Converting leguminous green manure into biochar

Geoderma 232–234 (2014) 581–588

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Converting leguminous green manure into biochar: changes in chemical composition and C and N mineralization Chi-Peng Chen a, Chih-Hsin Cheng a,⁎, Yu-Hsuan Huang a, Chien-Ten Chen b, Chao-Ming Lai c, Oleg V. Menyailo d, Liang-Jen Fan e, Yaw-Win Yang e a

School of Forestry and Resource Conservation, National Taiwan University, Taipei 106, Taiwan Department of Agronomy, National Chung Hsing University, Taichung 500, Taiwan Department of Agricultural Chemistry, National Taiwan University, Taipei 106, Taiwan d Institute of Forest SB RAS, Krasnoyarsk 660036, Russia e National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan b c

a r t i c l e

i n f o

Article history: Received 25 March 2014 Received in revised form 19 June 2014 Accepted 20 June 2014 Available online xxxx Keywords: Biochar Green manure Carbon sequestration Nitrogen mineralization Nuclear magnetic resonance

a b s t r a c t Leguminous green manure is an important source of nitrogen (N) and carbon (C) in cropping systems. The fast turnover of leguminous green manure enables it to release N quickly, but limits its effectiveness in maintaining soil organic C content. Converting leguminous green manure into biochar facilitates its use as a soil amendment. In this study, we assessed how the conversion of leguminous green manure (Sesbania roxburghii) into biochar altered its chemical composition and subsequent C and N mineralization. Biomass was charred along a temperature gradient from 200 to 500 °C. Using nuclear magnetic resonance and near-edge X-ray adsorption fine structure spectroscopy, we found that both C and N became enriched in aromatic and heterocyclic aromatic structures in biochar, and this structural change led to a reduction in C and N mineralization rates. The mineralized C decreased from 32.7% of the added C of raw biomass to b0.5% of that of biochar at charring temperatures above 400 °C. N release shifted from N mineralization in raw biomass to N immobilization at charring temperatures at 500 °C. As such, soil amended with biochar produced at charring temperatures exceeding 400 °C demonstrated a 25% decrease in dry shoot biomass compared with unamended soil. The results indicated that the C stability of leguminous green manure can be achieved by converting raw material into biochar, but that the charring process may limit it to providing N. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Leguminous green manure is a standard management tool in cropping systems, because it provides nitrogen (N) to subsequent crops. While the practice of applying leguminous green manures is declining in the modern cropping system due to the availability of synthetic fertilizers and the cost and time required producing a green manure crop, they remain extensively used in low-input cropping systems in tropical and subtropical areas (Crews and Peoples, 2004). The increasing costs of synthetic fertilizers and the deterioration of soil properties, in the absence of legumes, have prompted many farmers to reconsider leguminous green manure (Crews and Peoples, 2004). Leguminous green manure provides plants with N and replenishes soil organic carbon (C). However, the fast C cycle of fresh leguminous green manure may limit its effectiveness in maintaining soil organic C content in intensive cropping systems; furthermore, leguminous green manure is not frequently used in current cropping systems ⁎ Corresponding author at: R207 Forestry Hall, School of Forestry and Resource Conservation, National Taiwan University, Taipei 106, Taiwan. E-mail address: [email protected] (C.-H. Cheng).

http://dx.doi.org/10.1016/j.geoderma.2014.06.021 0016-7061/© 2014 Elsevier B.V. All rights reserved.

(Wander et al., 1994). Converting leguminous green manure into biochar can be applied as an alternative for maintaining or increasing soil organic C stocks. This is because the converted biochar is a stable soil organic amendment that resists microbial decomposition (Lehmann et al., 2006; Sohi et al., 2010). Numerous recent studies have shown that biochar application can also enhance soil C stock, soil fertility, and crop yields (Chan et al., 2008; Kimetu et al., 2008; Major et al., 2010; Van Zwieten et al., 2010). The change in the biochar C structure, as a result of charring process, is understood (Baldock and Smernik, 2002; Hammes et al., 2006; Keiluweit et al., 2010), but relatively little information has been published on the relative change of N structure (Hilscher and Knicker, 2011; Knicker, 2010; Wang et al., 2012). Some studies (e.g. Hilscher and Knicker, 2011; Knicker, 2010) proposed that N structure of biomass is altered from a peptide-like N form to a heterocyclic N forms under charring process. However, research regarding how changes in the N structure affect N mineralization and N availability is even scanter (Wang et al., 2012). In addition, only few biomass materials have been detected but not including green manure material that with high N content. A close examination of how the conversion of raw green manure material into biochar alters its chemical structure and subsequent C

