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Nutrient transformation during aerobic composting of pig manure with biochar prepared at different temperatures ab

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Ronghua Li , Quan Wang , Zengqiang Zhang , Guangjie Zhang , Zhonghong Li , Li Wang & d

Jianzhong Zheng a

College of Natural Resources and Environment, Northwest A&F University, Yangling 712100, People's Republic of China b

Key Laboratory of Plant Nutrition and the Agri-environment in Northwest China, Ministry of Agriculture, Yangling 712100, People's Republic of China c

College of Food Science and Engineering, Northwest A&F University, Yangling 712100, People's Republic of China d

College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China Accepted author version posted online: 11 Sep 2014.Published online: 07 Oct 2014.

To cite this article: Ronghua Li, Quan Wang, Zengqiang Zhang, Guangjie Zhang, Zhonghong Li, Li Wang & Jianzhong Zheng (2014): Nutrient transformation during aerobic composting of pig manure with biochar prepared at different temperatures, Environmental Technology, DOI: 10.1080/09593330.2014.963692 To link to this article: http://dx.doi.org/10.1080/09593330.2014.963692

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Environmental Technology, 2014 http://dx.doi.org/10.1080/09593330.2014.963692

Nutrient transformation during aerobic composting of pig manure with biochar prepared at different temperatures Ronghua Lia,b , Quan Wanga , Zengqiang Zhanga,b∗ , Guangjie Zhanga , Zhonghong Lic , Li Wanga and Jianzhong Zhengd of Natural Resources and Environment, Northwest A&F University, Yangling 712100, People’s Republic of China; b Key Laboratory of Plant Nutrition and the Agri-environment in Northwest China, Ministry of Agriculture, Yangling 712100, People’s Republic of China; c College of Food Science and Engineering, Northwest A&F University, Yangling 712100, People’s Republic of China; d College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China a College

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(Received 10 December 2013; final version received 4 September 2014 ) The effects of the corn stalk charred biomass (CB) prepared at different pyrolysis temperatures as additives on nutrient transformation during aerobic composting of pig manure were investigated. The results showed that the addition of CB carbonized at different temperatures to pig manure compost significantly influenced the compost temperature, moisture, pH, electrical conductivity, organic matter degradation, total nitrogen, NH+ 4 –N and NH3 variations during composting. Compared with control and adding CB charred at lower temperature treatments, the addition of CB prepared over 700°C resulted in higher pH (over 9.2) and NH3 emission and lower potherb mustard seed germination index value during the thermophilic phase. Peak temperatures of composts appeared at 7 days for control and 11 days for CB added treatments. During 90 days composting, the organic matter degradation could be increased over 14.8–29.6% after adding of CB in the compost mixture. The introduction of CB in pig manure could prolong the thermophilic phase, inhibit moisture reduce, facilitate the organic matter decomposition, reduce diethylene triamine pentaacetic acid (DTPA) extractable Zn and Cu contents in pig manure composts and increase ryegrass growth. The study indicated that the corn stalk CB prepared around 500°C was a suitable additive in pig manure composting. Keywords: compost; biochar; carbonized temperature; pig manure; nutrient transformation

1. Introduction The disposal of the pig manure in pig farms is a serious problem in China due to its large number of live pigs in the world.[1] Composting is considered as an effective method to treat the animal droppings by transforming the organic matter into compost, which is believed to be a stable, pathogen-free product often used as a soil amendment.[2] Charred biomass (CB), biochar has been used as an amendment to improve the compost quality and to shorten the maturity process during the composting process since recently.[3–10] Generally, CB is defined as the carbonaceous product obtained when biomass is subjected to heat treatment (up to 900°C) in an oxygenlimited environment and when applied to an agronomic process as an amendment.[11] The impact of CB as an amendment depends on its physiochemical properties, such as developed micropores structure, large surface area, recalcitrant environmental stability and high adsorption capability.[12,13] And these physiochemical properties of CB were believed to be mainly controlled by the feedstock types and pyrolysis temperature.[12–14] For example, the

*Corresponding author. Email: [email protected] © 2014 Taylor & Francis

feedstock selected affect several properties with agronomic implications, including pH, ash content, H/C ratio, surface area, cation exchange capacity and plant nutrient elements’ content.[13] And the CB does not possess similar physiochemical properties even when derived from the same feedstock due to differences in production conditions.[13] In general, CB pyrolysed at higher temperatures has higher pH and ash content [15,16] and higher surface area and adsorption capacity.[17,18] Moreover, environmental stable CB was achieved due to the decrease of the volatile matter or labile organic compounds at higher charring temperatures.[19] In addition, more stable CB enhanced the nutrients availability for plants.[20] Although CB shows potential as an additive in manure composting, and many researches had proved that CB could be used as amendments to improve the microorganisms’ nitrogen assimilation and to reduce nitrogen loss in composting,[3,6,10] to reduce the heavy metals, such as Cu and Zn, mobility [3,4,6] to decrease the water-soluble carbon content,[7] to enhance microbial activity [5,8] and to facilitate the composting process. [7,10] And all these reported CB used as additives

