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Transformation and transport mechanism of nitrogenous compounds in a biochar ‘preparation- returning to the field’ process studied by employing an isotope tracer method Liyun Liu, Zhongxin Tan, and Zhixiong Ye ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03172 • Publication Date (Web): 07 Jan 2018 Downloaded from http://pubs.acs.org on January 7, 2018

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ACS Sustainable Chemistry & Engineering

Transformation and transport mechanism of nitrogenous compounds in a biochar ‘preparation- returning to the field’ process studied by employing an isotope tracer method Liyun Liu, Zhongxin Tan*, Zhixiong Ye College of Resources and Environment, Huazhong Agricultural University, No.1 lion hill street in Hongshan District, Wuhan, 430070, P. R. China Abstract: Due to biochar’s excellent physical and chemical properties such as rich void ration and large specific surface, it could improve soil by conserving water and fertilizer and providing breeding grounds for soil microorganism, and therefore has been attracting researchers’ interests for its potential as a soil amendment. In this work, elemental analysis-stable isotope ratio mass spectrometry (EA-IRMS) and X-ray photoelectron spectroscopy (XPS) were conjointly employed to investigate the migration and transformation mechanism of biochar nitrogenous compounds in the ‘preparation-returning’ process. EA-IRMS data indicated that during the preparation process, the nitrogen retention rate in biochar first dropped sharply (300–400 °C), then became stable (400–500 °C), and finally decreased slowly (500–800 °C) with increasing pyrolysis temperature. After returning biochar to soil, the measurable total nitrogen in biochar migrated to soil and plant displayed a nitrogen mass distribution rate in the order of: biochar after returning (88.40–90.42%) >soil (8.81– 10.07%) >plants (0.77–1.53%). In addition, the pyrolysis temperature was negatively related to the nitrogen mass distribution rate in biochar, soil, and wheat. On the other hand, the pyrolysis atmosphere had little effect on the nitrogen retention rate in biochar before returning and the nitrogen mass distribution rate in biochar after returning to the field. XPS results suggested that alkaloid-N, free amino acid-N,

*

Corresponding author: Tel.: +86-15827583169; E-mail address: [email protected] 1

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protein-N, and NH4+-N in wheat straw were gradually transformed into pyridine-N, amino-N, pyrrole-N, quaternary-N, NH4+-N, NO2--N, and NO3--N in biochar during the biomass pyrolysis process. Biochar produced at 300 °C was in a transition stage that included all nitrogenous compounds present in wheat straw as well as biochar produced at the lower temperatures (≤500 °C). At higher temperatures, inorganic nitrogen species were more abundant and displayed higher contents. For pyrolysis temperatures ≤500 °C, biochars prepared under both N2 and CO2 atmospheres comprised similar nitrogenous compounds and contents. Moreover, the changes in nitrogenous compounds and nitrogen release patterns during the process of returning biochar to the field were not significantly different. Key words:

15

N labelled biochar, Pyrolysis temperature, Pyrolysis atmosphere,

Nitrogen mass distribution, Nitrogen release

Introduction Biochar refers to the carbon-rich product afforded from heating biomass in a closed system under a limited oxygen supply.1 The historic use of biochar can be traced back to 2000 years ago when ancient civilizations indigenous to the Amazon basin used it to enrich soil fertility.2 Due to its inherent alkali characteristics, biochar can increase the pH of acid soil. 3,4 Notably, soil quality and nutrient availability to plants can also be boosted by applying biochar to soil.5 Therefore, biochar shows great potential in improving the physicochemical characteristics and fertility of soil.6 After returning to soil, biochar can physically adsorb NH4+-N, a process closely linked to its large surface area and complex pore structure. In addition, the returning of biochar to soil has been shown to increase soil nutrient retention and microbial biomass, improve N2 fixation in cover crops, decrease the need for irrigation, and sequester C from the atmosphere.7 Thus, returning biochar to the field has many benefits for soil and crops alike. However, conflicting works have reported that the addition of biochar to soil decreases,8,9 increases

10,11

and has no effect

12

on N

mineralization, making this topic of research highly controversial. According to a report by Chen et al.,13 reduced phenolic moieties in biochar at low pyrolysis 2