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and N mineralization is essential. From an agronomical viewpoint, this information could enable the effective use of both raw green manure material and converted biochar in a manner that is economically profitable and environmentally sustainable. In this study, a leguminous biomass (Sesbania roxburghii) was used as a feedstock material to examine how its conversion altered its chemical structure and subsequent C and N mineralization. The objectives were (1) to determine changes in the chemical structure of raw green manure material and biochar charred along a temperature gradient ranging from 200 to 500 °C; and (2) to evaluate the C and N mineralization of raw green manure material and biochar using laboratory incubation and pot experiments. 2. Materials and methods 2.1. Materials Leguminous plant biomass (S. roxburghii) was collected from the Taoyuan District Agricultural Research and Extension Station in Northern Taiwan. Leguminous green manure in Taiwan is commonly made from Sesbania, which is widespread in tropical and subtropical regions (Becker et al., 1995; Palm and Sanchez, 1991). Such leguminous green manure is typically incorporated into soil during the early or middle blooming stages, approximately 50–80 days after planting. In this study, Sesbania was harvested 50 days after planting during its early blooming stage, and had a mean height of 120 cm. The fresh plant biomass was clipped at the ground level, oven dried at 65 °C to a constant weight, and chopped into 3-cm lengths before charring. The dry plant biomass was charred at 200, 300, 400, and 500 °C. Approximately 30 g of dry plant biomass was packed into a 300-mL stainless steel container, loosely sealed with a lid, and charred inside a muffle furnace. The temperature reached the desired temperature at 30 min, and was held constant at that temperature for 2 h. More than 30 containers were conducted at each charring temperature. The average recovery rates of biochar by mass were 96 ± 2% (mean ± standard deviation), 60 ± 2%, 41 ± 1%, and 35 ± 1% at 200, 300, 400, and 500 °C, respectively. Biochar produced at each of the four experimental temperatures was pooled into a single sample, ground, and passed through a 1-mm sieve for further use and analysis. The biochar samples produced at 200, 300, 400, and 500 °C were denoted as BC200, BC300, BC400, and BC500, respectively. An unconverted raw comparison sample was created by grinding raw biomass and passing it through a 1-mm sieve.

and a synchrotron-based near-edge X-ray adsorption fine structure spectroscopy (NEXAFS) of C 1s K-edge and N 1s K-edge. The solidstate 13C NMR spectra were measured on a Bruker Avance III 400 NMR spectrometer operating at a 13C frequency of 100 MHz with standard ramp cross-polarization magic angle spinning (CPMAS) spectroscopy. The powder sample was packed in a 4-mm diameter zirconia rotor and spun at a speed of 10 kHz. A contact time of 3 ms and a pulse delay of 1 s were used to measure all spectra, and 10,000 scans were accumulated. The chemical shift (δ) regions assigned to the major types of C were 0–45 ppm for alky C, 45–110 ppm for O-alkyl C, 110–145 ppm for aryl C, 145–165 ppm for O-aryl C, and 165–190 ppm for carboxyl C (Baldock and Smernik, 2002). For NEXAFS spectra, both C 1s K-edge and N 1s K-edge NEXAFS spectra were obtained on Beamline 24A at the National Synchrotron Radiation Research Center, Hsinchu, Taiwan. The sample was scanned at a pressure of less than 1 × 10−8 Torr. The energy steps for C 1s K-edge NEXAFS were executed at every 0.5 eV between 275 and 282 eV, at every 0.1 eV between 282 and 295 eV, at every 0.2 eV between 295 and 322 eV, and at every 0.5 eV between 322 and 340 eV. The N 1s Kedge NEXAFS spectra were measured following the completion of the C 1s K-edge NEXAFS scan. The energy steps were executed at every 0.5 eV between 390 and 397 eV, at every 0.1 eV between 397 and 410 eV, at every 0.2 eV between 410 and 420 eV, and at every 0.5 eV between 420 and 440 eV. The photon energy scale was calibrated against an intense 1s-π transition of the HOPG graphite that occurs at 285.38 eV. The dwell time for each energy point was 1 s. The incident beam intensity was recorded with a gold mesh reference monitor (I0), and the NEXAFS signal of the sample was detected in total electron yield (ITEY). The NEXAFS spectra were obtained by forming the ratio ITEY/Io for the sample current mode. The adsorption band assignment for the C 1s K-edge NEXAFS spectra of raw material and biochar was based on Solomon et al. (2009), Heymann et al. (2011) and Cheng et al. (2014), where 284.9–285.5 eV was assigned for the C 1s-π*C_C transition of aromatic C, 287.3 eV for the C 1s-π*C\H transition of aliphatic C, and 288.0–288.5 eV for the C 1s-3p/σ*C_O transition of carboxylic C. The adsorption band assignment for the N 1s K-edge NEXAFS spectra was based on Leinweber et al. (2007) and Kiersch et al. (2012), where 398.6 eV was assigned for nonpeptide C_N in aliphatic imine and aromatic pyridine and pyrazines; 399.6 eV for aromatic purine, pyrazole and imidazole; and 401.3 eV for amidic N. The fine structures beyond 290 eV transitions in the C 1s K-edge spectra and 403 eV transitions in the N 1s K-edge spectra tend to be broad and overlap with each other; therefore, only the main C and N 1s-π* and C 1s-3p/σ* transitions were used for interpretation of the NEXAFS spectra.