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in composting were often produced from plant residuals, such as bamboo,[3,4,6] eucalyptus wood,[7] konara oak [8] and pine chips, [10] charred at 300–550°C. The CB charred at different temperatures and originated from different feedstock may present different performances on the agronomic process, and therefore, the CB originated from different pyrolysis temperatures and different feedstock is still need to be studied to meet desirable characteristics for soil and composting applications for the sake of nutrient conservation and soil amendment.[16,19,21,22] The grain crop straw production in China was estimated to be over 5.84 × 108 tonnes in 2010, and more than 2.43 × 108 tonnes of crop straw was originated from corn planting. And the yield of CB from corn straw would be over 7.37 × 107 tonnes which showed great potential in agricultural utilization.[23] However, to date detailed studies on the effects of CB prepared from corn stalk at different charring temperatures on nutrient transformation during aerobic composting of pig manure are still need to be fully explored. The aim of the present work was to investigate the effects of the corn stalk CB prepared at different pyrolysis temperatures as additives on nutrient transformation during aerobic composting of pig manure. The changes of temperature, moisture content, pH, electrical conductivity (EC), seed germination, nitrogen transformation, and Cu and Zn contents in relation to organic degradation were studied in composting as well. The effect of the final composts on ryegrass (Lolium perenne) growth was also investigated. The results of this study were assumed to provide practical information to guide the exploitation of CB in composting application.

oxygen condition with a heating rate at 10 K/min to a desired temperature and held for 15 min. The pyrolysis temperatures employed were 250–300, 450–500, 600–700 and 750–900°C, and the CB product was then labelled as CB300, CB500, CB700 and CB900, respectively. In order to prevent an insufficient mixing being occurred during the composting, the CB was ground to size less than 1 mm, the corn stalk collected from the local farmland was crushed to less than 5 mm, and the fresh pig manure was air dried at room temperature and ground to size less than 1 mm before use. The previous analysis showed that the heavy metals in pig manure were mainly Zn and Cu, other metals such as Pb, Hg, Ni, Cd and As were nearly undetectable. Basic properties of composting materials are given in Table 1.

2.2.

Composting procedure and sampling

In all, 15 kg dry pig manure was mixed with 30 kg dry corn stalk powder. CB prepared at different temperatures was added to the mixture with a proportion of 2.5% in dry weight. Deionized water was used for watering the dry mixture manually to set the water content around 65 ± 1%. The sample without CB was denoted as the control. After being fully mixed by hand, 40 kg ( ∼ 100 L) of each mixture was put into an experimental cuboid-shaped laboratory polyvinyl chloride composter (height 82 cm, width 40 cm and length 40 cm) with a volume of 130 L (Figure 1). The composter was enveloped by asbestos cloth (10 mm thickness) and foamed plastic textile (40 mm thickness) to form the thermal insulation layer. Air was pumped from the bottom into the composter with a constant air flow of about 0.35 L h−1 kg−1 dry weight during the thermophilic phase. And the air flow rate was automatically increased to around 0.50 L h−1 kg−1 dry weight during the curing phase. The compost temperatures and the room temperatures were recorded after the compost was automatically mixed at 9:00 am, 15:00 and 21:00pm, and the temperatures were averaged daily. During the composting, no water was added. Each treatment was in triplication during the composting. Samples were periodically collected up to 90

2. Materials and methods 2.1. Collecting and processing of raw materials Fresh pig manure was collected from a national pig breeding farm with a holding inventory of 2000 pigs in Shannxi Province, China. The CB was purchased from the Bioene Biological Energy, Guangzhou, China. The CB was prepared by carbonizing corn stalk under a limited Table 1. Basic properties of composting materials. Parameter pH EC (mS cm−1 ) Cu (mg kg−1 ) Zn (mg kg−1 ) Ash (g kg−1 ) C (g kg−1 ) N (g kg−1 ) H(g kg−1 ) O(g kg−1 ) SA (mol kg−1 ) BET (m2 g−1 )

