Straw biochar hastens organic matter degradation and produces nutrient-rich compost

Bioresource Technology 200 (2016) 876–883

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Straw biochar hastens organic matter degradation and produces nutrient-rich compost Jining Zhang a,b, Guifa Chen a,b, Huifeng Sun a,b, Sheng Zhou a,b,⇑, Guoyan Zou a,b,c a

Eco-environmental Protection Research Institute, Shanghai Academy of Agricultural Sciences, Shanghai 201403, PR China Shanghai Engineering Research Centre of Low-carbon Agriculture (SERCLA), Shanghai 201415, PR China c Shanghai Co-Elite Agricultural Sci-Tech (Group) Co., Ltd, Shanghai 201106, PR China b

h i g h l i g h t s  Straw biochar improved water-soluble nutrient ions contents during composting.  Straw biochar enhanced the degradation of organic matter and maturity.  The product with biochar addition became mature after composting for 42 days.  A final concentration of 10–15% straw biochar was optimal.

a r t i c l e

i n f o

Article history: Received 19 August 2015 Received in revised form 5 November 2015 Accepted 7 November 2015 Available online 12 November 2015 Keywords: Composting Crop straw Straw biochar Pig manure Water-soluble nutrients

a b s t r a c t Biochar derived from wheat straw was added to pig manure in amounts equivalent to 5%, 10%, or 15% (w/ w, wet weight). The ratios of NH+4/NO3 and of UV light absorption at a wavelength of 254 nm (SUV254) and dissolved organic carbon (DOC) indicated that compost with 10–15% biochar became more mature and more humified within 42 days of composting, and the content of DOC and the concentration of NH+4 in such compost decreased by 37.5–62.0% and 4.0–20.9%, respectively, compared to the corresponding levels in the control. Addition of biochar lowered the pH and increased electrical conductivity by 7.0– 37.5% compared to the control and also increased the concentrations of water-soluble nutrients including PO34 (5.6–7.4%), K+ (14.2–58.6%), and Ca2+ (0–12.5%). It is therefore recommended that straw biochar be added to pig manure at 10–15% by weight. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Animal manure and crop residues, especially straw, are waste materials from modern agricultural ecosystems. In China, the total excrement produced by livestock in 2011 was about 2.2 billion tonnes and the yield of straw was about 0.7 billion tonnes (China Agricultural Statistics Yearbook, 2012). Composting kills pathogens present in these waste materials and generates humus. In fact, the composting of organic materials rich in nitrogen (animal manure and kitchen waste, for example) together with organic materials low in nitrogen (straw, rice husk, dry leaves, newsprint, sawdust, etc.) – collectively referred to as bulking agents – produces better manure by adjusting its moisture content and its carbon-tonitrogen (C/N) ratio. Mixtures of animal dung and straw, rather ⇑ Corresponding author at: Eco-environmental Protection Research Institute, Shanghai Academy of Agricultural Sciences, Shanghai 201403, PR China. Tel.: +86 21 37195163; fax: +86 21 62202503. E-mail address: [email protected] (S. Zhou). http://dx.doi.org/10.1016/j.biortech.2015.11.016 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.

than either of the components alone, are therefore more suitable for composting (Das et al., 2011; Zhu, 2007). Biochar produced by pyrolysis of carbonaceous material, which increases the degradation of organic matter (Dias et al., 2010; Enders et al., 2012; Sánchez-García et al., 2015) and adsorbs gaseous NH3 and water-soluble NH+4 (Hua et al., 2009; Malin´ska et al., 2014; Steiner et al., 2010), could be an ideal addition to a composting system. For example, when poultry manure was mixed with wood biochar, as much as 73% of the organic matter present initially was degraded (Dias et al., 2010); when mixed with sawdust and coffee husk, the corresponding figures were 65% and 84%, respectively. Addition of wood biochar led to faster humification and increased the contents of aqueous fulvic-acid-like compounds and humic-acid-like compounds by 13–26% and 15–30%, respectively, compared to the control (Zhang et al., 2014). Steiner et al. (2010) reported a process of composting poultry manure that involved adding wood biochar and found that addition of 20% (w/ w) biochar hastened the rate of biodegradation and reduced the emissions of ammonia by 52%. Adding bamboo biochar to sewage