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temperatures (300 °C) act as an electron source to enhance denitrification. Conversely, oxidized quinonoid moieties in biochar at high pyrolysis temperatures (800 °C) act as an electron sink and reduce total denitrification. Thus, biochars under different pyrolysis conditions possess distinct redox-active components that mediate the electron transfer in microbial denitrification. Dunnigan et al.14 reported that biochar produced by pyrolysis contained traces of polycyclic aromatic hydrocarbons (PAH) that are harmful to plants and may be introduced into the soil when returning biochar to the field. Although biochars are generally low in inorganic-N, this can provide diazotrophs with a competitive advantage in colonising their large surface area. This factor combined with their potential for NH4+ exchange with the soil solution could modify soil-N availability to plants and stimulate nodulation and fixation.5 Moreover, biochar application has been shown to stimulate nitrogen fixation in the legume-Rhizobia spp. symbiotic system.15, 16 The pyrolysis temperature is an important factor that affects the quality of biochar, nutrient content, and nature of nitrogenous compounds. Previous studies

17

have shown that the nitrogen content in biochar was negatively correlated with pyrolysis temperature, especially the research of Tian (Table 1). Biochar material at 300 °C is a transition state from straw to biochar, still contains abundant fiber structure, so this kind of biochar material can’t be treated as the product of complete pyrolysis of straw and therefore retains most of straw nitrogen element. As confirmed in Table 1, biochar-N decreases considerably at 300-400 °C, while the reduction of biochar-N is slowly in the range of 400-800 °C. It's also worth noting that, biochar prepared at 400 °C contains more nitrogen nutrient compared with biochar at 800 °C. This occurs because nitrogen-containing gases (i.e. NH3, HCN and HCNO)18 begins to evaporate at low temperatures

19,20

and thus, this phenomenon becomes more

evident and also the nitrogen retained in biochar decreased as the pyrolysis temperature increases. Table 1 also showed that CO2 gas had certain inhibitory effect on the volatilization of HCN and HCNO in biochar,21 so the nitrogen retention rate in biochar prepared under CO2 atmosphere was higher than that of N2 atmosphere from 400 °C to 600 °C. On the other hand, Yuan et al.22 also reported a decreasing tendency 3



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about the plant-available ammonium-N and nitrate-N in biochar with temperature from 300 °C and 700 °C. According to rice production and grain-straw ratio in China (i.e. straw yield/crop yield), 25 rice straw yield are 6911 kg·hm-1, respectively. When the nitrogen content in rice straw in China are 0.91%,26 and the nitrogen retention rate in biochar is 64.94% under the pyrolysis condition of CO2 atmosphere and at 400 oC, the nitrogen of biochar per hectare produced from rice straw are 36.76 kg·hm-1. In China, the applied amount of seasonal nitrogen fertilizer to rice are generally 85.85 kg·hm-1, the biochar nitrogen mass per hectare (400 °C and CO2 atmosphere) is equal to 42.82% of the amount of seasonal nitrogen fertilizer. Therefore, pyrolysis temperature and pyrolysis atmosphere play a vital role for nitrogen retention in biochar from the rice straw, and biochar prepared in proper work conditions (such as 400°C, CO2 atmosphere) still reserves much more nitrogen, so it still has certain fertility value. To date, literature on the mechanism of nitrogenous compound transformation during biochar preparation is scarce. Consequently, we explored the nitrogen transformation mechanism in a biochar ‘preparation-returning to the field’ process by adopting an isotopic tracer method which key role was to label the wheat straw raw material in biochar with 15N. The ‘preparation-returning field’ process mainly includes two important process: 15

15

N-labelled biochar was first derived from the pyrolysis of

N-rich wheat straw and subsequently, the prepared biochar was returned to soil to

improve its physicochemical properties and increase its nutrient content. Thus, first, the 15N content was determined by EA-IRMS to investigate the transfer and transport mechanism of nitrogen nutrients from the cultivation of

15

N-rich wheat straw to

biochar preparation, returning to the field, and crop growth. Next, the distribution of nitrogenous compounds and their relative contents were characterized by XPS. Finally, a comprehensive analysis of the mass distribution and nitrogenous compound transformation characteristics of nitrogen was used to explore the transformation of biochar nitrogenous compounds and nitrogen delivery. This can provide theoretical guidance for better utilization of biological carbon/ nitrogen nutrients. 4


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Material and method Experimental design 15

N isotope tracer experiment (Fig.1): first, wheat was cultivated in pot with 15N

isotope fertilizer to produce

15

N-labelled wheat straw. Next, the wheat straw was

pyrolyzed at set working conditions (Table 2) to manufacture

15

N-labelled biochar

which was then mixed with potting soil to cultivate wheat plants (potting experiment). Subsequently, the

15

N content of soil, wheat straw, and biochar samples before and

after returning to the field was measured by EA-IRMS, while the nitrogenous compound content in the wheat straw and biochar samples, before and after returning to the field was determined by XPS. The analytical data confirmed optimal nitrogenous compounds in biochar for returning to the field. Thus, this data can be used as a solid theoretical foundation for the regulation and preparation of biochar with rich optimum nitrogenous compounds during the pyrolysis process.

