Nitrogen availability in biochar-based fertilizers depending on activation treatment and nitrogen source

Waste Management 158 (2023) 76–83 Available online 13 January 2023 0956-053X/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). Nitrogen availability in biochar-based fertilizers depending on activation treatment and nitrogen source Raúl Castej´ on-del Pino * , María L. Cayuela, María S´ anchez-García, Miguel A. S´ anchez-Monedero Department of Soil and Water Conservation and Organic Waste Management, CEBAS-CSIC, Campus Universitario de Espinardo, 30100 Murcia, Spain ARTICLE INFO Keywords: Agricultural waste Olive tree pruning N-doped biochar Available N Nitrogen use efficiency Oxidizing treatment ABSTRACT Different activation and N-doping treatments were used to produce biochar-based fertilizers (BBFs) with increased N concentration and slow N release. Pristine biochars were produced by pyrolysis of olive tree pruning feedstock at low and high temperatures (400 and 800 ◦C). These biochars were activated either by ultrasonication, or oxidation with hydrogen peroxide (H 2 O 2 ) or nitric acid (HNO 3 ) to increase their N retention potential. Subsequently biochars were enriched with N with either urea or ammonium sulfate. The activation of low-temperature biochars with HNO 3 was the most effective treatment leading to new surface carboxylic groups that facilitated the later enrichment with N. When treated with urea, BBFs reached 7.0 N%, whereas the H 2 O 2 activation only allowed an increase up to 2.0 N%. The use of urea as the external N source was the most efficient for incorporating N. Urea treated biochars had a water-soluble fraction that represented up to 14.5 % of the total N. The hydrolyzable N fraction, composed by amides and simple N heterocycles originated by the N-doping treatments, and nitro groups generated from HNO 3 activation, represented up to 60 % of the total N. This study relates the N chemical forms in the new BBFs to potential N availability in soil. The presence of water-soluble, hydrolyzable and non-hydrolyzable N implied that these BBFs may supply N that would be progressively available for plants, acting as slow-release fertilizers. 1. Introduction Biochar-based fertilizers (BBFs) are a new type of engineered fertilizers, where biochar is formulated with mineral fertilizers or beneficial microorganisms, for precise plant nutrition and the enhancement of soil properties derived from biochar application (Calabi-Floody et al., 2018; Rombel et al., 2022). These new BBFs have been proposed as a strategy to increase nitrogen use efficiency (NUE), to improve soil properties, and to sequester C in soil simultaneously, in comparison to conventional mineral fertilizers (Joseph et al., 2021; Melo et al., 2022; Zhou et al., 2021). The co-application of biochar and synthetic fertilizers in soil has been the traditional approach utilized to exploit the benefits of their interaction. Biochar applied in combination with mineral N fertilizers has the potential to increase NUE by reducing N 2 O emissions and NO 3 – leaching, improving N uptake by plants (Marci´ nczyk and Oleszczuk, 2022), and increasing crop yield, as compared to the application of synthetic fertilizer alone (Bai et al., 2022; Melo et al., 2022). One of the most recognized mechanisms behind this interaction is the adsorption of NH 4 + on the biochar surface through cation exchange capacity sites, which regulates the release of N and prevent N losses (Cai et al., 2016). BBFs represent a new approach to optimize biochar-fertilizer interactions. Different methods have been developed for their production. The most common strategy is a physical mixture of biochar, synthetic fertilizer and an adhesive or a coating material (Sim et al., 2021). Recently, a new promising approach is the production of engineered biochars, where N is incorporated into the structure of the BBFs, either by preor post-pyrolysis processes (Ndoung et al., 2021). A variety of methods can be used as pre-pyrolysis treatments: direct pyrolysis of N-rich feedstock (Piash et al., 2021), using NH 3 as the pyrolysis carrier gas (Yu et al., 2018), or preloading a feedstock with an exogenous source of N such as ammonium nitrate (Zhu et al., 2018) or urea (Wang et al., 2019a). These N-doped biochars are typically used for the removal of contaminants in water or soil (Shakoor et al., 2021; Yu et al., 2022), but with limited agricultural applications. N-enriched biochars produced by this strategy usually contain most of the N in a stable and recalcitrant form that is hardly available for plants. Rasse et al. (2022) also identified the insufficient sorption capacity of biochar * Corresponding author. E-mail address: [email protected] (R. Castej´ on-del Pino). Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman https://doi.org/10.1016/j.wasman.2023.01.007 Received 13 October 2022; Received in revised form 15 December 2022; Accepted 6 January 2023 Waste Management 158 (2023) 76–83 77 as a limitation for the production of BBFs with enough N concentration and the capacity to perform as slow release fertilizer. Post-pyrolysis treatments to produce N-rich biochars are heterogeneous, and produce different end materials with slow-release performance (Marci´ nczyk and Oleszczuk, 2022). In order to optimize N incorporation, a previous activation step is recommended (Rasse et al., 2022). The activation treatments can be separated between physical and chemical (Jellali et al., 2022): i) physical modification is related to the increase in the specific surface area and the total pore volume by ballmilling, ultrasonication, and CO 2 or steam activation, ii) chemical modification by acidic, salt and alkaline solutions, or a mixture of chemicals, to increase the number of functional groups. Oxygenated functional groups facilitate surface chemical interactions, notably increasing the active binding sites (Yang et al., 2019). Either acid, alkaline or oxidizing modifications enhance the formation of O-functional groups such as carboxylic, lactonic, phenolic, and carbonyl groups, which can interact with N (Wang and Wang, 2019). The complex mechanisms behind the retention of N into biochar represent an opportunity to produce controlled N-release fertilizers with different N bioavailability (Bakshi et al., 2021). In this work, we propose the production of a new type of engineered fertilizers by the chemical incorporation of mineral N fertilizers on the activated surface of biochar and we relate the chemical characterization of the incorporated N forms in biochar with their bioavailability. To achieve this aim, we produced BBFs through the post-pyrolysis treatments of biochars, which were pyrolyzed at two highest treatment temperatures (HTT) of 400 and 800 ◦C. The surface of these biochars was activated to maximize their N retention capacity either by sonication or by oxidation with hydrogen peroxide or nitric acid. Urea and ammonium sulfate were used as N sources to enrich the biochars. We postulated that (i) the physicochemical activation influences the N enrichment of the BBFs and (ii) the N chemical forms incorporated with the added N sources may define the N bioavailability in BBFs. 2. Materials and methods 2.1. Biochar production Olive tree pruning was selected as feedstock for the production of biochar. Pruning consisted of tree branches with a diameter of 5–15 mm, which were separated from leaves. Feedstock was collected in an olive orchard located in the South-East of Spain. Prior to biomass pyrolysis, the feedstock was air-dried to a moisture content below 10 % and milled to a particle size of <6 mm. Dried and milled feedstock was slowpyrolyzed (a temperature ramp of 5 ◦C min −1 and 2 h of residence time) in a rotatory tube furnace (RSR-B 80/500/11, Nabertherm, Germany) at two HTT of 400 and 800 ◦C (B-400 and B-800, respectively) in an oxygen-free environment, maintained with an Ar flux. Further information on the pyrolysis conditions and the physicochemical properties of feedstock and biochars was provided in a previous study by S´ anchez-García et al. (2019). Pristine biochars were washed twice by stirring for 10 min with deionized water (1:10 w/w), dried at 105 ◦C, milled and sieved (<1 mm). 2.2. Synthesis of the BBFs Three activation treatments were applied to biochars prepared at both temperatures: ultrasonication (treatment 1) and oxidation either with hydrogen peroxide (treatment 2) or nitric acid (treatment 3). Treatment 1: Ultrasonic treatment at room temperature for 0.5 h using a 1:10 mass ratio of biochar:deionized water in an ultrasonic device set at 80 W and 50 Hz (LT −80 PRO, TQTECH., Spain). Treatment 2: 5 % H 2 O 2 treatment following a methodology modified from Mia et al. (2017). This treatment was performed at 80 ◦C for 1.5 h, and shaken at 80 rpm (1:25 w/v) in a water bath (WNB 14, Memmbert, Germany). Treatment 3: 40 % HNO 3 treatment following a methodology modified from Güzel et al. (2017). This treatment was performed at 80 ◦C for 1 h and shaken at 50 rpm (1:25 w/v) in a water bath (WNB 14, Memmbert, Germany). After the activation treatments, samples were washed with deionized water three times and dried at 105 ◦C until a constant weight was obtained. Eventually, the activated biochars were supplemented with N either as urea (U) or ammonium sulfate (AS). Biochars were submerged (1:10 w/v) in a 2 M urea or ammonium sulfate solution at 80 ◦C for 1.5 h in a thermoreactor (ECO 16, Velp Scientifica, Italy). After the N-doping treatments, samples were washed with deionized water three times and dried at 80 ◦C until a constant weight was obtained. The complete description of the different biochar-based fertilizers and the identification codes are summarized in the Supplementary Material in Table S1. 