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sludge reduced the amount of nitrogen lost during the composting process: 9% biochar (w/w) reduced nitrogen loss by 64% (Hua et al., 2009). Straw is widely available in the world and especially in China. Straw serves not only as a bulking agent in composting but is also as a product of pyrolysis for resource recycling. Instead of burning straw in situ, turning it into biochar could make it a tool for carbon sequestration (Lehmann et al., 2006): burning straw in the open emits CO2, CH4, and N2O—the greenhouse gases linked to global warming—which is why burning straw is illegal in many Asian countries. Secondly, turning straw into biochar and using it as a soil amendment proved better than directly incorporating it into soil by ploughing in terms of higher yields and improved physicochemical properties of soil (Liu et al., 2012). Thirdly, the biochar commonly added during the composting of organic matter is wood biochar, which, however, has low ash content (Brewer et al., 2011; Spokas et al., 2011) and thus did not provide macro- and micronutrients in sufficient quantities after composting (Glaser and Birk, 2012; Sánchez-García et al., 2015), and mineral and organic fertilizers had to be added to facilitate crop growth and development (Schulz and Glaser, 2012). Biochar based on animal manure, on the other hand, is richer in essential nutrients than plant-based biochars (Sarkhot et al., 2012). However, when used in composting, its high pH leads to emissions of ammonia and carbon dioxide (Enders et al., 2012; Sarkhot et al., 2012). Little is known about the interaction between straw biochar and compost quality; however, the quality of compost usually depends on the level of plantavailable nutrients. It was with this background that straw biochar pyrolyzed at medium temperature was mixed with agricultural waste (pig manure and straw) to be composted to investigate its effect on biodegradation of organic matter. The present experiment also sought to understand how biochar affects compost quality and water-extractable nutrients to arrive at the most optimal proportions in which straw biochar, animal manure, and straw should be mixed to obtain high-quality biofertilizer.

Table 1 Characteristics of the raw materials used for composting. Items

Swine manure

Wheat straw

Straw biochar

Moisture content (%) pH EC (ms cm 1) DOC (g kg 1) DON (g kg 1) C (%) N (%) C/N Water-soluble NH+4 (g kg 1) Water-soluble K+ (g kg 1) Water-soluble Na+ (g kg 1) Water-soluble Ca2+ (g kg 1) Water-soluble Mg2+ (g kg 1) Water-soluble NO3 (g kg 1) Water-soluble PO34 (g kg 1) Water-soluble Cl (g kg 1)

68.9 ± 0.5 7.52 ± 0.1 1.88 ± 0.1 5.07 ± 0.0 0.93 ± 0.08 34.4 ± 0.9 2.39 ± 0.2 14.4 0.88 ± 0.04 3.12 ± 0.37 0.50 ± 0.25 0.64 ± 0.04 0.56 ± 0.01 0.02 1.58 ± 0.19 0.46 ± 0.19

10.1 ± 0.3 5.30 ± 0.1 4.40 ± 0.1 21.6 ± 0.8 0.37 ± 0.02 44.0 ± 3.3 0.30 ± 0.0 146 0.10 ± 0.01 14.4 ± 0.62 0.88 ± 0.12 1.60 ± 0.00 2.54 ± 0.51 0.01 1.25 ± 0.21 13.1 ± 0.39

3.50 ± 0.1 7.32 ± 0.2 5.91 ± 0.1 22.9 ± 0.1 0.06 ± 0.00 62.6 ± 2.6 0.72 ± 0.0 86.9 0.09 ± 0.00 20.6 ± 0.59 1.25 ± 0.20 0.80 ± 0.00 BD 0.01 1.09 ± 0.03 19.8 ± 0.84

BD: below the limit of detection.

for aerating the mixtures: fresh air was pumped into a tube at the bottom of each vessel at the rate of 0.03 m3 h 1 kg 1 (wet basis). A time-based aeration control system was adopted for intermittent supply of oxygen. The core temperature of each composting vessel or pile was recorded daily using temperature sensors. Each mixture was turned over manually every 7 days during composting, and moisture content was maintained at 65% by adding tap water as required before 42 days. Samples were collected from each vessel every 7 days during the first 42 days and every 3 weeks thereafter (a total of nine samplings). However, active aeration was stopped after 21 days to control the temperature drop inside the composting piles. The entire composting process, including the maturation phase that began after 42 days, took 105 days. Although the compost materials were not aerated after 21 days, each mixture was turned manually every 7 days to ensure that the required quantities of oxygen were supplied.