2.2. Properties of raw material and biochar To determine the content of organic C (OC), N and H contents of raw material and biochar, the samples were further ground using a ball grinder (Oscillating Mill MM400 by Retsch, Newtown, PA, USA) and measured using an elemental analyzer (Perkin Elmer 2200, MA, USA) after removing the inorganic C with 0.1 M HCl. The ash content was determined using the loss on ignition method, in which the samples were put in a crucible and ignited at 550 °C for 2 h (Cheng et al., 2008; Hammes et al., 2006). The oxygen content was calculated by difference from OC, N, H, and ash content (Wang et al., 2012). The OC, N, H, O, and ash content were presented on a dry basis. The pH values of raw material and biochar were measured in a 1:20 raw material or biochar/water mixture after shaking for 1 h. The inorganic N content was measured by mixing 0.5 g of raw material or biochar with 40 mL of 0.001 M CaCl2 for 1 h. The ammonium and nitrate concentrations in the solution were determined using ion-selective electrodes (HI4101 and HI4113, Hanna Instruments, RI, USA). For each chemical analysis, measurements were replicated thrice and the values were the mean of the three measurements. The chemical structures of raw material and biochar were examined using a solid-state 13C nuclear magnetic resonance (NMR) spectrometer

2.3. C and N mineralization of raw material and biochar Laboratory incubation experiments were conducted to assess the short-term C and N mineralization of the raw material and biochar amended in a highly weathered cropland soil. The soil was a Typic Kandiudult taken from the Datu Tableland of Central Taiwan (Cheng et al., 2013). The field from which the soil sample was taken had a long history of cropping, and had been rotated with sweet potato (Ipomoea batatas), peanut (Arachis hypogaea), and sesame (Sesamum indicum) in recent years. The top 15 cm of soil was collected in October 2011 following the sweet potato harvest. The soil was brought to the laboratory, air dried, and sieved through a 2-mm sieve. A hydrometer method was used to measure the soil texture, and was composed of 41% clay, 34% silt, and 25% sand. An elemental analyzer (Perkin Elmer 2200, MA, USA) was used to analyze the soil C and N content, which were 7.5 mg C g−1 and 0.9 mg N g−1, respectively. The soil pH value in a 1:2.5 soil/water mixture was 6.1, and the potential cation exchange capacity, which was measured using 1 M NH4OAc at pH 7, was 3.7 cmol (+) kg−1. The soil 0.001 M CaCl2 extractable NH4-N and NO3-N in a 1:10 soil/solution mixture were 3.6 and 14.6 μg N g−1, respectively.