2.3. Characterization of BBFs An automatic elemental analyzer (CHNS-932, LECO, USA) was used to measure the elemental content of carbon (C), hydrogen (H) and nitrogen (N) in the samples. The sulfur concentration was below the limit of detection (0.2 %). Before analysis, the samples were ball-milled to <0.5 mm. Ash content was analyzed according to ASTM D1762-84 (2021) following the next equation: Ash (%) =(calcined material (g) / dried material (g)) ×100. Oxygen (O) concentration was determined by subtraction (O (%) =100 – C (%) – H (%) – N (%) – ash (%)). Electrical conductivity (EC) and pH were determined in a 1:20 (w/v) watersoluble extract after the sample was shaken for 2 h, centrifuged for 20 min at 4000 rpm, and paper-filtered. An X-ray Photoelectron Spectrometer (XPS) analysis was performed to obtain the surface functional group distribution and atomic elemental composition of the BBFs, using an XPS system (K-Alpha, Thermo Scientific, UK) with a monochromatic Al K α radiation source. The quantitative analysis of the XPS results was performed by calculating the integral of each peak, where baselines were adjusted following the Shirley method. The peaks were detected in Lorentzian-Gaussian type curves. The XPS data were processed with the available software XPSPEAK 4.1. Before XPS analysis, samples were ball-milled <0.5 mm. Fourier Transform Infrared (FTIR) spectroscopy was performed with a Perkin-Elmer 1430 spectrophotometer to evaluate the surface functional groups. The KBr pellet method was used for this. Briefly, 1–2 mg of the sample were mixed with 200 mg of dried KBr and compressed to form pellets. 10 scans were recorded per sample. Before the IR analysis, the samples were ball-milled <0.5 mm. Three N fractions were defined depending on its bioavailability, as referred to the grade of N availability to plants: water-soluble N (shortly available N for plants), hydrolyzable N (gradually available N) and nonhydrolyzable N (hardly available N). Water-soluble N was determined in a deionized water extraction, which was shaken for 2 h (1:20 w/v), centrifuged for 15 min at 4000 rpm, filtered (<0.45 μ m), and measured by an aqueous elemental analyzer (multi N/C 3100, Analytik Jena, Germany). Hydrolyzable and non-hydrolyzable N fractions were analyzed following an acid digestion according to Wang et al. (2012). This procedure allows us to separate the bio-reactive N pool from the chemically stable N of the BBFs. Briefly, a 0.5 g sample of biochar was treated with 25 mL of 6 M HCl and 0.1 % phenol plus 2 drops of octyl alcohol in a 100 mL Pyrex cation digestion tube. The 1:50 mixture was blended with a vortex mixer, covered with a reflux funnel, and placed on an Al digestion block (Bloc Digest 12, J.P. Selecta, Spain) for 24 h at 105 ◦C. Non-hydrolyzable N was determined by elemental analysis in the biochar after the acid-digestion, whereas hydrolyzable N was estimated as follows: Hydrolyzable N =Total N – non-hydrolyzable N – water-soluble N. Additionally, mineral N-fractions were determined as follows: NO 3 – and NO 2 – were analyzed by ion chromatography (ICS 2100, Dionex, USA) in the same water extract used for water-soluble N. NH 4 + was extracted R. Castej´ on-del Pino et al. Waste Management 158 (2023) 76–83 78 by shaking the samples for 2 h (1:20 w/v) with 2 M KCl, centrifuging for 15 min at 4000 rpm, and filtering (<0.45 μ m). NH 4 + was measured by a colorimetric method based on Bethelot’s reaction (Sommer et al., 1992). Two replicates were performed in each analysis to calculate the means and the standard deviations. 3. Results 3.1. Elemental and physicochemical characterization of BBFs Biochar-based fertilizers were prepared from olive tree pruning biochars (B-400 and B-800), which were characterized by a rich organic composition, and low N and ash concentrations (Table 1). Nitric acid oxidation was the most effective treatment for incorporating N into the biochar structure. The oxidation with HNO 3 , without any additional source of N, increased N concentration from 0.68 to 4.31 % in B3-400 and from 0.81 to 1.97 % in B3-800. The addition of either urea or ammonium sulfate further increased N concentrations up to 7.02 % and 5.39 % in B3U-400 and B3AS-400, respectively. The addition of extra N was less effective in biochars prepared at 800 ◦C, where the N concentration slightly increased up to 2.79 % and 1.62 % in B3U-800 and B3AS800, respectively. At both HTT, the use of urea as an added N source was more effective than ammonium sulfate in incorporating N into the BBF structure. The activation with H 2 O 2 slightly facilitated the incorporation of the added N in biochars produced at 400 ◦C (B2U-400 and B2AS-400), in comparison to the non-activated biochar (BU-400). The incorporation of N in H 2 O 2 -treated biochar was even less effective in biochars prepared at 800 ◦C. The sonication treatment did not show any significant effect neither in 400 nor in 800 ◦C biochars. The Van Krevelen diagram can group biochars according to their degree of aromaticity and the content of O-functional groups (Figure S1). For all modified biochars, molar H/C ratios were under the 0.7 limit established by the IBI Biochar Standard (2015). BBFs were grouped according to pyrolysis HTT. The aromaticity of the biochars was mostly affected by the