2. Methods 2.1. Properties of pig manure, wheat straw, and wheat-straw biochar Pig manure was collected from a local pig farm and wheat straw, from an experimental field of the Zhuanghang Field Station, Shanghai Academy of Agricultural Sciences, China. Chopped wheat straw (1–3 cm lengths) was used as a bulking agent. Wheat-straw biochar was prepared at 500–600 °C in a vertical charcoal furnace (ECO-5000, Wuneng Environment Co., Ltd., Hangzhou, Zhejiang province, China). The main physicochemical characteristics of the raw materials are given in Table 1. 2.2. Composting system Composting was carried out in 15 L laboratory vessels filled with fresh pig manure (1.5 kg), wheat straw (450 g, that is 30% by wet weight of the pig manure), and wheat-straw biochar (75 g, 150 g, or 225 g, that is 5%, 10%, or 15% by wet weight of the pig manure). A mixture of pig manure and wheat straw (that is, without the biochar) served as a control. The treatments, each replicated twice, were labeled SB (for straw biochar) followed by the percentage of biochar added, as in SB0, SB5, SB10, and SB15, corresponding to the mixing ratios on dry-weight basis of 6:4:0, 6:4:1, 6:4:2, and 6:4:3 (pig manure: straw:biochar, respectively). The composting materials were mixed thoroughly and the initial moisture content of each treatment was adjusted to 65 ± 5% (w/ w, wet weight). A whirlpool pump and a gas-flow meter were used

2.3. Analytical methods Collected samples were homogenized manually, and freezedried using standard methods. The freeze-dried mixtures were ground fine enough so that all particles were smaller than 75 lm for determining the contents of carbon and nitrogen (using Vario EL cube, Elementar Co., Germany). Aqueous extracts were obtained by shaking the fresh samples with deionized water (1:10, w/v) at 200 r min 1 for 8 h on a horizontal shaker. The pH of the supernatant was measured with a digital pH meter (PB-21, Sartorius AG, Germany) and electrical conductivity (EC) of the supernatant was measured with an EC meter (DDSJ-318, Jingke Co., China). Thereafter, the supernatant of each sample was passed through a polytetrafluoroethylene filter (pore size 0.45 lm) and the dissolved organic matter (DOM) content was determined. The total dissolved organic carbon (DOC) content of the DOM was measured with an Apollo 9000 total organic analyzerTM (Teledyne Tekmar, USA). Dissolved organic nitrogen (DON) in the DOM was oxidized with alkaline potassium persulfate (AWWA, Clesceri et al., 1998) and its concentration was determined using an ultraviolet (UV) spectrophotometer (DR 5000, Hach, USA) at a wavelength of 254 nm (SUV254). The concentrations of water-extractable NH+4, Ca2+, Mg2+, NO3 , PO34 , and Cl were measured using a 930 Compact IC Flex ion chromatograph (Metrohm, Switzerland) and water-extractable K+ and Na+ were measured using a FP640 flame spectrophotometer (Jingke Co., Shanghai, China).

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3. Results and discussion 3.1. Evolution of the composting temperature Temperatures recorded from the core of each composting vessel over the first 21 days are shown in Fig. 1. Initially, the temperatures were at a level suited to the mesophiles, then rose to a level suited to the thermophiles within 12 h, and fell again to reach a level close to ambient temperature during the initial 7 days. The higher temperatures were as follows: SB15, 66.3 °C; SB10, 63.7 °C; SB5, 56.7 °C; and SB0, 54.3 °C. Subsequently, all mixtures were turned over manually every 7 days, which resulted in the temperatures rising once again owing to secondary fermentation induced by increased availability of organic components and by increased microbial activity. The second peak was attained on day 8, the temperatures being as follows: SB15, 47.0 °C; SB10, 45.3 °C; SB5, 43.6 °C; and SB0, 43.1 °C. After the second peak, the temperatures began to decrease once again and reached the ambient level after 14 days. The third peak was attained on day 15; however, the temperatures were not significantly greater than the two earlier peaks. Each time, SB15 recorded the highest temperature and SB0, the lowest. The higher temperatures reached in biochar-amended vessels were probably due to the faster composting made possible by the denser substrates: addition of biochar filled the pore spaces (Zhang and Sun, 2014) and thus reduced the heat loss that occurs because of greater air space. Moreover, adding biochar not only led to faster uptake of oxygen (Zhang et al., 2014) but also increased the relative abundance of Actinobacteria (Khodadad et al., 2011), which are generally able to degrade more organic material when it is amended with biochar, ultimately stimulating rapid heat generation during the thermophilic phase. The variation in temperature was consistent with earlier reports (Sánchez-García et al., 2015; Zhang and Sun, 2014). As the experiment continued, a decrease in metabolic rate was observed. Furthermore, the compost-derived materials clogged the pores within the biochar, thereby reducing its surface area (Prost et al., 2013). Hence, addition of biochar had no significant effect on heat release during the later phase of composting. 3.2. Variation in the C/N ratio Carbon and nitrogen are used by microorganisms for energy production and cell growth during composting, which results in considerable changes in the C/N ratio, which can be used to assess