In the C mineralization experiments, 50-g samples of unamended soil (control) or soil amended with 0.5 g of raw material or biochar were placed in 500-mL glass jars (Cheng et al., 2008). The incubation experiments were replicated four times, and incubated at 60% maximum water holding capacity at 25 °C in the dark inside a growth chamber. The experiments were initiated immediately after wetting. The evolved CO2 was trapped in 10 mL 0.5 M NaOH, which was placed in a small vessel inside the incubation jars. The NaOH vessels were removed and replaced with a new vessel after 3 days, weekly for 1 month, and biweekly after 1 month. The trapped CO2 was precipitated by an excess of 1 M BaCl2 solution and back-titrated with standard 0.5 M HCl to quantify the evolved CO2. The total incubation time was 95 days. It was assumed that priming effects were not caused by soil amendment (Cross and Sohi, 2011). The mineralized C in raw material or biochar was calculated as the difference in the amount of cumulative CO2-C emission in the raw material or biochar amendments and the amount of cumulative CO2-C emission in the control treatment (unamended soil). Nitrogen mineralization was evaluated using a sequential leaching method, following the procedures developed by Gavi et al. (1997) and Wang et al. (2003). The glass column used in this study was 5.5 cm wide and 10 cm in height, and was conically tapered with a Teflon stopcock at the bottom. Samples of 150 g of unamended soil and soil amended with 1.5 g of raw material or biochar were thoroughly mixed and packed to a height of 7 cm in a glass column, with a soil bulk density of 1 g cm−3. The top of the soil column was covered with a glass fiber filter, and the bottom neck of the column was filled with glass wool to prevent particulate movement during the leaching process. The soils were incubated at a 60% maximum water holding capacity at 25 °C in the dark inside a growth chamber. The N mineralization experiments were repeated three times on the unamended soil, soil amended with raw material, soil amended with BC300, and soil amended with BC500. Each column was leached with 100 mL of 0.001 M CaCl2 solution at 7, 14, 21, 28, and 35 days after the beginning of the incubation (Laird et al., 2010). The leachate from each column was analyzed for NH4-N and NO3-N. Ammonium was analyzed using the indophenol blue method (Dorich and Nelson, 1983) and nitrate was analyzed using the method proposed by Cataldo et al. (1975). The leached inorganic N in each column was determined as the combination of NH4-N and NO3-N, and calculated based on the mass balance of N between leached inorganic N and the residual inorganic N that was not leached before incubation (week 0). It was also assumed that soil amendment did not cause priming effects. In the raw material and biochar, the net mineralized N was calculated as the difference between the leached inorganic N and that in the control treatment (unamended soil). A positive value indicated a net N mineralization and a negative value indicated a net N immobilization (Burgos et al., 2006; Wang et al., 2003). 2.4. Pot experiments A greenhouse experiment was conducted to assess the effects of soil amendment on plant yield and N uptake. The pot experiments were conducted in a greenhouse at the Experimental Farm at National Taiwan University, Taipei, Taiwan. Water spinach (Ipomoea aquatica) was planted in pots 14 cm in diameter and 10 cm deep. Each pot was filled with 1 kg of unamended soil (the same soil as that used in the incubation experiment), soil amended with 10 g of raw material, or soil amended with 10 g of biochar. Treatments with and without added fertilizer were created. For the fertilization treatments, NPK fertilizer (NH4NO3–Ca(H2PO4)2–K2SO4) in solution was added before sowing and at 20 days after germination at rates based on surface areas equivalent to NPK at 30, 30, and 30 kg ha− 1 for each application. The pot experiments were replicated four times using a completely randomized design. Five seeds were sown in each pot, and subsequently thinned to 2 seedlings one week after emergence. Shoots were clipped after 42 days

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and dried at 65 °C until a constant weight was reached. The dry shoot biomass was weighed and then ground before an elemental analyzer (Perkin Elmer 2200, MA, USA) was used to analyze the N content. The plant N uptake was calculated by multiplying the dry shoot biomass by the N concentration. 2.5. Statistical analysis All statistical analyses in this study were performed using SAS 9.1. The C and N mineralization differences among treatments were analyzed using repeated measures analysis of variance (ANOVA), and the measurement date was included as a repeated factor. In the pot experiments, one-way ANOVA was used to analyze statistical differences in dry shoot biomass and plant N uptake between treatments for each set with and without added fertilizer, and compared using Tukey's post hoc test at a 0.05 probability level. 3. Results 3.1. Composition and structure of raw material and biochar Changes in the elemental compositions of OC, N, H, and O were notable during the charring process (Table 1). The OC concentration rose progressively as the charring temperature increased, rising from 487 mg OC g− 1 in raw material to 595 mg OC kg− 1 in BC500. By contrast, N, H, and O contents declined as the charring temperature increased. These changes in elemental composition were reflected in changes in elemental ratios. The C/N ratio rose with the charring temperature, whereas the O/C and H/C ratios decreased with the charring temperature. The ash content increased with the charring temperature, rising from 100 g kg− 1 in raw material to 250 g kg− 1 in BC500 (Table 1). The pH also increased with the charring temperature, rising from 5.2 in raw material to 10.7 in BC500. Changes in inorganic NH4-N and NO3-N concentrations showed the opposite trends with the increase of charring temperature. The NH4-N concentration decreased as the charring temperature increased, decreasing from 0.81 mg g−1 in raw material to 0.02 mg g−1 in BC500, whereas the NO3-N concentration increased from 0.63 mg g−1 in raw material to 4.43 mg g−1 in BC500. The solid-state 13C NMR spectra revealed detailed changes in the chemical structure between the raw material and biochar samples (Fig. 1). The 13C NMR spectra for raw material and BC200 revealed resonances of O-alkyl C (δ = 45–110 ppm), carboxyl C (δ = 165– 190 ppm), and alkyl C (δ = 0–45 ppm), which are typically assigned to cellulose, hemicellulose, and cuticle compositions, respectively (Baldock and Smernik, 2002). Charring led to the loss of the O-alkyl C and carboxyl C structures. After the charring process, the spectra were dominated by a large resonance in aromatic C (δ = 110–145 ppm) combined with a relatively minor signal in the alky C structure. The alkyl C structure was formed in BC300 and BC400, but was destroyed at the higher charring temperature of 500 °C. Thus, the 13C NMR spectra for BC500 exhibited a dominant aromatic C peak at 128 ppm. Table 1 Elemental concentration, ash content, pH and inorganic NH4-N and NO3-N concentration of Sesbania raw material and biochar charred at 200, 300, 400, and 500 °C. OC