Pig manure 8.37 1.33 1175 2174 246 422 29.8

± 0.02 ± 0.03 ± 11.02 ± 42.05 ± 0.34 ± 2.37 ± 0.30 ND ND ND ND

Corn stalk 6.75 0.03 3.27 22.0 70.2 543 11.5 66.1 302

± 0.01 ± 0.01 ± 0.11 ± 0.13 ± 4.40 ± 3.72 ± 0.03 ± 2.33 ± 2.08 ND ND

CB300 7.43 0.79 9.13 12.6 144 582 7.09 58.7 208 2.73 10.5

± ± ± ± ± ± ± ± ± ± ±

0.01 0.01 0.01 0.05 2.37 3.30 0.02 1.78 13.08 0.02 0.62

CB500 8.21 1.58 11.7 16.1 235 623 8.36 43.7 184 2.27 18.6

± ± ± ± ± ± ± ± ± ± ±

0.03 0.01 0.10 0.09 15.71 1.37 0.06 4.41 0.06 0.03 1.32

CB700 10.86 8.86 15.1 21.3 253 640 10.1 28.2 66.4 0.72 28.0

± ± ± ± ± ± ± ± ± ± ±

0.04 0.01 0.09 0.13 4.01 4.12 0.06 3.04 5.13 0.07 9.68

CB900 11.39 11.3 17.0 26.6 275 671 9.91 2.6 35.9 0.18 35.1

± ± ± ± ± ± ± ± ± ± ±

0.03 0.03 0.12 0.21 3.36 3.72 0.03 1.02 3.13 0.03 4.33

Note: Values indicate mean ± standard deviation based on the samples with four times replication. ND in table indicates not detected.

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2 1: Air intake/discharging pip

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3: Axial support system 4: Temperature detector

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5: Input / discharging hole 6:Thermal insulation layer 7: Air pump automatic control system.

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Figure 1. Schematic diagrams (a) and digital photo (b) of composting reactor. 1, Air intake/discharging pipe; 2, air distribution system, 3. axial support system; 4, temperature detector; 5, input/discharging hole; 6, thermal insulation layer; and 7, air pump automatic control system.

days. The collected samples were divided into two portions. One portion was stored at 4°C before analyses; the other was freeze-dried, crushed to pass through a 0.15-mm nylon sieve and thoroughly mixed before analyses. 2.3. Germination test The germination test was carried out by using potherb mustard (Ardisiasquamulosa Presl) seed to evaluate the phytotoxicity.[24] Compost extracts were obtained by shaking 10 g (in dried weight basis) fresh compost samples with the right amount of deionized water in a 1:5 solid:water ratio (m/V) for 4 h at room temperature in the dark, and the aqueous extracts were obtained by centrifugation and filtration through a 0.45-μm Gelman membrane filter. The extracts were used as germination media. A

Whatman No. 1 filter paper was placed inside a sterilized and disposable Petri dish and was wetted with 5 mL of each germination solution, and 40 potherb mustard seeds were placed on the paper. Deionized water was used as the control. The Petri dish was placed in an incubator and stored in the dark at 25°C for 2 days. After incubation, the number of germination seeds and the primary root length were measured and expressed as a percentage of the control as the germination index

GI =

(seed germination rate in treatment) × (root length in treatment) (seed germination rate in control) × (root length in control)

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2.4. Pot experiments The yellow loamy soil (yellow brown earth) samples were collected from the farmland in Northwest A & F University Test Station with a sampling depth of 0–20 cm. The physical and chemical properties of the soil samples were pH 8.7, 45.2% of clay particle size less than 0.01 mm, organic matter 19.1 g/kg, EC 0.3 mS/cm, total nitrogen − 1.1 g/kg, NH+ 4 –N 2.4 mg/kg, NO3 N 5.6 mg/kg, Olsen-P 11.6 mg/kg, available K 173.6 mg/kg, total Cu 48.1 mg/kg, total Zn 63.1 mg/kg, diethylene triamine pentaacetic acid (DTPA) extractable Cu 0.7 mg/kg and DTPA extractable Zn 0.2 mg/kg. A hard polyethylene pot with height 23 cm, i.d. of the bottom 19 cm and the mouth 30 cm was used as the container. Deionized water was used for watering the pots before the experiment. The pot experiments were conducted as described in the literature by Hua et al. [4]. Soil and composts were naturally air-dried in laboratory, and ground to pass 1 mm nylon sieve before being mixed. Different composts were incorporated into the soil at 5 wt.% application dosage, which was proved to be an appropriate application rate for the grass planting in the previous study. For the blank treatment, only soil was added in the pot without any fertilizer addition. Each pot contained 2 kg soil/compost mixtures, and the substrate was incubated for 15 days with a humidity of ∼ 80% field capacity after setting the treatments. The grass ryegrass (L. perenne) was used as the test plant, and the seeds of 60 grains were planted per pot. All the pots were placed in a greenhouse with controlled temperature (18–20°C) and controlled humidity (55–60%). After 35 days, the numbers and the stem lengths of the survival grass seedlings were recorded, and the grass roots and stems were separated and were put into paper bags for being dried at 60°C until constant weight was reached. Samples were weighted for the determination of dry biomass. The dry grass stems were then stored in a dehydrator before analyses.