pyrolysis temperature, regardless of the activation treatment, except for the HNO 3 treatment, which caused a significant decrease in the H concentrations in the biochars. BBFs made at 800 ◦C were grouped between 0.1 and 0.2H/C ratios, whereas those at 400 ◦C were found between 0.5 and 0.6. Similarly, molar O/C ratios were also grouped according to pyrolysis temperature in the 0.05–0.20 range in biochars made from B-800, and 0.20–0.45 in biochars derived from B400. Surface activating treatments also had a strong impact on the O/C molar ratios of the BBFs, which was more evident in biochars made at 400 ◦C. Furthermore, the biochars activated with HNO 3 showed the highest molar O/C ratio at both HTT, caused by the increase in O and the decrease in C concentrations. The source of N did not affect the molar ratios. Pristine biochars produced at 400 ◦C and 800 ◦C had a basic pH of 8.32 and 10.69, respectively (Table 1). Both oxidizing treatments, HNO 3 and H 2 O 2 , decreased the pH values, especially in biochars modified with HNO 3 , reaching 3.40 for B3-400 and 4.53 for B3-800. Generally, ureatreated biochars showed higher pH values (between 5.46 and 8.99) than ammonium sulfate-enriched biochars (between 4.65 and 7.0). Although most BBFs had a circumneutral pH, B3U-400 and B3AS-400 were moderately acidic, with a pH of 5.46 and 4.65, respectively. The electrical conductivity (EC) of pristine biochars was low for both B-400 and B-800, with a similar value of 0.73 and 0.59 dS/cm, respectively (Table S2). The biochar’s EC increased with the N content, especially when ammonium sulfate was used. 3.2. Surface functionalization of BBFs The surface functionalization of the BBFs was assessed by both XPS and FTIR spectroscopy. The FTIR spectra of pristine biochar (B-400) were dominated by the presence of two main bands at 3430 and 1600 cm −1 (Fig. 1), corresponding to the stretching vibrations of O – H and aromatic C – – C/C – C bonds, respectively (Johnston, 2017). In the case of B-800, these bands were less intense, especially the one at 1600 cm −1 (Figure S2). The oxidation treatments enhanced the O – H band, especially HNO 3 treatments. Furthermore, the FTIR spectra of the HNO 3 - treated biochars (B3-400, B3U-400 and B3AS-400) were characterized by three peaks at 1720, 1532 and 1385 cm −1 , corresponding to C – – O stretching vibration from carboxylic acid groups (Li et al., 2014), asymmetric – NO 2 and symmetric – NO 2 , respectively (Fan et al., 2018; Qian and Chen, 2014) (Fig. 1). In contrast, the sonication treatments did not change the spectra as compared to the non-activated biochars. The differences observed in the FTIR spectra of the BBFs were driven by the oxidation treatments rather than the added N source, since ammonium sulfate and urea-enriched biochars had similar FTIR spectra. Table 1 Elemental content (C, H, O and N in %), ash (%) and pH of the pristine biochars produced at 400 and 800 ◦C, the HNO 3 -activated biochars without subsequent N addition (treatment 3) and the sixteen final materials doped with urea (U) or ammonium sulfate (AS) after ultrasonic (1), H 2 O 2 (2) or HNO 3 (3) treatments. %C %H %O %N %Ash pH Pristine biochars B-400 73.2 ±0.7 3.86 ±0.63 17.6 ±1.4 0.68 ±0.07 4.6 ±0.0 8.32 ±0.06 B-800 82.0 ±0.4 1.04 ±0.11 12.0 ±0.1 0.81 ±0.03 4.1 ±0.2 10.7 ±0.12 Only treated with HNO 3 B3-400 57.9 ±0.5 2.51 ±0.27 31.3 ±0.94 4.31 ±0.03 3.9 ±0.1 3.40 ±0.07 B3-800 76.2 ±0.7 0.88 ±0.14 18.0 ±1.16 1.97 ±0.07 3.0 ±0.5 4.53 ±0.17 Urea BU-400 69.0 ±0.7 3.16 ±0.07 20.3 ±0.6 1.51 ±0.00 6.0 ±0.0 8.05 ±0.01 B1U-400 69.5 ±0.0 3.54 ±0.07 18.3 ±0.6 1.61 ±0.04 7.0 ±0.7 7.82 ±0.08 B2U-400 68.2 ±2.0 3.51 ±0.07 23.9 ±2.3 2.04 ±0.02 2.3 ±0.2 6.98 ±0.13 B3U-400 56.8 ±1.0 2.68 ±0.19 32.4 ±0.5 7.02 ±0.02 1.1 ±0.3 5.46 ±0.06 BU-800 84.7 ±0.7 0.93 ±0.08 9.3 ±0.4 1.53 ±0.04 3.6 ±0.1 8.62 ±0.18 B1U-800 83.1 ±0.1 1.28 ±0.30 10.5 ±1.1 1.73 ±0.67 3.3 ±0.2 8.99 ±0.11 B2U-800 80.4 ±0.8 0.88 ±0.01 14.1 ±2.1 1.33 ±0.01 3.3 ±1.2 8.29 ±0.05 B3U-800 76.9 ±0.8 1.09 ±0.07 17.3 ±0.2 2.79 ±0.02 1.9 ±0.7 7.18 ±0.01 Ammonium sulfate BAS-400 68.4 ±1.6 3.07 ±0.07 25.7 ±1.4 0.88 ±0.07 2.0 ±0.3 6.90 ±0.15 B1AS-400 66.5 ±1.1 3.24 ±0.07 28.1 ±1.0 0.98 ±0.02 1.2 ±0.0 6.64 ±0.02 B2AS-400 70.5 ±0.6 2.97 ±0.03 23.9 ±0.9 1.03 ±0.02 1.5 ±0.3 6.04 ±0.18 B3AS-400 57.5 ±0.5 2.67 ±0.21 34.0 ±0.5 5.39 ±0.02 0.2 ±0.3 4.56 ±0.01 BAS-800 86.3 ±0.2 0.92 ±0.09 7.7 ±0.0 0.90 ±0.06 4.2 ±0.3 7.00 ±0.07 B1AS-800 84.1 ±1.4 0.89 ±0.09 9.3 ±0.6 0.97 ±0.03 4.6 ±0.7 7.01 ±0.04 B2AS-800 82.6 ±0.2 1.02 ±0.07 12.4 ±0.1 0.73 ±0.04 3.3 ±0.2 6.77 ±0.02 B3AS-800 75.4 ±0.0 1.25 ±0.03 19.4 ±0.1 1.62 ±0.02 2.4 ±0.2 6.16 ±0.08 R. Castej´ on-del Pino et al. Waste Management 158 (2023) 76–83 79 The only difference was the peak at 1590–1600 cm −1 , which showed a rise after the urea treatments, due to the formation of amide or amine groups (Bamdad et al., 2021; Shi et al., 2020). Only minor differences were found in BBFs produced with B-800 due to their high recalcitrant structure (Figure S2). The only difference was observed in the HNO 