1. Introduction
The poultry industry is the largest and fastest growing livestock sector in the world, producing substantial amounts of meat and eggs for human consumption, but, at the same time, it produces a huge quantity of chicken manure (CM) waste [
1]. Handling and storage of the manure generated from chicken rearing is problematic due to its bulky nature, high moisture content, and very offensive smell. Furthermore, application of CM to soil causes pollution to the environment (soil, water bodies, and the atmosphere), increases pest incidence, and can cause burns to plants [
2]. Thus, the need for further processing of the manure to make handling and storage easier and to reduce its toxic effect before land application for crop production is necessary for sustainable use [
3].
Carbonization is a popular processing method suitable for converting fresh manure into agriculturally useful amendments [
4]. Carbonization of CM leads to drastic reduction in odor, weight, and volume of the manure, thereby making handling and application to soil convenient [
5,
6]. Other notable advantages of carbonization include that the process requires less space, is relatively fast, eradicates potential pathogens and pharmaceutically active compounds, and reduces emissions of gases as compared to techniques like composting [
7]. CM carbonization is in line with the effort to attain sustainable development through efficient use of resources and reduction of waste, while at the same time reducing the negative effects on the environment.
The decades of work on CM carbonization for the production of agriculturally useful amendment have established the importance of temperature conditions during CM carbonization [
8,
9]. Temperatures around 300–500 °C have been recommended as sufficient for thoroughly carbonizing the manure into value-added material that is useful as organic N fertilizer for soil fertility improvement and crop productivity [
3,
10]. However, confounding results have been reported with regards to the ability of CM to supply nitrogen (N) in soils after having been carbonized around these recommended temperature ranges, especially the upper limit temperature. For instance, Tagoe et al. [
10] observed an increase in total N content of soybeans and a 43% increase in seed yield of the plant after application of CM that was carbonized at 500 °C. In addition, Chan et al. [
3] observed a 96% increase in radish yield and a significant increase in tissue N content over that of the control for CM, which was carbonized at 450 and 550 °C. With these results, they established that carbonized CM produced around these temperatures can supply sufficient plant nutrients, including N, for sustainable plant productivity. On the contrary, Ishimori et al. [
11] reported that CM carbonized at 528 °C alone cannot support the normal growth of plants after they observed that
Oryza sativa and
Brassica rapa komatsuna were not significantly different in their growth in plots of carbonized CM and the control. Steiner et al. [
12] found lower concentrations of N in tissues of plants fertilized with CM that was carbonized at 500 °C when compared to the unfertilized control, and concluded that “N in carbonized CM is not plant available”. Despite the differential results of the N supply ability of carbonized CM, research has not been conducted to clarify this issue by elucidating the mechanism(s) governing the N supply ability of CM after being carbonized around these temperature conditions and to possibly suggest a new threshold temperature that will ensure the sustainability of the good N fertilizer value of CM after carbonization.
Thermal treatments of N-rich organic materials can increase the recalcitrance of the material to decomposition as a result of cyclization of aliphatic chains and decrease in the quantity of hydrolysable organic N due to the formation of heterocyclic N structures [
8,
13,
14,
15]. This drastic alteration of the materials during heat treatments significantly affects their nutrient supply ability [
16,
17]. Hence, thorough microscopic examination of the surface functionality and N speciation coupled with basic chemical property analysis of any N-rich organic material during carbonization for the purpose of N supply is a prerequisite.
Since the start of CM carbonization in 2008 for use as a soil amendment [
3], several authors have tried to calibrate suitable temperature conditions for the production of agriculturally useful organic N fertilizer by evaluating the effects of temperature conditions on carbonized CM yield, elemental composition, pH, electrical conductivity (EC), cation exchange capacity (CEC), and other basic compositional properties [
4,
8,
18]. However, little has been done to understand the spectroscopic transformations of the surface functional group and the speciation of the organic N in CM during carbonization and the subsequent effects of these transformations on N availability in soil. As indicated above, the temperature conditions set by the mere use of the basic physicochemical properties as a yardstick have resulted in discrepant results in the N supply ability of the carbonized product [
3,
10,
11,
12]. We believe that a combination of chemical and microscopic examination of the composition, surface functional group, and N speciation in CM after carbonization will provide a much clearer exposition on the underlying mechanism(s) responsible for affecting the ability of the carbonized CM to supply N in soil and how the process can be manipulated in order to ensure the preservation of the good N supply ability of the manure. This information can be used to establish a new threshold carbonization temperature, which will serve as a guide during CM carbonization for the preservation of the good N supply ability of the manure for sustainable use as organic N fertilizer.
Therefore, the objective of this study was to investigate the alteration in chemical composition, surface functional groups, and N speciation in CM carbonized at different temperatures, as well as their influence on N supply ability of the manure in order to determine a threshold carbonization temperature for preserving the N fertilizer integrity of CM for sustainable use as organic N fertilizer.
2. Materials and Methods
2.1. Sample Source and Preparation
Fresh chicken manure was collected at the poultry section of the Field Science Center, Gifu University. The fresh manure was dried at 65 °C in a conventional oven for 24 hours and then stored in air-tight plastic prior to carbonization. Fifty grams (50 g) of dried CM was placed in a ceramic crucible and carbonized in a muffle furnace. Two batches of carbonized CM were produced at the respective temperatures of 350, 375, 400, 425, 450, and 475 °C. During carbonization, the temperature was raised to the desired values at a heating rate of 5–10 °C per minute, and was thereafter kept running for a period of 1½ hours. After carbonization, the samples were allowed to cool, and the yield of the carbonized manure was recorded. The duplicate samples for each treatment were homogenized with a mortar, pestle, and sieve to pass through a 2 mm mesh. The samples were stored in air-tight plastic containers prior to the experiment. The carbonized CMs are hereafter referred to as CCM350, CCM375, CCM400, CCM425, CCM450, and CCM475 for CM carbonized at 350, 375, 400, 425, 450, and 475 °C, respectively. The un-carbonized chicken manure will still be referred to as CM.