2.5. Analytical methods The BET (Brunauer–Emmett–Teller) surface area of the CB sample was measured by N2 adsorption multilayer theory using a TriStar 3000 surface area analyzer (Micromeritics, USA). The total surface acidity (SA) of the CB sample was examined with a conventional titration method, as conducted by Mukome et al. [12]. A Thermo Flash EA-1112 Elemental Analyzer was used to determine the total carbon, hydrogen, nitrogen and oxygen element contents in samples. Ash content of samples was determined by weight loss after heating to 750°C in an oxygen atmosphere for 4 h.[12] Fourier transform infrared spectroscopy (FTIR) absorbance spectra of KBr pellets prepared with CB were recorded between 400 and 4000 cm−1 with 64 time scans averaged with a resolution of 4 cm−1 (Bruker Tensor 27 FTIR Spectrometer). The surface morphology of CB samples was studied using

a scanning electron microscope (SEM) using a Hitachi − S-4180 FE-SEM operating at 5 kV. NH+ 4 –N, NO3 N, moisture, EC and pH of the compost were determined with fresh samples. Moisture content was determined by the sample weight loss dried at 105 °C in an oven for 24 h. EC and pH of the samples were determined by using a pH and − ion analyser.[2] NH+ 4 –N and NO3 N were extracted in a shaker at 160 r/min for 1 h with 50 mL of 2 mol/L KCl (solid:extractant, 1:5 (m/V)). The extracts were filtered through a qualitative filter paper. NH+ 4 –N was determined by the Indophenol Blue colorimetric method. NO− 3 N was determined by sulphanilamide and N-1-naphthylethylene diamine dihydrochloride reaction after cadmium reduction to NO− 2 N.[25] Ammonia in compost was determined by using an ammonia-gas-sensing electrode (pNH 3-1, Huier, Hangzhou, China) and a standardized curve method, with a detection range of 10−5 –10−1 mol/L and with a detection limit of 10−6 mol/L. Total nitrogen, organic matter and Cu and Zn contents were evaluated with air-dried samples. Total nitrogen was determined by Kjeldahl methods.[25] Organic matter (OM) was measured with dried samples incinerated at 550°C for 24 h in a muffle furnace.[2] The DTPA-extractable and total contents of Cu and Zn in compost at 1st and 90th day were measured using the method described in the literature.[3] Cu and Zn contents were determined by an atomic absorption spectrophotometer (Hitachi Z-2000). In order to reveal the impact of the CB addition in compost on the Cu and Zn uptake of ryegrass seedlings, one portion of dry ryegrass stems were digested by concentrated HNO3 –H2 O2 . During the Cu and Zn analyses throughout the study, about 30% paralleled replication of samples was used as a quality control procedure and spiked reference samples (GBW-08501) were used for quality assurance and quality control. All the treatments were repeated four times. Statistical analysis was performed by SPSS 18.0 software in a one-way analysis of variance test with a probability defined at 0.05 to separate treatment means for the mass data.

3. Results and discussion 3.1. Influence of pyrolysis temperature on biochar properties The basic properties of CB prepared at different temperatures are summarized in Table 1. The properties of CB such as pH, ash content and surface area increased with increasing charred temperature.[15–17, 26] And the decrease in the H/C ratio indicates the improvement in the CB environmental stability.[19] The H and O contents decreasing with pyrolysis temperature would lead to the decrease in the SA of CB.[12] Figure 2 shows the micrographs obtained by SEM for four CB samples and the raw corn stalk sample. Overall, the plant cellular structure was clearly visible in the raw corn stalk (Figure 2a), while CB particles were much finer and the edges and corners in the surface were

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Figure 2. SEM pictures of the raw corncob powder (a), CB300 (b), CB500 (c), CB 700 (d), CB900 (e) and the FTIR analysis of the samples (f).

become invisible with increasing carbonized temperatures. The similar phenomena were found by other researchers also during the preparation of CB at different pyrolysis temperatures by using orange peel, pine needle, sugar cane bagasse and wood as raw materials.[27,28] The evolution of FTIR spectra of CB as a function of charring temperature is shown in Figure 2f. The broad peak at 3428 cm−1 represented the bonded hydroxyl of phenol group. The peak around 2936 and 2885 cm−1 was indicative of aliphatic−CH stretching. The peaks at 1609 were assigned to C=O stretching of the carboxylic carbonyl group and conjugated C=C bond.[18] The peak at 1510, 1440 and 1160 cm−1 related to C=C stretch and

C−O−C bonds associated with aromatic components in cellulose, hemicellulose and lignin.[29] The raw corn stalk and CB300 spectra displayed a number of peaks, indicating the complex in nature. Pyrolysis temperature impacted the functional groups on the CB, with CB produced at a lower temperature showing more functional groups, which is in accordance with decreasing O and H contents given in Table 1. Compared to raw corn stalk, with the charring temperature increase, the peaks of CB at 3428, 2936, 2885, 1609, 1510 and 1160 cm−1 reduced, and the peak at 2912 cm−1 (−CH and carboxylic groups) disappeared. While the progressive aromatization is obvious for CB starting from the intermediate temperature, as evidenced from

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the ratios between the C=O and aromatic C=C stretching bonds noticeably decreasing with increasing pyrolysis temperature.[29] Approaching the high temperature, the clearly visible bonds that remain from the bark and leaf chars are those attributed to C=C aromatic ring stretching (1609–1440 cm−1 ) and aromatic C—H out-of-plane bending (885–750 cm−1 ). These results confirmed that the remaining CB structure at high temperature is predominantly aromatic rings during pyrolysis at high temperature. Overall, the FTIR analysis showed that the reduction in all

the assigned oxygen functional groups and the increase in the more recalcitrant and aromatic structures were formed in the CB with increasing temperature.[30,31]

3.2.