3 - treated biochars, which exhibited a peak at 1385 cm −1 , corresponding to N – O stretching vibration from – NO 2 groups. XPS was used to describe the surface chemical composition of C, O and N content by studying the binding energies of C1s, O1s and N1s photoelectrons of the carbonaceous materials (Table S3). C1s were Fig. 1. a) Region between 4000 and 750 cm −1 of infrared spectra of pristine biochar made at 400 ◦C (B-400), biochar only treated with HNO 3 (B3-400) and nontreated biochar or activated with treatments 1, 2 and 3 (ultrasonic, H 2 O 2 and HNO 3 , respectively) with final N-enrichment with urea (BU-400, B1U-400, B2U-400 and B3U-400). b) FTIR spectra of B-400, B3-400 and BBFs made at 400 ◦C treated with ammonium sulfate. Fig. 2. N1s XPS over the binding energy range 410–395 eV of pristine biochar made at 400 ◦C (B-400), B-400 with urea treatment (BU-400), B-400 with HNO 3 treatment (B3-400) and B-400 with HNO 3 and urea treatments (B3U-400). R. Castej´ on-del Pino et al. Waste Management 158 (2023) 76–83 80 decomposed into three peaks at 284.5 eV (aromatic and aliphatic C – C/ C – H bonds), 286.1–286.3 eV (C – O) and 288.2–288.5 eV (C – – O) (Zhang et al., 2021). The total C1s concentration did not change significantly between treatments. In all cases, the XPS results were dominated by the C – – C/C – C signal (aromatic and aliphatic C), which represented up to 80 % of the total C concentration. However, after the HNO 3 and H 2 O 2 treatments, the C – – C/C – C relative percentage decreased due to the oxidizing effect, while the peaks corresponding to functionalized C, such as C – O and C – – O, increased. This effect was clearly observed in HNO 3 -treated biochars, B3-400 and B3-800. The functionalization of surface was also observed in the O1s spectra, which presented three peaks at 531.1–531.3 eV (C – – O bonds), 532.5–532.7 eV (O – C – – O) and 533.7 eV (C – O) (Zhang et al., 2020) (Table S3). In general, the relative percentage of O functional groups only showed minor differences between the samples. Nitric acid treatments (B3-400 and B3-800) produced an increase in O – C – – O and C – O stretching vibration in comparison to pristine biochars. BBFs prepared with urea, presented a higher percentage of C – – O, which increased gradually with the abundance of N in the samples, as compared with those prepared with ammonium sulfate. In the N1s spectra (Fig. 2), three peaks were detected in all the samples at 398.7 (pyridinic N bonds), 399.9–400.1 (amides, amines and pyrrolic N) and 401.2–401.5 eV (graphitic N) (Leng et al., 2020; Wang et al., 2019b). In the case of HNO 3 -treated biochars, an additional peak associated to the bonding stretching of –NO 2 was observed at 405.5 eV in BBFs made with biochar at 400 ◦C (Fig. 2, Table S3), and at 406.5 eV in samples made with biochar at 800 ◦C. Fig. 2 also shows that amide and pyrrolic N increased after both HNO 3 and urea treatments. The pyridinic N peak slightly increased after HNO 3 treatment, but increased more strongly after the urea treatment. The graphitic N peak hardly changed by the treatments, and lastly, the presence of ammonium was not proven (Fig. 2). 3.3. N availability in BBFs N in the BBFs was separated into three different fractions according to its availability: water-soluble N, hydrolyzable N, and nonhydrolyzable N (Fig. 3). N in pristine biochars was mostly nonhydrolyzable. Modified biochars not only presented a high percentage of N, but also exhibited an enhanced proportion of bioavailable N, especially those prepared with HNO 3 and/or urea. Ultrasonic-treated biochars showed a high proportion of non-hydrolyzable N, similar to pristine biochars. The oxidation with H 2 O 2 increased the concentration of hydrolyzable N in B2U-400, as compared to BU-400 and B1U-400. BBFs with the HNO 3 treatment showed the highest content of potentially bioavailable N (water-soluble and hydrolyzable N), especially those produced from biochar made at 400 ◦C, which reached 39.3 and 24.4 N g kg −1 of potentially bioavailable N in B3U-400 and B3AS-400, respectively. The added N source highly influenced hydrolyzable N, as urea was more efficient for incorporating N than ammonium sulfate, and especially for incorporating hydrolyzable N forms. N in ammonium sulfate samples was basically non-hydrolyzable, except with nitric acid activation and subsequent ammonium sulfate treatment. In urea-treated biochars, the concentration of water-soluble N was higher than the sum of the inorganic soluble N. For instance, the water-soluble N of B3U400 was 10.19 N g kg −1 , whereas the sum of NO 3 – -N and NH 4 + -N only reached 1.49 N g kg −1 (Table S2), meaning that other soluble N sources apart from the mineral forms, dominated the water-soluble fraction. Pyrolysis HTT affected N availability of the BBFs, as those made with B-800 showed not only lower total N concentrations, but also lower water-soluble and hydrolyzable N than those produced with B-400. Only BBFs treated with nitric acid showed a relevant bioavailable N fraction in the BBFs produced with B-800, with the ammonium sulfate treatment being unsuccessful in increasing N concentration in biochar produced at high pyrolysis temperatures. 