2.2. Physical and Chemical Properties of the Samples
The yields of all of the carbonized CM (CCM) were expressed as the mass fraction of the original CM feedstock. Ash content was determined by dry combustion at 750 °C for 5 hours [
19]. The pH and EC were measured in a deionized water suspension (1:10 w/v) after shaking at 200 rpm for 1 hour and allowed to stand for 1 hour. C, H, and N were determined by a JMA 102 auto-sampler CHN elemental analyzer (J–Science Lab co., Ltd, Kyoto, Japan). The amount of oxygen (O) was calculated by difference. The results of C, H, N, and O were used to calculate atomic H/C and O/C ratios for the preliminary evaluation of the relative degrees of aromaticity (H/C ratio) and stability (O/C ratio) and the possible type of reaction taking place during the carbonization of CM at different temperatures. The concentration of ammonium nitrogen (NH
4+-N) was extracted in a 2 M KCl solution and was determined by steam distillation. The nitration of salicylic acid method [
20] as reported by Logah et al. [
21] was used to measure the nitrate nitrogen (NO3
−-N) content after extraction of the samples with 0.5 M K
2SO
4.
2.3. Surface Functional Group Analysis
The functional groups on the surface of CM and CCM samples were investigated by Fourier-transform infrared spectroscopy (FTIR). Potassium bromide (KBr) pellets that contained 1% (w/w) of each sample were prepared with a cylindrical piston under pressure and vacuum. FTIR analysis of all of the samples was conducted by JASCO 4100 spectrometer (JASCO Corporation, Tokyo, Japan) at wavelengths ranging from 4000 to 400 cm−1 and a resolution of 2 cm−1. The spectrum of the pure KBr pellet was recorded before every measurement for spectra correction.
2.4. Nitrogen Forms and Speciation
The existing nitrogen structures in CM and CCM products were characterized by X-ray photoelectron spectroscopy (XPS). A Quantera SXM-GX scanning X-ray microprobe spectrometer (Ulvac-PHI.inc, Kanagawa, Japan) was used to measure the N1s spectrum of the samples, with a monochromatic AlKα (1486.6 eV) X-ray source operating at 25 W with a spot size of 100 μm in diameter at an electron takeoff angle of 45º. The spectrometer was run at pass energies of 120 and 55 eV to obtain survey and high-resolution spectra, respectively. Prior to introduction in the spectrometer, the samples were mounted on a double-sided carbon tape. All of the samples were analyzed under identical conditions. Deconvolution and quantification of the composition of each N form in the samples from their XPS spectra was done with MultiPak 8.2 software (Ulvac-PHI.inc, Kanagawa, Japan). The binding energies (BE) of all samples were referenced to the C1s peak at 284.8 eV in order to compensate for sample charging. Spectra were fitted based on Gauss–Lorentzian line shapes with Shirley-type background subtraction. The fitted areas of peaks reflected the relative contents of different N-containing compounds.
2.5. Measurement of Nitrogen Supply Ability
The potential of CM and CCM products as N fertilizer was evaluated in a 42 day laboratory aerobic incubation study. A total of 100 g (oven dry basis) of air-dried 2 mm mesh-sieved soil was placed in a 400 mL plastic container, and deionized water was added to bring the soil to a 60% water holding capacity. The respective treatment samples were added at an equal N application rate of 120 kg N/ha, and the soil was mixed. Containers with soil but without amendments were used as controls. Three replicates were prepared for each treatment per sampling time. The initial weights of all of the samples were recorded, covered with pin hole parafilm, and incubated at 30 °C. At weekly intervals, the samples were removed from the incubator, aerated, and rewetted to their initial weight by adding deionized water.
To determine the N supply abilities of the different treatments, triplicates of each sample treatment were removed at 0, 7, 14, 21, 28, and 42 days, and their mineral N contents (NH
4+-N and NO3
−-N) were assessed. Samples of 10 g each for NH
4+-N and NO3
−-N for each treatment were weighed for analysis. In order to determine the moisture contents of the soils at each sampling time, 10 g samples were weighed into aluminum foil cups of known weight, dried in an oven at 105 °C for 48 hours and their weight recorded for moisture content calculations. NH
4+-N samples were extracted with a 2 M KCl solution, and the extracts were analyzed by steam distillation, while the NO
3−-N samples were extracted with a 0.5 M K
2SO
4 solution and analyzed following the nitration of salicylic acid method [
20] as reported in Logah et al. [
21]. At each sampling time, the mineral N content of the control soil (
Table A1 and
Figure A1) was subtracted from each treatment at that particular date. All of the results were reported on a dry weight basis.
2.6. Statistical Data Analysis
The data obtained from the incubation studies were statistically analyzed by one-way analysis of variance (ANOVA) using Microsoft Excel 2010. Significant means were separated using Tukey’s honest significant difference post hoc comparison. Regression analyses were performed between the initial properties of the different manures and their total mineral N contents released in the 42 day incubation.