoproposed close or belowby the[16] to consider upper a compost limit proposed bybeing mature. [16] to consider a compost being mature. 700 600 Ammonia N, ppm 500 400 300 S 200 FB 100 0 0 12 19 26 33 47 62 75 90 Time, d Figure Figure4. 4. Evolution of of Evolution ammonia N content ammonia of samples N content taken of samples fromfrom taken the composted pilespiles the composted (sawdust = (sawdust S=and forest biomass = FB). S and forest biomass = FB). In order to accelerate the composting process, it would be necessary to increase the N content in the original material by adding a N source to reduce its initial C/N ratio. In this sense, the practice used by some farmers of drying the material resulting in the CBP and reusing it in the barn could help achieve this objective. However, effort should be made to achieve the appropriate conditions to ensure the sanitation of material and avoid animal health problems. Another alternative to achieve this sanitation, when lignocellulosic materials are used, could be to lengthen the process (>6 months). The progression of each chemical compound of these piles over time, expressed with the corresponding regression equation, is shown in Table 5. The relationship was statistically significant in both piles for OM, organic N, ammonia N and the C/N ratio, the slope being negative for OM, ammonia N and the C/N ratio, and positive for organic N. In those variables, the slope comparison showed that there were no statistical differences between treatments (data not shown). Moreover, in the sawdust pile, the relationship exists also for EC, moisture and total N, and in the forest biomass pile, for pH. Processes 2021, 9, 546 10 of 12 Table 5. Regression equations in each composting pile (sawdust = S; forest biomass = FB) between chemical variables and time (d) of composting period. S FB Item Equation R2 p-Value RMSE Equation R2 p-Value RMSE pH 9.19 − 0.009d 0.41 0.064 0.343 7.25 + 0.006d 0.53 0.026 0.178 EC, mS/cm 3.66 + 0.023d 0.95 0.001 0.176 2.03 + 0.001d 0.02 0.745 0.252 Moisture,% 59.9 − 0.146d 0.79 0.002 2.468 59.9 − 0.047d 0.25 0.174 2.681 OM, % DM 91.0 − 0.041d 0.90 0.001 0.499 85.3 − 0.027d 0.65 0.028 0.688 N, % DM Organic 0.88 + 0.003d 0.68 0.006 0.061 1.14 + 0.001d 0.48 0.039 0.037 0.06 − 0.001d 0.89 0.001 0.008 0.02 − 0.001d 0.75 0.002 0.004 Ammonia C/N 52.0 − 0.145d 0.84 0.004 2.286 37.5 − 0.050d 0.73 0.015 1.073 P, % DM 0.26 + 0.001d 0.63 0.107 0.019 0.28 − 0.001d 0.27 0.292 0.028 K, % DM 1.83 + 0.004d 0.71 0.073 0.107 1.76 + 0.002d 0.29 0.270 0.127 3.4. Agronomic Suitability of the Materials Both CBP materials contain high amounts of OM and a low nutrient content in comparison with cattle manure. Thus, they must be considered organic fertilizers to provide OM to the soil. The CBP review by [18] regards this material as a green waste compost, and stated that in the long term, the use of CBP as manure can result in considerably higher amounts of OM and a larger accumulation of N than cattle manure. The C/N ratio of both CBPs was high but within the wide range, from 10.5 to 49.3, reviewed by [18]. In addition, high ratios are expected when wood-derived materials are considered [11]. The incorporation in the soil of materials with a high C/N ratio can initially lead to a N inorganic immobilization because it is used by microbes, making it temporarily inaccessible to crops. This negative effect could happen if the material of the S pile was used due to its less stable OM, but not in the case of the FB pile material with a more stable OM. However, in accordance with the Spanish regulation RD506/2013 on fertilizing, both materials would be unmarketable because of a C/N ratio greater than 20. Thus, this final composting process did not bring about any meaningful change to CBP materials. Only if sanitation was achieved could this additional work be justified to add safety in cattle farms. 4. Conclusions From the agronomic point of view, although both materials had a high C/N ratio to be registered as soil fertilizers, they present potentially valuable characteristics when it comes to organically amending the soil, as they are high in organic matter content and low in nutrients. Further, the organic matter of the forest biomass pile was well stabilized (more than 50%). Therefore, as sawdust and forest biomass are very likely to improve soil properties when added to the land, further studies focusing on improving the composting management should be made to achieve effective composting, assuring an adequate C/N ratio and the sanitation of this material in the case of its reutilization as a compost-bedded pack. Author Contributions: Conceptualization, L.C. and A.F.; formal analysis, L.L. and C.G.; investigation, L.L., C.G., M.L. and T.B.; resources, M.L., L.C., A.F. and T.B.; data curation, L.L., C.G. and M.L.; writing—original draft preparation, L.L., C.G. and T.B.; writing—review and editing, L.L., L.C., A.F. and T.B.; visualization, L.L., A.F. and T.B.; supervision, M.L., L.C., A.F. and T.B.; project administration, L.C. and A.F.; funding acquisition, L.C. and A.F. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund, reference project AGL2015-68373-C2-1-R, Madrid, Spain. Processes 2021, 9, 546 11 of 12 Institutional Review Board Statement: Animal procedures were approved by the Institutional Animal Care and Use Committee (reference CEEAH 9963) of the Universitat Autònoma de Barcelona (Spain) in accordance with the European directive 2010/63/EU. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors would like to thank the UAB Farms and Experimental Fields Service for supporting the experimental work activities. Conflicts of Interest: The authors declare no conflict of interest. References 1. 68% of the World Population Projected to Live in Urban Areas by 2050. Available online: https://www.un.org/development/ desa/en/news/population/2018-revision-of-world-urbanization-prospects.html (accessed on 18 February 2021). 2. Boogaard, B.K.; Oosting, S.J.; Bock, B.B.; Wiskerke, J.S.C. 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