80 SB0 SB05 SB10 SB15

60

o

Temperature ( C)

70

50

40

30

20 0

7

14

21

Composting days Fig. 1. Changes in the core temperature of the compost substrate over time.

the maturity of compost. After 42 days of composting, the total organic carbon (g kg 1) decreased as follows: from 433.8 to 418.9 in SB15, 421.5 to 394.2 in SB10, 416.2 to 370.8 in SB5, and 389.8 to 341.3 in SB0 (Fig. 2)—the higher the proportion of biochar, the greater the content of total organic carbon. Total nitrogen content increased during composting and was maximum in the control; it ranged from 19.0 g kg 1 to 25.6 g kg 1. The corresponding figures for other treatments were as follows: SB5, 17.7–24.5 g kg 1; SB10, 17.3–24.4 g kg 1; and SB15, 16.7–23.1 g kg 1. As expected, the higher the proportion of biochar, the lower the nitrogen content of the compost. The C/N ratio (for C and N in the solid form) decreased in all the treatments: from 20.5 to 13.3 in SB0, 23.5 to 15.1 in SB5, 24.4 to 16.2 in SB10, and 26.0 to 18.1 in SB15. These results fit very closely the results obtained by Selvam et al. (2012), who reported that the C/N ratio for C and N in the solid form declines during composting; Dias et al. (2010) also obtained similar results in composting poultry manure and wood biochar that had been pyrolyzed at 300– 450 °C. This decrease with composting time was probably due either to the mineralization of the substrate or to the increase in total nitrogen following the degradation of carbon (Huang et al., 2004; Zhang et al., 2014). Recalcitrant carbon and the reduced mineralization of compost derived from the added biochar might explain the higher C/N ratio at any stage of the process in the trials with added biochar. The final C/N values on day 42 were 15.1–18.1 in the treatments with added biochar, indicating that the degree of maturity of the compost (Hachicha et al., 2012) met the standards stipulated for organic manures. 3.3. Variation in the SUV254/DOC ratio DOC is the most readily available biologically active compound produced in composts. DOM from mature compost is low on biodegradable organic matter but rich in organic macromolecules. The contents of DOC in the compost increased gradually from their levels (g kg 1) in the raw materials to those recorded on day 7, as follows: from 5.4 to 13.5 in SB0, 4.6 to 12.3 in SB5, 4.1 to 11.0 in SB10, and 3.9 to 10.0 in SB15, only to fall so that the corresponding levels at the end of day 42 were 1.3, 1.2, 1.1, and 0.8 g kg 1 (Table 2). These results, however, are different from the reported by Dias et al. (2010), who observed a continuous decline throughout the entire composting process. It should be noted, however, that the present study was based on straw, which is different from sawdust or pure biochar used in the study by Dias et al. (2010). Straw is rich in easily hydrolysable carbohydrates, which rapidly give rise to water-soluble carbohydrates and other soluble carbon fractions during the early stage of composting. Finally, the relative stability of biochar resulted in the lowest concentrations of DOC observed in the treatments with added biochar in the present experiment. The SUV254/DOC ratio indicates the degree to which the aromatic carbon network is condensed, a high ratio indicating greater condensation of the aromatic humic constituents. The ratios of SUV254/DOC for all treatments are shown in Fig. 3. The initial values were 1.1–1.3 L mg 1 m 1 and increased gradually during the composting process. The value for SB15 was the highest within the first 28 days of the process. Furthermore, the values increased progressively in the order SB0 < SB5 < SB10 < SB15. The higher values observed in the treatments were attributed to the addition of straw biochar, which contained a greater number of surface oxygen functional groups, which increased its capacity to bind humic acids to its surface (Wang et al., 2014). Moreover, the aromatic psystems in the biochar were receptive to the electron donor–acceptor interactions, which may be crucial to the high adsorption capacity of biochar as well as being instrumental in the persistence of natural organic matter (Keiluweit and Kleber, 2009). Most