Biochar production The soil samples comprised surface soil (0–20 cm) collected from the experimental base of Huazhong Agricultural University; the basic physicochemical properties of the soil are listed in Table 3. The soil was air dried and passed through a 10-mesh sieve to produce homogeneous soil particles. Next, the soil was enclosed in an experiment pot (diameter = 160 mm; height = 220 mm), watered with deionized water to maintain a moisture content of 18%–22%, and uniformly wheat seeded (all wheat mentioned henceforth is of the E-wheat 596 variety). Once the wheat germinated and reached a height of 3 cm, 15 mL NH4Cl solution (1 wt%)-isotope marker (15N, 99%) mixture was sprayed every 24 h. After cultivating for 20 d, the 15

N-labelled wheat straw was harvested, washed, and dried to constant weight at

65 °C. It was then sealed and stored for further experimental use. The biochar samples were prepared by pyrolysis of

15

N-labelled wheat straw at

temperatures of 300, 400, 500, 600, and 800 °C under CO2 or N2 atmosphere. Prior to the preparation of biochar, wheat straw was cut into 2-cm segments. The furnace was first purged with the pyrolysis gas to displace the air and the heating process was then 5



initiated at an initial temperature of 20 °C and a heating rate of 20 °C∙min-1. The residence time was set to 20 min with continuous ventilation (flow rate of gas intake = 5 L·min-1) to maintain anoxia or an oxygen-limited environment in the pyrolysis furnace. After pyrolysis, the samples were cooled naturally to room temperature (25 °C) and then weighed to determine the yield (Table 2). The biochar was ground, passed through a 1-mm sieve, placed into a bag, and labelled as CO2-300, CO2-400, CO2-500, CO2-600, CO2-800, N2-300, N2-400, N2-500, N2-600, and N2-800 according to the pyrolysis temperature (300–800 °C) and atmosphere (N2 or CO2).

Biochar incubation in soil The 15N-labelled wheat straw was evenly mixed with soil at a mass ratio of 5:100 (biochar mass according to the yields of the same wheat straw mass). A blank control (C), in which no biochar or wheat straw was added to the soil, was also prepared. Therefore, twelve different treatments were used in the potting experiment: addition of the different biochars to the soil, addition of wheat straw to the soil, and no biochar or wheat straw added to the soil. Subsequently, the cultivated wheat was labelled as BC-300, BC-400, BC-500, BC-600, BC-800, BN-300, BN-400, BN-500, BN-600, BN-800, WS, and C, respectively. BC and BN indicate whether the biochar was treated under carbon dioxide or nitrogen atmosphere, respectively, while the pyrolysis temperature is indicated by the number succeeding the hyphen. Experiments were performed in triplicate for each set of conditions. The mixture was watered with deionized water and the wheat was sown on the surface of the soil. The pot culture was then placed in an artificial climate incubator with the daylight intensity (12 h) set to 3 class and 70% humidity and the night intensity (12 h) set to 0 class and 75% humidity. Other environmental factors, such as the daytime (25 °C) and night-time (18 °C) temperatures were also controlled. No fertilizer was applied during the growth period and the soil moisture was maintained at 18–22%. The growth period lasted for 31 d. At the end of the incubation experiment, the wheat was harvested, dried to constant weight, and subsequently weighed and sealed. The biochar was removed from the soil of the wheat root (i.e., 6


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rhizosphere soil) and sealed after drying. Before XPS and EA-IRMS determination, biochar was milled and passed through a 100-mesh sieve to produce homogeneous fine particles.

Elemental analysis-stable isotope ratio mass spectrometry (EA-IRMS) EA (Elementar Vario PYRO cube, Germany) and stable IRMS (100) were employed to analyse the wheat straw and biochar samples before and after returning to the field to confirm the nitrogen content and 15N abundance in the samples.