4. Discussion 4.1. Influence of activation treatments on the N enrichment of BBFs In agreement with our first hypothesis, the activation of biochar before the addition of an external source of N was a suitable strategy for enhancing N retention. Whereas the addition of N source to nonactivated biochars achieved N concentrations of up to 1.5 %, the combination of the oxidation with HNO 3 and the addition of N sources produced BBFs with up to 7.0 % N. Activation with H 2 O 2 , and especially HNO 3 , are known to increase biochar surface functional acid groups. Li and Li (2019) described that HNO 3 treatment produced carboxyl, carbonyl and hydroxyl groups, as a consequence of the oxidation of aromatic and aliphatic C on the biochar surface. In our case, an enhancement of – COOH and – OH groups were observed in the FTIR spectra of biochar oxidized with either HNO 3 (B3-400) or H 2 O 2 (B2-400) (Figure S3). In addition, the XPS analysis confirmed an increase in these functional groups in B3-400. The degree of oxidation was unequal for both oxidizing treatments, as HNO 3 oxidation was stronger than H 2 O 2 , Fig. 3. Water-soluble N, hydrolyzable N and non-hydrolyzable N of pristine biochars, biochars treated with HNO 3 , and the sixteen BBFs, expressed in N g kg −1 . U and AS represent urea and ammonium sulfate treatments, respectively. 1, 2, and 3 correspond to ultrasonic, H 2 O 2 and HNO 3 activations, respectively. 400 ◦C and 800 ◦C are the pyrolysis HTT of the biochars. R. Castej´ on-del Pino et al. Waste Management 158 (2023) 76–83 81 as reflected by the increase in the intensity of the – COOH peak observed in the FTIR spectra of B3-400 compared with B2-400 (Figure S3). The increase in surface acidic functional groups in the oxidized biochars improved its reactivity, favoring chemical and electrostatic interactions with the added N sources to produce the enriched BBFs, in accordance with Wang et al. (2020). Figure S4 was included in Supplementary Material summarizing the main chemical reactions produced by the activation treatments and the added N source. The addition of urea led to the formation of amides, which represents one of the largest N fractions incorporated into the BBFs’ surface, as reflected in the enhancement in the amide/pyrrolic peak observed in the XPS analysis (Fig. 2), and in the FTIR peak at 1590–1600 cm −1 (Fig. 1). The interaction between the carboxylic acid groups of the biochar with amine-N compounds to form amides has already been described for ethylenediamine (Zahedifar et al., 2021) and urea (Chhatwal et al., 2020). Amine-N may interact either by a nucleophilic reaction with a carbonyl group, or by a condensation reaction with carboxyls, forming amides in both cases (Li and Li, 2019). Another fact that demonstrates the formation of amides after urea treatment was the perceptible carboxyl peak in B2-400 and B2AS-400, and its inexistence in B2U-400 (Fig. 1). This information suggested the conversion of carboxyl to an amide peak in the urea samples and not in the ammonium sulfate ones. The urea treatment also increased pyridinic N concentration as observed in the XPS analysis (Fig. 2). O-containing functional groups can react with amines through Maillard reactions, producing pyridinic and other heterocyclic N groups (Chen et al., 2018). The formation of pyridinic N after the urea treatment was previously reported to occur at high temperatures during pyrolysis (Hulicova-Jurcakova et al., 2009), with the highest pyridinic N formation observed at 600 ◦C (Wang et al., 2019b). In our study, we obtained a large proportion of pyridinic N, in the range between 21.4 and 47.9 %, in BBFs treated with urea, even at relatively low temperatures (80 ◦C). BBFs obtained with ammonium sulfate as the external N source showed similar FTIR and XPS peaks than those treated with urea, but the amount of the N forms differed as compared with urea-treated BBFs. The XPS analysis of biochars treated with ammonium sulfate showed lower amide/pyrrolic peaks than urea-treated biochars, suggesting a minor formation of amides in ammonium sulfate BBFs. In the case of B3U-400, which had the highest concentration of carboxylic acid groups, it showed a higher amide/pyrrolic peak than B3AS-400, confirming that it did not form a significant amount of amide groups (Table S3). Biochar has traditionally been considered to adsorb NH 4 + mainly through H-bonding and electrostatic interactions (Cai et al., 2016), however, the XPS analysis demonstrated the absence of NH 4 + in both urea and ammonium sulfate treatments, evidencing that biochar cation exchange capacity was not an important mechanism for N retention in our case. KCl extracts further demonstrated the absence of significant concentrations of NH 4 + in cation exchange sites (Table S2), which contradicts the traditional view. Other less-studied