Variation of temperatures, moistures, pH and EC during composting Peak temperatures of composts appeared at 7 days for control and 11 days for CB added treatments (Figure 3a). During the thermophilic phase, the maximum temperatures

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Figure 3. Variation of temperature (a), moisture content (b), pH (c), EC (d), organic matter (e) and C/N (f) during the composting process.

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Environmental Technology in relation to the increase in different CB preparation temperatures were 64.6°C, 70.9°C, 72.2°C, 72.8°C and 71.9°C. The thermophilic temperatures were rapidly reached over 55°C because of microbial respiration, indicating that adequate feedstock compositions could support aerobic microbial degradation. After the thermophilic phase, the temperature in all the treatments gradually decreased before reaching room temperature. The results in this study were in accordance with the temperature variations reported in sludge or poultry manure composted with the CB from bamboo,[3,6] eucalyptus wood,[7] konara oak [8] and pine chips.[10] The addition of CB prepared at different temperatures in pig manure compost appeared to improve the quick temperature rise and to prolong the thermophilic phase during composting. Figure 3b shows the effect of CB on the moisture content. The moisture contents of all the samples decrease throughout the entire composting process: from 66.4% to 31.4% in control, from 66.3% to 33.6% in the samples with CB. During the 90 days composting, the moisture decrease could be buffered by the addition of CB. The moisture contents in samples with CB were higher than in the control, the contents were also increased with the CB carbonized temperatures. The results might be attributed to the waterholding capacity of the CB, and the capacity might be increased with the carbonization temperatures which could increase the surface area.[26] Compared with the control, the addition of CB had a significant effect on compost pH. The addition of CB increased the initial pH of the composting mixture from 8.1 to 9.4 (Figure 3c). The initial pH increased with the CB carbonization temperatures. This was possibly because the higher temperature resulted in the higher pH value of CB,[14–16] which is given in Table 1. Wong et al. [32] and Fang and Wong [33] also pointed out that the addition of alkalinity substance (lime and fly ash) could increase the initial pH of the composting mixture to nearly 9.2. During the composting, pH of the control increased from 8.1 to 8.6 and then levelled to 7.7. The initial pH increase in compost was known to the occurrence of ammonification in the early compost stage.[32,33] The decrease in pH at the later stage might be due to the formation of low-molecular weight fatty acids and CO2 during OM degradation.[10,34] EC is usually measured during composting because it reflects the salinity of the composting product and its suitability for plant growth.[33] The EC values of all treatments showed a steady increase with the composting time in the early stage (Figure 3d). The EC increase is possibly caused by the net loss of weight compost and the release of soluble salts through the OM decomposition.[34] For all the treatments, EC values were lower than the standard of 4 mS/cm provided by Garcia et al. [35], indicating the low plant growth inhibition when applied to soil.

3.3.

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Changes in organic matter and C/N

The organic matter content in the mixture experienced a relatively fast decline and then decreased steadily during composting (Figure 3e). This might be caused by the mineralization of organic matter under the aeration condition.[36] With the increase in CB preparation temperatures, organic matter contents reduced by 14.2%, 16.3%, 16.9%, 18.0% and 18.4%, respectively. The total organic matter loss of the composting materials of all treatments slightly increased with the CB preparation temperatures, indicating that the addition of biochar in compost mixtures could increase the organic matter degradation.[3,6,7] The results were in accordance with the observations in eucalyptus grandis biochar,[8] and konara oak tree biochar [7] added poultry litter composting. The decrease in C/N ratio with composting time could be used as an indicator of the stability of the composting process.[37] In the study, C/N ratio variation is shown in Figure 3f. During the composting, the C/N ratios of all the treatments increased in the first several days. And then, the C/N ratios gradually decreased to around 20 which showed a satisfactory maturation of the composts.[33] The increase in C/N ratios at the beginning might be attributed to the ammonia volatilization, while the decrease at the later stage could be caused by organic matter degradation and nitrogen compound mineralization.[34]