X-ray photoelectron spectroscopy (XPS) XPS can be utilized to identify pyridinic-type nitrogen, pyrrolic nitrogen, quaternary/graphitic nitrogen, pyridinium-like nitrogen, and pyridinic-N+– Onitrogen with binding energies of 398 eV, 400 eV, 401 eV, 400–401 eV, and 402.5 eV, respectively.25 In this work, XPS (MULT1LAB2000) was used to determine the nitrogen compounds distribution in the wheat straw and biochar samples before and after returning to the soil. All XPS spectral peaks were fitted with CASAXPS software using Gaussian-Lorentzian (30% Lorentzian) line shapes.26,27 Best fits were evaluated using a root-mean-square measure where the line shape, peak width [full width at half maximum (FWHM)], and binding energy were adjustable parameters. Shirley backgrounds were used in all the fits to narrow the scan spectra.28

Calculation and statistical analysis In the atmosphere,

15

N is a stable isotope with stable and homogeneous

abundance. Its atomic abundance (0.3663) is also considered as the

15

N standard

isotope abundance.29 Generally, the nitrogen isotopic composition of the sample (δ15N) is expressed as: ‰ = ⁄ − 1 × 1000

(1)

where where and represent the 15N atomic abundance in the sample and atmosphere, respectively. The biochar yields (Table 2) are expressed as: 7



!" 2

where 1

!" %

=

$%&'()* + ,(-). 0.*), +

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× 100%

(2)

represents biochar mass in grams and 13 3 2

represents the mass of wheat straw for biochar preparation. The retention rate of nitrogen in biochar (Fig.2) is expressed as:

4 =

where

!" %

!" %

$%&'()* +×4$%&'()* % ,(-). 0.*), +×4$%&'()* %

× 100%

(3)

represents the percentage mass of nitrogen in biochar and

represents the percentage mass of nitrogen in wheat straw.

The content of one nitrogenous compound in biochar (Fig.4 and 7) is expressed as: 56!+67 " 768 !6 = : where :

!" 12⁄92

!" 12 ⁄92

!" 12⁄92

;;6!+67 " 768 !6

× ;;6!+67 " 768 !6

!" %

(4)

represents the total nitrogen (TN) content in biochar and

!" %

represents the percentage mass of one

nitrogenous compound that accounts for the total nitrogen in biochar. The content of one nitrogenous compound in wheat straw (Fig.4 and 7) is expressed as: 56!+67 " 768 !6 3 3 12⁄92 = :3 3 12⁄92 × ;;6!+67 " 768 !6 3 3 %

(5)

where :3 3 12⁄92 represents the total nitrogen content in wheat straw and ;;6!+67 " 768 !6 3 3 % represents the percentage mass of one nitrogenous compound that accounts for the total nitrogen in wheat straw. 15

N mass in biochar after returning to the field is expressed as:

1< %= >?$%&'()* 12 = 1< %= $%&'()* 12 − 1< %= 0&%@ 12 − 1< %= A@)=. 12 where 1< %= $%&')(* 12 represents field,

1< %= 0&%@ 12

represents

15

(6)

N mass in biochar before returning to the

15

N

mass

in

soil

from

biochar,

1< %= A@)=. 12 represents 15N mass in the plant sample from biochar. 8


and

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The mass distribution rates of 15N in biochar after returning to the field, in soil or plant are expressed as: ;B:< %=C % = where 1< %= C 12 represents

15

DE< %= C +

DE< %= $%&'()* +

× 100%

(7)

N mass in biochar after returning to the field in

soil, or plant and 1< %= $%&')(* 12 represents 15N mass in biochar before returning to the field. Experimental data were analysed by Statistical Product and Service Solutions Software (SPSS, version 19.0). All significant differences were reported at the 0.05 probability level and all experimental data were presented as mean values ± standard (SD) deviations, in triplicate.

Results and discussion Analysis of

15

N speciation and mass retaining ratio in biochar

before returning to the field 15

Effect of pyrolysis temperature on the N mass retaining ratio and

nitrogenous compound distribution in biochar returning to the field A decreasing trend in percentage nitrogen content was observed when wheat straw was pyrolyzed at different temperatures (300–800 °C) under limited oxygen (N2 or CO2) (Table 2). This suggests that nitrogen was drained and not enriched. Data in Table 2 also indicate that the biochar yield and the

15

N content decreased with an

increase in pyrolysis temperature under both N2 and CO2 atmospheres. Moreover, Fig.2 illustrates that the retention rate of nitrogen in biochar was on the decline. This decline was greatest at a temperature range of 300 °C–400 °C, remained relatively stable at 400 °C–500 °C, and was the lowest at 500 °C–800 °C. This occurs because as the pyrolysis temperature increases, carbonization proceeds to completion while nitrogen continues to volatilize. Therefore, the percentage nitrogen content retained in the biochar gradually decreases. 9