mechanisms, involving the retention of NH 4 + with O-functional groups, may be relevant as suggested by Takaya et al. (2016) for hydrochar and low temperature biochars. Apart from the interaction of the added N with carboxylic groups, the incorporation of nitro groups in the modified biochars represented up to 50 % of the N functional groups in B3-400 and B3-800, as measured in the XPS analysis (Table S3). The nitration of the aromatic rings to form nitro groups represents another N retention mechanism derived from the oxidation with nitric acid, as reported by Sajjadi et al. (2018). In concentrated nitric acid solution, nitro-aromatic compounds are formed between NO 2 – and aromatic C rings by electrophilic aromatic substitution (Yang et al., 2019). The oxidation with HNO 3 also originated an increase in pyrrolic and pyridinic N, as observed in the N1s spectra (Fig. 2). The increase in heterocyclic N alongside the formation of nitro groups has been previously reported (Lu et al., 2021; Zheng et al., 2020). Li and Li (2019), in a biochar pre-treated with H 3 PO 4 , found graphitic N enrichment with increasing the concentration of HNO 3 treatment up to 25 %, but, in our case, even using higher HNO 3 concentration, we did not find any significant increase. BBFs treated with urea showed a remarkable amount of watersoluble N, with inorganic N being a minor fraction of the total watersoluble N (Fig. 3, Table S2). This result suggested the availability of organic N in urea form. Sajjadi et al. (2019) noted H-bonding between carboxyl and hydroxyl groups and urea. Aside from the chemical bonding of N, activated biochars may retain N by other physicochemical mechanisms. Otieno et al. (2021) found that the surface area and porous structure of biochar can contribute in retaining ammonium in solution, but in our case, the XPS and FTIR spectra rejected significant NH 4 + concentrations. In fact, neither NO 3 – nor NH 4 + were the main watersoluble N forms in urea-treated biochars, suggesting that a considerable amount of urea was trapped in the biochar pores. Liu et al. (2019) described how the specific surface area diminished after the urea application, suggesting that urea particles filled the micro and mesopores of the biochar. Nevertheless, HNO 3 -BBFs, which showed the highest water-soluble N contents, are characterized by decreasing both surface area and total pore volume (Chac´ on et al., 2020; Yang et al., 2019), and the transformation of the microand mesopores to macropores (Güzel et al., 2017; Vu et al., 2017). These macropores may have been responsible for the physical retention of urea, as previously reported by Liu et al. (2019). The ultrasonic treatment was less effective for incorporating N than the oxidation treatments. Sonication has been previously used to clean the biochar surface, increasing its porosity (Sajjadi et al., 2019). These authors used sonication as a pretreatment for the activation of biochar with urea as a strategy to increase its adsorption capacity. The final N concentration of biochars were in the 1.80 and 2.23 % range, which is in line with the N concentrations of the sonicated BBFs in our study (between 0.97 and 1.73 %). Pyrolysis HTT also influenced N enrichment, as lower temperatures of biochar production led to higher N retention. Condensed aromatic C is the most common structure found in biochars produced at a high pyrolysis temperature, in which aliphatic structures and functional groups (such as carbonyls, carboxyls, ketones and esters) are almost absent (Hassan et al., 2020). Due to the high chemical stability of high temperature biochars, oxidizing treatments were more effective in low pyrolysis-temperature biochars. Our results are in agreement with Singh et al. (2020), who found, in a study comparing urea sorption in biochars made at different temperatures, the highest values for biochar made at 450 ◦C, associated to the optimum equilibrium between functionality, porosity and cation exchange capacity. Nevertheless, the application of activation treatments is essential for biochar produced at a high temperature, where the combination of HNO 3 and urea treatments showed a remarkable hydrolyzable N concentration despite the highly stable C structure. 4.2. N Bioavailability in BBFs N-doping of biochar through surface functionalization with N compounds has been mostly used as a strategy to remove pollutants from the environment (Yang et al., 2019). The adsorption properties of N-doped biochars rely on the presence of amines, pyrrolic and pyridinic N, that contain free electrons that can interact with organic and inorganic pollutants (Wang and Wang, 2019). In the case of BBFs, the added N needs to be plant-available to behave as a slow or controlled N release fertilizer. For this reason, a different approach is needed to reduce the recalcitrance of the added N and increase its bioavailability. In this study, we hypothesized that the added N source may influence the availability of the N in BBFs. The urea treatments showed a high amount