N

H

Oa

Ash

C/N

H/C

O/C

pH

−1

Raw material BC200 BC300 BC400 BC500

487 477 587 577 595

48 43 41 35 34

NH4–N NO3–N mg g−1

mg g

53 44 28 16 8

312 325 194 182 113

100 110 150 190 250

10.1 11.1 14.2 16.6 17.6

0.11 0.09 0.05 0.03 0.01

0.6 0.7 0.3 0.3 0.2

5.2 5.5 7.1 9.0 10.7

0.81 0.64 0.06 0.02 0.02

0.63 0.37 0.83 1.21 4.43

a Calculated from weight difference: O (mg g − 1 ) = 1000 − OC (mg g − 1 ) − N (mg g−1) − H (mg g−1) − ash (mg g−1).

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pyrrolic N at 399.6 eV in association with the reduction of amidic N structure as the charring temperature exceeded 300 °C (Table S1). Compared to the N 1s K-edge spectra for raw material and BC200, the BC500 spectra displayed lower adsorption intensity in amidic composition but higher adsorption intensity in nonpeptide C_N and heterocyclic aryl N structures.

128 105

175

74

33

3.2. C and N mineralization patterns

*

*

BC500

* BC400

BC300

BC200

Raw material

Significant differences in CO2-C emission were detected between soils amended with raw material and biochar and soils that were not (Fig. 3a). Soils amended with biochar had a significantly lower CO2-C emission (P b 0.05) compared with soil amended with raw material, with the exception of soil amended with the BC200 sample. When the charring temperature exceeded 400 °C, cumulative CO2-C emission was not significantly different from the control soil (P N 0.05). At the end of the 95 day incubation period, the mineralized C in raw material and BC200 were 32.7% and 37.2%, respectively, relative to the added OC (Table 2). Only 2.9% of the added OC was mineralized in BC300, and less than 0.5% of the added OC was mineralized in BC400 and BC500. Similar to the C mineralization results, significant differences in leached inorganic N (NH4-N + NO3-N) were also detected (Fig. 3b and Table 2). Soil amended with raw material exhibited the greatest leached inorganic N (P b 0.05) than soils that were and were not amended with biochar, and a net N mineralization occurred. The leached inorganic N in soil amended with BC300 was not significantly different to that in the control treatment (P N 0.05); thus, a relatively insignificant effect on N mineralization/immobilization was exerted. In contrast, soil amended with BC500 had significantly lower levels of leached inorganic N (P b 0.05) than did the control treatment, and a net N immobilization was measured. At the end of the 35 day incubation period, the mineralized N was 12% and b1% relative to the added N in raw material and BC300, respectively (Table 2). However, immobilized N at −12.9% relative to the added N was observed in BC500. 3.3. Pot experiment

250

200

150

100

50

0

-50

Chemical shift (ppm) Fig. 1. Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy of Sesbania raw material and biochar charred at 200, 300, 400, and 500 °C. The chemical shift regions assigned to the major types of C were 0–45 ppm for alky C, 45–110 ppm for O-alkyl C, 110–145 ppm for aryl C, 145–165 ppm for O-aryl C, and 165–190 ppm for carboxyl C. Asterisk refers to spinning sidebands.

The main characteristic of C 1s K-edge NEXAFS spectra showed well resolved absorption bands near photo energy at 285, 287.3 and 284.4 eV (Fig. 2a). The absorption band near 285 eV was related to aromatic C and progressively increased with the charring temperature. The band near 287.3 eV that related to aliphatic C also increased with the charring temperature, but was mainly found in BC300 and BC400, and not in BC500. In contrast, the band at 288.4 eV that related to carboxylic C gradually decreased as the charring temperature increased. The changes of band height ratios can also be referred to Table S1 in the supplementary material. Overall, the C 1s K-edge NEXAFS spectra exhibited a similar trend consistent with that of 13C NMR spectra and the C 1s K-edge spectra for BC500 showed the least adsorption intensity in carboxylic C but the highest adsorption in aromatic C, whereas the C 1s K-edge spectra for raw material and BC200 samples exhibited the opposite trend. The N 1s K-edge NEXAFS spectra revealed substantial changes between the raw material and biochar samples (Fig. 2b). The spectra for raw material and BC200 indicated that most N was bound in an amidic structure with a dominant band at 401.3 eV. The spectra then changed to the formation of pyridinic-C/nonpeptide C_N at 398.6 eV and

