Chemical structures and characteristics of animal manures and composts during composting and assessment of maturity indices

Changes in physicochemical characteristics, chemical structures and maturity of swine, cattle and chicken manures and composts during 70-day composting without addition of bulking agents were investigated. Physicochemical characteristics were measured by routine analyses and chemical structures by solid-state 13C NMR and FT-IR. Three manures were of distinct properties. Their changes in physicochemical characteristics, chemical structures, and maturity were different not only from each other but also from those with addition of bulking agents during composting. Aromaticity in chicken manure composts decreased at first, and then increased whereas that in cattle and swine manure composts increased. Enhanced ammonia volatilization occurred without addition of bulking agents. NMR structural information indicated that cattle and chicken composts were relatively stable at day 36 and 56, respectively, but swine manure composts were not mature up to day 70. Finally, the days required for three manures to reach the threshold values of different maturity indices were different.


Sample preparation and samplings
For composting, the manures were put in a cool and ventilated place to allow them to dry naturally until their moisture contents reached about 60% (w/w). Then 12 kg each of three manures were composted separately in impervious plastic boxes (44 L volume, 53cm length x 38cm width x 22cm height), with three replicates for each manure. All the boxes were maintained in a temperature adjustable incubator for 70 days. The incubator was opened to ventilate for 15 minutes, and temperature recorded at the same time every day. The incubator temperature was adjusted to 1°C below that of manure composts to reduce heat loss. During the first month, the manure composts were turned and mixed every day to provide aeration, and water was immediately added to maintain a moisture content of 60% (w/w). After one month, they were turned and mixed every other day to promote aerobic decomposition. The triplicate samples of each manure or compost were collected at day 0, 3, 7, 16, 24, 30, 36, 44, 56 and 70. A portion of the samples were air dried and ground through a 100-mesh sieve for NMR, FT-IR and elemental analysis, and the remaining portion stored in a refrigerator at -20 ℃ for other analyses

Chemical analyses
The elemental compositions (%C, %N, %H, and %O) were determined by an Elementar Vario EL cube elemental analyzer through dry combustion at 1150 °C [1].

FT-IR spectroscopy
The FT-IR analyses were conducted on a Nicolet 8700 IR spectrometer. Each S3 sample was prepared by grinding 3 mg of the sample with 300 mg of oven-dried KBr in a vibrating puck mill and then compressing about 150 mg of the mixture into a translucent pellet under a hydraulic compressor. The pellet was immediately placed on a sample holder, and a spectrum ranging from 4,000 to 400 cm −1 was recorded under the conditions of 4 cm -1 wavenumber resolution and 32 scans. Pure KBr spectra were used as the background [2]. The major peaks (intensity and wavenumber) were found by using OMNIC software.

Statistical analysis
The FT-IR absorbance spectra were processed with OMNIC 8 spectroscopy software. The spectra were normalized by setting the highest value of each spectrum to 1 absorbance unit.

Elemental compositions during composting
The changes in elemental compositions of the three manures and composts at different composting phases are shown in Table 1. Carbon was most abundant in all of the three manures. Swine manure had an obviously a higher carbon content (37.5%) than those of cattle manure (30.8%) and chicken manure (27.0%). Also chicken manure contained a higher nitrogen content (5.3%) than those of swine manure (4.7%) and cattle manure (2.9%). Swine manure had an obviously higher hydrogen content (6.4%) than those of cattle manure (3.8%) and chicken manure (4.2%). Moreover, chicken manure contained a higher oxygen content (37.6%) than those of swine manure (34.0%) and cattle manure (32.1%). Consequently, the C/N ratios of chicken S4 manure (5.1) and swine manure (8.0) were lower than that of cattle manure (10.7).  much for all the three manure composts. All the differences described above were significant based on Duncan's multiple range tests at P < 0.05.

FT-IR analysis S1 Fig FT-IR spectra of swine, cattle and chicken manure composts during composting
The FT-IR spectra of the cattle, swine and chicken manure composts at different stages (0, 7, 24, 44, and 70 days) exhibited similar spectral patterns (S1 Fig 1), except the differences in the intensity of certain bands. The main absorbance bands were those around 3300-3400 cm -1 , attributed to the intra-molecular and inter-molecular S6 hydrogen-bonded OH groups in phenol, carbohydrate and carboxylic acid compounds, as well as N-H stretching in amines and amides [6]. The two distinct small peaks at 2930 and 2854 cm -1 were ascribed to the stretching of aliphatic CH 3 and CH 2 , respectively [7]. A pronounced peak at 1650 cm -1 was attributed to the C=O stretching of amide groups (amide I) and aromatic C=C vibrations, as well as to symmetric stretching of COOgroups. A small peak at 1540cm -1 was assigned to vibrations in aromatic rings [8] and Spectral differences were observed among different manure composts. Compared with the spectra of the swine and cattle manure composts (S1 Fig(a) and (b)), the band around 3400 cm -1 became less intense and shifted toward a lower wavenumber at 3300 cm -1 in the spectra of the chicken manure composts (S1 Fig (c)). A small sharp peak at 2520 cm -1 was observed only in the spectra of chicken manure composts, which can be attributed to S-H stretch of aromatic or nonaromatic mercaptans and sulfides. Spectral differences were also observed in the 1540-1430 cm -1 region. The bands at 1540 cm -1 were distinct in the spectra of swine manure composts but smaller in the spectra of cattle and chicken manure composts. However, the bands at 1430 S7 cm -1 were stronger in the spectra of chicken manure composts but weaker in the spectra of cattle and swine manure composts. The bands at 870, 765 and 612 cm -1 were distinct in the spectra of chicken manure composts but weaker in those of cattle and swine manure composts. These bands are preferentially assigned to vibrations in proton-substituted aromatic rings.
The FT-IR spectra did not differ noticeably among different composting stages.
Only slight reduction of peaks in the aliphatic region at 2930 and 2854 cm -1 for cattle manure composts during the thermophilic phase was observed (S1 Fig (b)), possibly due to preferential biodegradation of aliphatic structures [12]. In addition a decrease of peak intensity in the aromatic region at 1650 cm -1 for chicken manure composts (S1 Fig (c)) and an increase of peak intensity in the polysaccharide region at 1030 cm -1 in swine (S1 Fig (a)) and chicken manure composts were observed as the composting proceeded.

Evolution of elemental composition during composting
The elemental composition evolution of manures and composts during composting in the present study was similar to that of most other studies [13,14], showing the decreased trends of C and N contents. However, Sá nchez-Monedero et al. [15] and Huang et al. [16] observed the increase trend of N in humic acids as composting proceeded. They attributed the increase of N to incorporation of N in humic acids by condensation of lignin with proteins and formation of refractory, complex structures between nitrogen and the humic 'core'. The decreased trend of C/N S8 ratios for swine and cattle manure composts during our composting was also similar to that of other studies [13,[17][18][19]. Nevertheless, the change of C/N ratio for chicken manure composts in many studies was in disparity, with some showing the increase trend [13,20] in agreement with our results but others indicating the decrease trend [17,21]. Antil et al. [21] ascribed the decrease of C/N ratio to the decreased carbon content and increased N content in their study. The chicken manures and composts in the present study had greater N and pH but lower C/N ratio than swine and cattle manures and composts. The high pH in chicken manure composts contributed to faster losses of N relative to C via NH 3 volatilization [22] and thus as composting proceeded, the C/N ratio of chicken composts increased.