of hydrolyzable N, while ammonium sulfate BBFs only reached a relevant N concentration when HNO 3 was used as the activation treatment. Approximately two thirds of the N in urea BBFs were potentially bioavailable in the soil. In the case of the most efficient BBFs (B3U-400), about 15 % of the total N was soluble in R. Castej´ on-del Pino et al. Waste Management 158 (2023) 76–83 82 water, mostly in organic forms, as the inorganic N available for plants represented 2.1 % of the N. This indicates that urea was either trapped in pores as found by Liu et al. (2019); or retained by hydrogen bonding interaction in accordance with Sajjadi et al. (2019) and Chen et al. (2020). Our results suggest that a small amount of NO 3 – -N and NH 4 + -N would be plant-available at first, and the hydrolysis of the trapped urea could start some days later in the short term (Sigurdarson et al., 2018). A large proportion of the added N could be slowly available for plants, since about 50 % of N in B3U-400 was hydrolyzable in acid, which is considered a fraction that could be mineralized over time in soil (Otto et al., 2013). Amides and nitro groups found on the biochar’s surface may be the most relevant functional groups included in the N hydrolyzable fractions of BBFs. Piash et al. (2021) described that in a soil incubation experiment with an amide-rich biochar, most of the amide content was released after the first 120 days, being available for plant use in a reasonable crop growing period. We found that acid digestion degraded most nitro groups, as confirmed by FTIR, where the peaks associated to nitro groups (1385 and 1532 cm −1 ) reduced their intensity after hydrolysis (Figure S5). Accordingly, Marvin-Sikkema and de Bont (1994) previously reported that microorganism degrade numerous nitro-aromatic compounds in soil. The aromatic N contained in biochar is generally considered unavailable (Jassal et al., 2015). However, our results suggest that a small fraction of the heterocyclic N of BBFs, especially low molecular weight pyrrols and pyridines, may also be included in the hydrolyzable N fraction. Even though aromatic N in biochar is generally considered to be mostly recalcitrant (Jassal et al., 2015), Wang et al. (2012) reported that low molecular weight heterocyclic N can decompose in soil. The non-hydrolyzable N fraction represented about one third of the total N in urea BBFs. This remaining N, composed of graphitic and the non-hydrolyzable fraction of pyridinic and pyrrolic N, can be considered to be recalcitrant. Aromatic N is mostly classified as a stable form, although some authors revealed that aging, weathering and microbial activity could mineralize it, being available for plant nutrition (Williams et al., 2019). The biochar structure protects aromatic heterocyclic N, and it only becomes available when the biochar structure starts to degrade in soil. Mineralization of heterocyclic N may be possible on longer periods; however, the C ring could be opened by hydroxylation, producing aliphatic N compounds available for microbial use (Torres-Rojas et al., 2020). 5. Conclusions This study provided several modified biochars obtained from olive tree pruning with a potential use as slow-release N fertilizers. The biochar-based fertilizers contain heterogeneous chemical N forms, which may regulate the N release. In general, oxidation treatments enhanced the incorporation of N from the added N source, as compared to sonicated and non-activated biochar. Biochars treated with nitric acid showed the highest concentration of O-functional groups, which were responsible for an important part of N retention. Aside from the incorporation of external sources of N, the oxidation with HNO 3 increased N content by itself, forming nitro groups and pyrrolic N. The combination of HNO 3 and urea led to the highest N enrichment in the biochar-based fertilizers (BBFs). About two thirds of the N of the BBFs treated with urea would be potentially available for plants with different release rates. The water-soluble N, mostly in the form of trapped urea that would be firstly available, represented a fraction of 14.5 %. Hydrolyzable N groups, as amides, nitro groups and low weight molecular N that would be slowly released in soils for plants nutrition, represented approximately 50 % of the total N. These findings provide new insights to design future field experiments to evaluate the slow-release performance, the efficient use of the N of these engineered BBFs and their economic feasibility. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request. Acknowledgments We gratefully acknowledge the financial support of the projects RTI2018-099417-BI00 and PID2021-128896OB-I00 from the Spanish Ministry of Science, Innovation and Universities, co-funded with EU FEDER funds. The authors are very grateful to Mr. Mario Fon for editing the final version of the manuscript. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi. org/10.1016/j.wasman.2023.01.007. 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