In pots without added fertilizer, the BC400 and BC500 treatments exhibited a lower dry shoot biomass (P b 0.05) compared with the control treatment, with reductions of 28% and 25%, respectively (Fig. 4a). The raw material, BC200, and BC300 amendments were not significantly different from the control treatment, and produced an average of 0.87 g pot−1 of dry shoot biomass (Fig. 4a). By contrast, plant N uptake in the raw material, BC200, and BC300 treatments was higher (P b 0.05) than in the control treatment (Fig. 4b) and the BC400 and BC500 treatments did not significantly differ (P N 0.05) from the control treatment. With the addition of fertilizer, the dry shoot biomass and plant N uptake increased. The raw material treatment showed the greatest amount of dry shoot biomass and plant N uptake among the treatments (P b 0.05). The addition of fertilizer improved dry shoot biomass and plant N uptake in the BC400 and BC500 treatments, and no significant difference was found between the BC400, BC500, and control treatments (P N 0.05). 4. Discussion 4.1. Characterization of raw material and biochar This study demonstrated changes in the elemental composition of the Sesbania raw material during conversion into biochar. The higher OC content and lower O and H contents observed after charring were consistent with previous studies using different feedstock materials (Baldock and Smernik, 2002; Hammes et al., 2006; Keiluweit et al., 2010). The results reflect the thermal dehydration (loss of H2O),


(a)

288.4 287.3

(b)

BC500

285.0

399.6 398.6

585

BC500 401.3

BC400 BC400

BC300 BC300 BC200 BC200 Raw material

Raw material

280

285

290

295

300

305

Photon energy (eV)

396

398

400

402

404

406

408

410

Photon energy (eV)

Fig. 2. Near edge X-ray absorption fine structure (NEXAFS) spectra of (a) C 1s K-edge and (b) N 1s K-edge of Sesbania raw materials and biochar charred at 200, 300, 400, and 500 °C. Vertical dashed lines indicate the energy absorbance related to aromatic C at 285.0 eV, aliphatic C at 287.3 eV, and carboxylic C at 288.4 eV (a); and pyridinic N or non-peptidic C_N at 398.6 eV, pyrrolic N at 399.6 eV, and amidic N at 401.3 eV (b).

demethylation (loss of CH3), and decarboxylation (loss of CO2) during the charring process. However, the decrease of N content in biochar was inconsistent with certain studies (Keiluweit et al., 2010; Wang et al., 2012), but similar to chicken manure biochar (Koutcheiko et al., 2007; Uchimiya et al., 2011). The decrease in the N content likely occurred because the N structure in the leguminous raw material consists primarily of amines, amino acids, and amino-sugar, which may volatilize during the charring process (Koutcheiko et al., 2007). Some of the N was transformed to NO3-N in the BC400 and BC500 samples in this study. The enhanced NO3-N concentration in these samples may have derived from the thermal transformation of biomass N under thermal processes (Pratt et al., 2011). Because of changes in elemental composition, biochar exhibited a higher C/N ratio and lower H/C and O/C ratios than did raw material. The O/C and H/C ratios observed at 0.05–0.01 and 0.3–0.2, respectively, were typical for other biochars (Spokas, 2010). However, the C/N ratio at 14.2–17.6 was lower than other biochars produced from feedstock materials with high initial C/N ratios (e.g., wood and grass biomass). The increasing ash content with charring temperature suggests that mineral ash is selectively preserved toward gaseous and liquid products. Because the ash contains mostly bases such as Ca, Mg, and K, the high ash content in BC400 and BC500 may be responsible for the high pH (Table 1) (Brewer et al., 2011). The decreases in O-alkyl C and carboxylic C and the increase in aromatic C with increasing charring temperature was consistent with previous studies (Baldock and Smernik, 2002; Keiluweit et al., 2010). The alkyl C structure of the BC300 and BC400 samples resulted from the alkyl C condensation during thermal degradation of the biomass, which was also detected in other biochars produced at lower charring temperatures (e.g., b400 °C; Baldock and Smernik, 2002; Hammes et al., 2006; Keith et al., 2011; Nguyen et al., 2010) and biochars

produced by fast pyrolysis at a high charring temperature (e.g., 500 °C, Brewer et al., 2011). At higher charring temperatures, the alkyl C structure is destroyed and a condensed aromatic C structure is formed. The temperature-dependent structural alterations that occur with increases in charring temperature can also be characterized by the following stages: (1) transition char, (2) amorphous char, (3) composite char, and (4) turbostratic char (Keiluweit et al., 2010). Thus, biochars produced at lower temperatures may bear closer resemblance to its original feedstock, containing a less-ordered structure and a relatively low stability. In contrast, biochars produced at higher temperatures appear to show a greater degree of condensed and long-range ordered structure, leading to a higher stability. With increases in charring temperature, the N structure shifts from the amidic N dominance in raw material to the aromatic N system dominance in biochar (Fig. 2b). The changes in N structure observed in this study were similar to those revealed in previous studies, but were detected using different techniques—solid-state 15N NMR (Almendros et al., 2003; Kelemen et al., 2002; Knicker, 2010) and X-ray photoelectron spectroscopy (Kelemen et al., 2002; Koutcheiko et al., 2007). Both C and N structures became enriched in aromatic and heterocyclic aromatic structures in biochar. This change in the N structure in biochar can be expected to greatly influence N mineralization rates once amended in the soil. 4.2. C and N mineralization Compared to soil amended with raw material, soil amended with biochar, particularly biochar produced at charring temperatures exceeding 300 °C, exhibited lower CO2-C emission. Within 95 days of incubation, the mineralized C for biochar charred at temperatures higher than 300 °C was b2.9% of the added OC, which was within the