Variation of total nitrogen, NH4+ –N , NO− 3 –N and NH3 during composting The total nitrogen concentrations in the composting mixtures with CB300 and CB500 nearly increased throughout the entire composting. While in the control, CB700 and CB900 added treatments, the total nitrogen concentrations in the composting mixtures decreased significantly during the first 7 days and then increased afterwards (Figure 4a). The loss of total nitrogen at the beginning of the composting stage might be due to the loss of ammonia volatilization at relatively higher temperatures. From the point of biochar preparation, CB generated at 700°C and 900°C has the higher surface area than CB obtained at 300°C and 500°C (Table 1), and the resulting CB700 and CB900 with higher surface area values should have stronger NH3 adsorption ability than CB300 and CB500. However, in this study, the total nitrogen concentrations in CB 700 and CB 900 treatments at the beginning stage of the composting were not only lower than that in the CB300 and CB500 added treatments, even lower than that in the control. This might be caused by the fact that the CB generated under low temperature has more acidic groups in its structure and lower pH than CB prepared under high temperature (Table 1). Biochars generated at low pyrolysis temperatures could adsorb NH3 and inhibited NH3 volatilization effectively if acidic functional groups were present on the biochars surface or if the biochars had a low 3.4.

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− Figure 4. Total nitrogen (a), NH+ 4 –N (b), NO3 –N (c) and NH3 (d) variation during the composting.

pH,[16,17] and carbonization temperature around 500°C might be optimal for the preparation of biochar with a higher capacity for NH3 adsorption.[38] Hence, the total nitrogen concentrations in the composting mixtures with CB300 and CB500 were slightly higher than that in other treatments at the beginning of the composting stage. The increase in total nitrogen at the later stage in all treatments was possibly because of the continuous degradation of organic compounds.[3,10] The NH+ 4 –N contents increased during the first 14 days and the peak NH+ 4 –N contents were 1.1, 1.0, 1.0, 0.9, 0.8 g/kg, with the increase in CB preparation temperatures. After 14 days, the NH+ 4 –N contents rapidly decreased with composting, and only small amounts of NH+ 4 –N were detected in the final compost, which is given in Figure 4b. NH+ 4 –N contents of all treatments increased significantly with temperatures and pH at the beginning of composting, which might be due to ammonification.[33] After 14 days, the NH+ 4 –N contents decreased significantly in all treatments, this might be caused by the ammonia volatilization and the micro-organisms immobilization and − the NO− 3 –N formation.[2] The NO3 –N contents showed a steady increase in all treatments (Figure 4c). The increase in NO− 3 –N content indicated the composting development because the weakening of NH+ 4 –N was considered to be relevant with the increase in NO− 3 –N contents during the

aerobic composting process. The increase in NO− 3 –N contents in all the treatments was non-significant during the aerobic composting process (Table 2). The NH+ 4 –N concentrations in all the final composts were lower than 0.4 g/kg, which proved the maturity and stability of the final composts.[37] Ammonia concentrations peaked after 7 days during the composting (Figure 4d). Mean concentrations were significantly reduced in the CB300 and CB500 treatments, with the mean reduction of 26.6% and 55.4%, respectively. The emissions of NH3 from animal manure were influenced by pH and the NH4 + –NH3 transformation equilibrium. Generally, high pH favours the formation of ammonia and low pH benefits the ammonia protonation in the compost.[37] The amendments, especially CB700 and CB900 used in this study, had a pH value above 9, which favoured NH3 losses. However, the concentrations of NH3 in the CB300 and CB500 treatments were lower, which might be because the lower pyrolysis temperatures benefited the biochar preparation from biomass with better NH3 adsorption abilities.[14,17,38] 3.5.

Variation of the total and DTPA extractable Zn and Cu contents before and after composting During the composting, the variation of total Zn, Cu and DTPA-extractable Zn and Cu at the 1st and 90th day

*Snedecor F 5,∞ (p = 0.05). − Note: The effect of CB charring temperature on the compost temperature, moisture, pH, EC, organic matter, C/N, total nitrogen, NH+ 4 –N, NO3 –N, NH3 and GI.

1.01 No 2.90 Yes 0.18 No 3.30 Yes 2.39 Yes 2.74 Yes 4.66 Yes 21.68 Yes 3.17 Yes

14.74 Yes

0.24 0.23 0.29 0.21 0.23 14042.72 7760.48 2824.77 17,901.37 23,533.76 0.06 0.09 0.12 0.19 0.22 0.14 0.20 0.34 0.10 0.08 26.34 32.46 37.92 37.90 26.19 164.65 145.02 156.78 196.84 199.46 30.64 30.63 30.48 34.94 34.82 0.23 0.22 0.22 0.15 0.12 0.09 0.12 0.21 0.14 0.20 130.44 136.98 119.79 122.04 127.87

Treatments

Control 130.34 CB300 259.73 CB500 274.66 CB700 276.24 CB900 273.84 Minimum F = 2.21* F obtained 27.08 Significant Yes difference?

pH (Figure 2c) Moisture (Figure 2b) Temperature (Figure 2a)

Variance of the data

Table 2. Results of the one-way ANOVA analysis.