Fig.3 and Fig.4 present the XPS N1s energy spectra and the corresponding fitting of nitrogenous compounds in biochar, respectively. A comparison of the N1s spectra of wheat straw with those of N2-300–N2-800 biochars clearly indicates a change in nitrogen compounds during pyrolysis. Nitrogen in wheat straw exists mainly in four forms: alkaloid-N with a binding energy of 398.6 eV, free amino acid-N with a binding energy of 399.2 eV,30-32 protein-N with a binding energy of 399.7 eV, and NH4+-N with a binding energy of 401.3±0.4 eV,33 protein-N is the dominant species (highest content; 39240 mg/kg). The spectrum of N2-300, the biochar derived from low temperature pyrolysis of wheat straw, still exhibited the absorption peaks of the original nitrogenous compounds observed for wheat straw but with significantly decreased peak heights and areas (Fig.3). Moreover, a series of new absorption peaks at 398.7±0.2 eV, 399.4 eV, 400.3±0.2 eV, and 401.4±0.1 eV, attributed to pyridine-N,34,35 amino-N, pyrrole-N, and quaternary-N, respectively, were also observed. These peaks indicated that during pyrolysis, nitrogen becomes embedded into the carbon skeleton after cracking, condensation, and the recombination of protein, free amino acids, and other species to form more stable compounds (pyridine, pyrrole, and quaternary nitrogen). With an increase in the pyrolysis temperature from 300 °C to 400 °C and 500 °C (Fig.3; N2-400 and N2-500, respectively), the carbonization process intensifies and the original N-containing organic compounds in wheat straw are almost completely converted to pyridine-N, amino-N, pyrrole-N, quaternary-N, and NH4+-N. Two additional adsorption peaks at 403.6 eV and 405.2 eV also appeared as the pyrolysis temperature was further increased to 600 °C and 800 °C; these were assigned to NO2--N and NO3--N, respectively. The results demonstrated that the increasing temperature enriched the diversity of the nitrogenous compounds in biochar and increased the inorganic-N content simultaneously. Fig.4 illustrates that N2-300 was not precisely biochar but transition state matter between straw and biochar comprising almost all the nitrogenous compounds observed in straw and low temperature biochars, but at lower contents. The organic-N content (pyridine-N, amino-N, pyrrole-N, and quaternary-N) in biochars prepared at 300 °C–500 °C, notably in those prepared at 400 °C, was higher than that of high 10


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temperature (600 °C–800 °C) biochars. In addition, pyridine-N and pyrrole-N were the main nitrogenous compounds in low temperature (300 °C–500 °C) biochars. These results are consistent with the conclusions reported by Geng et al.35 N2-500 displayed the highest NH4+-N content, indicating that N2-500 exhibited the largest retention of original NH4+-N in wheat straw. NO2--N and NO3--N were fitted only in N2-600 and N2-800 and the content of both was higher in N2-600. This was strongly associated with the TN content in the biochar yields, namely, N2-600: 32.14% >N2-800: 22.86% (Table 1).

Effect of the pyrolysis atmosphere on the

15

N mass retaining ratio

and nitrogenous compound distribution in biochar before returning to the field Table 2 and Fig.2 reveal that at the same pyrolysis temperature, biochar prepared under CO2 and N2 atmospheres displayed similar changing trends in yield and nitrogen retention rate. This confirms that except for 800 °C biochars, the pyrolysis atmosphere has little effect on these two parameters. This was attributed to the activation energy of the CO2 gas at high temperatures that enables it to participate in the pyrolysis reaction (Fig.5). Thus, at this temperature, rather than providing a limited oxygen environment, CO2 affects the biochar yield and nitrogen retention rate. The XPS N1s energy spectra of biochar at various temperatures under CO2 and N2 atmospheres are displayed in Fig.6. These spectra illustrate that at the same pyrolysis temperature, there was little difference in nitrogenous compound distribution between biochars prepared under the two different atmospheres. These results are corroborated in Fig.4, where the influence of the pyrolysis atmosphere on nitrogen speciation distribution is also shown to be negligible. The two atmospheres only played a role in biochar preparation at temperatures plants (0.77–1.53%). Thus, the higher the biochar pyrolysis temperature, the higher the mass distribution rate of nitrogen in biochar and conversely, the lower the rate of mass distribution in soil and wheat plants. 14


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(4) The pyrolysis atmosphere had no significant influence on the nitrogen retention rate at the same pyrolysis temperature (

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