Cumulative leached N (mg N kg-1 soil)

Cumulative CO2-C emission (mg C kg-1 soil)

586


2500

(a)

2000 Control soil Raw material BC200 BC300 BC400 BC500

1500

1000

500

0 0

20

40

60

80

100

300

(b) 250 200 150 Control soil Raw material BC300

100 50

BC500

0 0

10

20

30

40

Days Fig. 3. (a) Cumulative CO2-C emission (mg CO2-C kg−1 soil) and (b) cumulative leached inorganic N (mg N kg−1 soil) of soils amended with Sesbania raw material or biochar charred at 200, 300, 400, and 500 °C incubated at 25 °C for 95 days for C mineralization and 35 days for N mineralization. Data show mean ± SE at each measurement date.

range reported for other biochars (Baldock and Smernik, 2002; Cheng et al., 2008; Hamer et al., 2004; Keith et al., 2011; Kuzyakov et al., 2009; Zimmerman et al., 2011). As mentioned above, this low C mineralization occurred because the charring process removed the most easily mineralized composition, leaving only the condensed and stabilized structure. The least amount of C mineralized was from the BC500 treatment, indicating that higher charring temperatures increased the condensation and recalcitrance of the OC (Keith et al., 2011; Nguyen et al., 2010). Our results were consistent with Spokas (2010), who suggested that the stability of biochar can be assessed by its molar O/C ratio, and that a low value represents a higher stability. We found a significant correlation between the charring temperature, the O/C ratio, and mineralized C in this study (r = 0.98 between the O/C ratio and mineralized C, data not shown). Charring leguminous raw material also exerted a noteworthy effect on the N cycle. Sesbania raw material exhibited a net N mineralization, which was reflected in its high N content and low C/N ratio (Palm and Sanchez, 1991), and N release shifted from a net N mineralization in the raw material amendment to a net N immobilization in the BC500 amendment. The lower level of leached inorganic N might be interpreted as resulting from the charring process: the thermal alteration of the N structure may have suppressed the microbial mineralization of the N compounds in the biochar. Although this may explain the results for the BC300 amendment, which exhibited the same leached inorganic N content as the control treatment, the explanation is insufficient to explain N immobilization in the BC500 amendment. Previous research has shown that N immobilization is induced by microbial immobilization, in that the added available C cannot be matched by a

sufficient amount of added available N (Deenik et al., 2010; Nelson et al., 2011). However, this microbial immobilization may not have occurred in the BC500 amendment, because BC500 contains little available C. Therefore, the N immobilization in the BC500 amendment was likely caused by the biochar sorption over native soil organic matter (Cross and Sohi, 2011; Dempster et al., 2012; Jones et al., 2011; Zimmerman et al., 2011). The sorption over native soil organic matter could produce a protective mechanism to reduce the mineralization of native soil organic matter and cause this negative N release. Fresh biochars produced at higher charring temperatures in particular are highly sorptive over soil organic matter because of their greater porous structure and positive charge (Cheng and Lehmann, 2009; Kasozi et al., 2010). The possibility of a toxic effect inhibiting the degradation of soil organic matter may have been another factor inducing N immobilization. However, this toxic effect appears more frequently in low-temperature biochars than in high-temperature biochars (Spokas et al., 2010), and is therefore less supportive of the results of this study. The lower dry shoot biomass of water spinach in pots amended with BC500 and BC400 was likely caused by N immobilization, which reduced plant N availability. However, the plant N uptake was not as greatly influenced as the dry shoot mass, and no significant differences were found among the BC400, BC500, and control treatments. This reduced effect on plant N uptake may imply that the observed N immobilization was a short-term effect, and may shift to N mineralization at later stages (Novak et al., 2010). Furthermore, the higher initial NO3-N content of BC400 and BC500 (Table 1) may also have provided plant available N. However, the initial NO3-N content could also leach out in the beginning of the experiment, when the root systems of water spinach are still limited. Unlike the BC400 and BC500 amendments, the higher dry shoot biomass yield and plant N uptake in the raw material and BC200 amendments likely resulted from the increasing N mineralization, which provided a higher N availability to improve productivity levels. The addition of fertilizer improved the dry shoot biomass of water spinach in BC400- and BC500-amended soils, suggesting that reduced dry shoot biomass in the unfertilized treatments resulted from a reduced plant N availability. However, synergistic effects between biochar application and fertilizer addition that were previously observed (Chan et al., 2008; Van Zwieten et al., 2010; Yamato et al., 2006) were not observed in this study. One possible explanation for this discrepancy is that the soil used in this study was occasionally amended with lime and manure, as its soil pH was increased from 4.4 in the adjacent forest (Cheng et al., 2013) to 6.1. Van Zwieten et al. (2010) indicated that biochar amendment increased biomass most in acidic soil, suggesting that improvement of soil properties in the current study may have reduced certain beneficial effects of biochar application.