EC (Figure 2d)

Organic matter (Figure 2e)

C/N (Figure 2f)

Total nitrogen (Figure 3a)

Downloaded by [Universite Laval] at 09:19 08 October 2014

NH+ 4 –N (Figure 3b)

NO− 3 –N (Figure 3c)

NH3 (Figure 3d)

GI (Figure 4)

Environmental Technology

9

is given in Table 3. With the addition of CB, the total Zn and Cu concentrations in the 1st day in all the treatments decreased because CB amendment has lower Zn and Cu contents. After 90 days composting, the total Zn and Cu contents were increased in all the treatments. In the study, the increases in total Zn and Cu concentrations were due to the decomposition of organic matters and moisture reduction because the compost reactors were sealed and no leachate loss occurred. However, the ratio of DTPA-extractable Zn and Cu contents to the total Zn and Cu contents was significantly decreased after 90 days. This was probably due to the fact that the biochar–pig manure mixture presented a large quantity of organic functional groups, which could effectively reduce the mobility of heavy metal ions through metal–humic complex formation.[7] The tendency of the extractable metal contents was consistent with the results in bamboo charcoaladded pig manure composts.[3] The decrease in the DTPA extractable Zn and Cu ratios indicated that the addition of CB to the composting mixtures could reduce the bioavailability of Zn and Cu because the DTPA extractable content of heavy metals could be used to assess the mobility of heavy metals in most cases. The percentages of DTPAextractable Zn reduction were 2.0%, 5.8%, 9.9%, 8.6% and 8.7% in the order of CB carbonization temperatures. And the percentages of the DTPA-extractable Cu reduction were 11.5%, 22.3%, 24.8%, 23.1% and 22.1% with the increase in CB carbonization temperatures. Although the reduction of DTPA-extractable Zn and Cu in this study was lower than the results obtained by Hua et al. [6] and Chen et al. [3], these results were comparable to some extent. For example, Chen et al. [3] also found that the ratio of DTPA extractable Cu was higher than that of DTPA extractable Zn during the pig manure aerobic composting with biochar. The deviation between the two studies could be explained by the fact that the CB were of different feedstock sources, chemical compositions and carbonization temperatures during the pyrolysis process.[14,16,19] The properties of different CB additives could affect the behaviour and performances of metals during composting.[2,33,36]

3.6. Change of germination index during composting Plant toxicity was usually used to estimate compost maturity and quality by seed germination assays. The GI obtained from each treatment clearly showed a tendency of gradual decrease in phytotoxicity with the composting time, which is presented in Figure 4. In the initial stage of composting, the lower GI values in all the treatments could result from the higher pH or the accumulation of toxic compounds such as alcohols, phenolic compounds, lowmolecular weight fatty acids, ammonia and toxic nitrogen compounds.[39] After 30 days of composting, the GI values in CB700 and CB900 treatments were still lower than

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R. Li et al.

Table 3. Total Zn, total Cu, DTPA-Zn and DTPA-Cu variation during the composting. Treatments Control CB300 CB500 CB700 CB900

Time (d) 1 90 1 90 1 90 1 90 1 90

Total Cu (mg/kg) 393.9 451.3 384.5 442.8 384.6 446.2 384.7 454.0 384.7 455.6

± ± ± ± ± ± ± ± ± ±

3.10a 0.88b 2.18a 4.13b 1.11a 6.09b 1.04a 2.63b 4.01a 10.03b

DTPA-Cu (%) 33.84 22.30 33.61 11.31 33.48 8.66 31.89 8.78 31.13 9.03

± ± ± ± ± ± ± ± ± ±

Total Zn (mg/kg)

0.18a 0.21b 0.24a 0.08c 0.11a 0.10d 0.03a 0.30d 0.12a 0.26d

739.2 846.8 721.5 831.0 721.5 837.8 721.7 851.9 721.9 845.5

± ± ± ± ± ± ± ± ± ±

10.20a 2.18b 6.11a 0.98b 5.33a 2.61b 2.94a 1.72b 3.55a 1.98b

DTPA-Zn (%) 20.82 18.86 20.12 14.33 19.88 10.03 19.12 10.51 18.87 10.16

± ± ± ± ± ± ± ± ± ±

0.24a 0.19b 0.11a 0.23c 0.07a 0.26d 0.15a 0.34d 0.19a 0.31d

Downloaded by [Universite Laval] at 09:19 08 October 2014

Note: Values indicate mean ± standard deviation based on the samples with four times replication. Data in a column with the same letter mean that there were no significant differences at p < 0.05.

0.25, even while the values of GI in control, CB300 and CB500 treatments after 30 days composting were 0.52, 0.72 and 0.84, respectively. As GI values should be higher than 0.5 in mature composts,[24] the results in Figure 5 indicate that the composts from the control, CB300 and CB500 treatments were matured after 30 days composting; while for the CB700 and CB900 added treatments, a longer composting time is needed before maturity. Overall, judging from the maturity indices established for the compost of different sources given in Table 4, after 90 days composting the final products in all the treatments could be classified as non-phytotoxic. Figure 5.

3.7.

GI variation during composting.