4.3. Implications Although the C stability of Sesbania green manure can be enhanced by converting raw biomass into biochar, this conversion may decrease the fundamental role of green manure, which is to provide N as a nutrient. The reduced plant N availability of biochar applications may necessitate different management strategies to those used in raw material application. Farmers may need to apply additional N fertilizer to offset this N deficit. This additional N fertilizer use could increase costs to the farmer and cause CO2 emission to be generated from inorganic fertilizer production. Changes in chemical structure during the charring process observed in this study can be the same to other feedstock materials. Thus, for any material that is intended to provide N, the charring process may decrease N availability, rendering it unsuitable for this specific purpose. However, the charring process is satisfactory for materials intended to enhance soil organic C, and may even enhance the long-term build-up of soil organic C stocks (Lehmann et al., 2006).


587

Table 2 Cumulative C and N mineralization at the end of incubation and the percentage of mineralized C and N relative to the added organic matter of Sesbania raw material and biochar charred at 200, 300, 400, and 500 °C. Treatment

Cumulative evolved CO2-C (mg C kg−1 soil)a

CO2-C from amendment (mg C)

Percentage of added OC (%)

Control soil Raw material BC200 BC300 BC400 BC500

560 c 2151 a 2336 a 728 b 588 c 544 c

1591 1776 168 28 −16

32.7 37.2 2.9 0.5 −0.3

a b

Cumulative leached inorganic N (mg N kg−1 soil)a

Mineralized N from amendment (mg N)

Percentage of added N (%)

193 b 258 a NDb 193 b ND 149 c

65 ND 0 ND −44

13.5 ND 0 ND −12.9

Value within each column followed by the same letters indicate treatments not significantly different at P b 0.05. Not determined.

The results of this study also indicate that the charring temperature is one of the deterministic parameters to produce biochar with different compositions and properties. A high charring temperature makes the biochar more condensed and recalcitrant compared with a low charring temperature. The change in chemical structure for biochar charred below 200 °C seems to be trivial, because rectification is generally said to occur at temperatures from 230 to 250 °C and torrefaction occurs at temperatures between 250 and 280 °C (Antal and Gronli, 2003).

5. Conclusion Leguminous green manure is an important source of N and C in cropping systems. In this study, we used Sesbania biomass as a feedstock material to examine changes in chemical structure and subsequent C and N mineralization resulting from the conversion of leguminous 2.5

Acknowledgment

Shoot biomass (g)

(a)

- Fertilizer + Fertilizer

A

2.0

AB BC

BC

BC

C

1.5 a

1.0

a

a

a

b

b

0.0 Control

Raw

BC300

BC400

BC500

(b) A

80

N uptake (mg)

BC200

B

B B

B

B

60

40 ab

20

a b

c

c

c

BC400

BC500

0 Control

Raw

BC200

BC300

This study was supported by the Ministry of Science and Technology of Taiwan and a cooperative grant from the Ministry of Science and Technology of Taiwan and the Siberian Branch of the Russian Academy of Sciences (project no. 101-2923-B-002-002). We gratefully acknowledge I-Yu Lin and Chang-Ya Cheng for their assistance with field and lab work. References

0.5

100

green manure into biochar. By using NMR and NEXAFS techniques, we found that both C and N became enriched in aromatic and heterocyclic aromatic structures in biochar, and this structural alteration led to reductions in the C and N mineralization rates of biochar. Compared to the unconverted raw material, mineralized C decreased from 32.7% of the added OC in raw biomass to b0.5% in biochar at charring temperatures exceeding 400 °C, and the N release pattern shifted from N mineralization in raw material to N immobilization in biochar. Soils amended with biochar at charring temperatures of 400 °C or 500 °C demonstrated a 25% decrease in plant dry shoot biomass compared to unamended control soil. Thus, our study indicated that the C stability of leguminous green manure can be achieved by converting raw biomass into biochar; however, the charring process may limit it to providing N. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.geoderma.2014.06.021.

Fig. 4. Effects of Sesbania raw material and biochar amendments on (a) dry shoot biomass and (b) plant N uptake of water spinach after 42 days of growth. Data show means ± SE. Means having a common letter(s) are not significant different at P b 0.05.

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