Effect of CB amended compost on ryegrass seedling growth

The effect of CB amended composts on ryegrass seedling growth in pot experiments is given in Table 5. Neither the control compost nor the CB amended composts had apparent influence on the ryegrass seedling survival. Compared with the blank treatment, yields of ryegrass seedlings treated with composts increased significantly, and the peak weights of biomass were obtained with the CB500-treated

compost. Biomass of ryegrass seedlings in CB-added composts was 17.1–52.5% higher than that in seedlings without the CB-treated compost, and was 82.6–137.9% higher than that in the blank treatment. The differences in yields of ryegrass biomass indicated that CB-amended compost effectively promoted ryegrass seedling growth. Hua et al. [4] also found that Fescue (Festuca arundinacea) and

Table 4. The maturity indices established for compost of different sources.

Treatments Control CB300 CB500 CB700 CB900 Standard value Maturity?

GI 1.12 ± 0.00 1.14 ± 0.09 1.30 ± 0.06 1.01 ± 0.06 0.99 ± 0.07 > 0.5 [24] Yes

−1 NH+ 4 –N (m g kg )

103.8 ± 3.5 111.4 ± 0.3 44.6 ± 6.5 4.55 ± 0.07 4.08 ± 0.02 < 400 [37] Yes

Maturity indices and values − NH+ 4 –N/NO3 –N 0.10 ± 0.02 0.10 ± 0.03 0.04 ± 0.01 0.004 ± 0.000 0.003 ± 0.000 < 0.16 [37] Yes

C/N

pH

18.34 ± 1.03 16.50 ± 0.39 15.57 ± 0.09 15.55 ± 0.09 17.24 ± 1.34 < 20 [37] Yes

7.74 ± 0.06 7.87 ± 0.10 7.74 ± 0.06 8.18 ± 0.04 8.28 ± 0.07 7–9 [37] Yes

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Environmental Technology Table 5.

Effect of CB amended compost on ryegrass (Lolium perenne) seedling growth in pot experiment.

Treatments Survival ratio (%pot−1 ) Root weight (g pot−1 ) Stem and leaves weight (g pot−1 ) Cu content in Stem and leaves (mg kg−1 ) Zn content in Stem and leaves (mg kg−1 )

Soil 98.3 0.22 0.32 5.7 11.7

± ± ± ± ±

1.0a 0.01a 0.03a 0.29a 0.33a

Control 99.0 0.39 0.45 17.0 28.4

± ± ± ± ±

CB300

0.8 a 0.01b 0.02b 2.30b 1.12b

98.8 0.54 0.65 9.5 14.5

± ± ± ± ±

0.5a 0.04c 0.04c 1.83c 0.23c

CB500 99.3 0.60 0.67 6.4 9.5

± ± ± ± ±

0.5a 0.02d 0.03c 1.56d 0.21d

CB700

CB900

± ± ± ± ±

97.3 ± 1.71a 0.45 ± 0.02f 0.52 ± 0.05e 6.6 ± 0.83d 9.8 ± 2.04d

97.8 0.50 0.58 6.5 10.0

2.2a 0.02e 0.01d 1.04d 1.68d

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Note: Values indicate mean ± standard deviation based on the samples with four times replication. Data in a row with the same letter mean that there were no significant differences at p < 0.05.

ryegrass (L. perenne) yields were significantly improved by the addition of bamboo biochar–sludge compost. The Cu and Zn concentrations in ryegrass stem and leaves increased as a result of the application of composted pig manure. The addition of CB in the compost could significantly reduce the Cu and Zn uptake in the ryegrass seedlings compared with the compost without CB addition. The contents of Cu and Zn in ryegrass stem and leaves were lower in the CB500 treatment than those in other treatments. 4. Conclusions An addition of CB carbonized at different temperatures to pig manure compost significantly influenced the nutrient transformation during the aerobic composting process. The addition of CB could prolong the thermophilic phase, improve organic matter decomposition and reduce DTPA extractable Zn and Cu contents in pig manure composts. Compared with control and adding CB charred at lower temperature treatments, the addition of CB prepared over 700°C resulted in higher pH and NH3 emission and lower potherb mustard seed GI value during the thermophilic phase. Judging from NH3 loss, GI progress, ryegrass seedling growth, and Cu and Zn contents in stem and leaves, CB prepared at temperatures around 500°C was a suitable amendment for pig manure composting. Acknowledgements Funding for this research was provided by National Natural Science Foundation of China (No. 41101288), Shannxi Province Natural Science Foundation of Research Projects (No. 2013JM3011) and Northwest A&F University Young Scholar Research Projects (2014YB064). Mr. Yanjun Yin in Northwest A&F University was thanked for his constructive advices. Jiaojiao Zhou, Xuan Wang and other students who took part in the 2013 Undergraduate Research Project of Northwest A&F University were also thanked for their help. The anonymous reviewers are gratefully acknowledged for their valuable comments and suggestions that helped to substantially improve this article.

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