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500 SB0

SB05

SB10

SB15

C (g kg-1)

450 400 350 300 35 0

7

14

21

28

35

42

49

56

63

70

77 SB0

84 SB05

91

98

SB10

105 SB15

N (g kg-1)

30 25 20 15 30

0

7

14

21

28

35

42

49

56

63

70

77 SB0

84

91

SB05

98 SB10

105 SB15

C/N

25 20 15 10 0

7

14

21

28

35

42

49

56

63

70

77

84

91

98

105

Composting days Fig. 2. Changes in carbon content, nitrogen content, and the C/N ratio during composting.

important, pure-straw biochar showed a greater cation exchange capacity (CEC), the range being 32–146 cmol kg 1 (Zhao et al., 2013). Higher CEC may be associated with the degree of humification (Steiner et al., 2010; Zhang and Sun, 2014) and the nutrientretaining capacity of compost. The control (SB0) took much longer (several weeks) to reach a similar degree of stability and maturity in the compost whereas SB5, SB10, and SB15 had stabilized and detoxified to a greater degree by day 28. This suggests that the decline in DOC and the increase in SUVA254/DOC could be the result of the degradation of the more labile fraction of DOC, which could increase the proportion of recalcitrant aromatics. 3.4. Variation in pH and electrical conductivity Changes in pH during the composting process are shown in Fig. 4. The pH increased throughout the composting process from its initial value of 7.3–7.5 to 8.9 in SB0 and 8.6 in all the others after 21 days. Generally, emissions of ammonia were predominant in the early stage of composting, the increased pH being mainly linked to the release of volatile NH3. The pH began to decline thereafter owing to the production of organic and inorganic acids from the decomposition of organic matter in the mixtures as a result of microbial activity (Huang et al., 2004). After 28 days, the pH was increased slightly with increasing concentrations of Ca2+, Mg2+, K+, and Na+, especially on day 63. The final composts were generally alkaline (pH 8.5–9.0), which is suitable for the growth of most plants. Electrical conductivity (EC) was related to the mineralization of the substrate and the concentration of the mineral fraction, indicating possible phytotoxic or phyto-inhibitory effects. As shown in Fig. 5, EC values were 2.0–2.5 ms cm 1 initially and had increased with time to 2.4–2.8 ms cm 1 by day 7, probably because

of the mineral salts released during the decomposition of organic matter (Silva et al., 2009) and because of the water loss by evaporation due to the higher temperatures during oxidation of organic matter. Thereafter, the EC values decreased, probably as a result of the volatilization of ammonia and precipitation of mineral salts. However, from day 42 the EC values increased continuously until the end of the experiment owing to the net loss of dry mass (Silva et al., 2009), because the lost water had not been replaced during the later stages of composting. The treatments with added biochar recorded higher EC values throughout the composting process: for example, on day 42 the values (ms cm 1) were 1.3 in SB0, 1.4 in SB5, 1.6 in SB10, and 1.8 in SB15. The straw biochar was rich in water-soluble salts (Brewer et al., 2011), leading to relatively high concentrations of mineral salts in the end product. However, the final EC values were below the upper limit of 4.0 ms cm 1 considered tolerable by plants of medium sensitivity (Lasaridi et al., 2006). The composting materials could be applied to soil to avoid potential toxicity and to serve as suitable manure during the seedling stage as well as later. 3.5. Effect of added straw biochar on water-soluble nutrients during composting Fig. 6 shows the concentrations of DON, NH+4, and NO3 throughout the composting process. In the control (SB0), the concentration of DON was 440.8–1226.3 mg kg 1 on day 7 and decreased to 525.3 mg kg 1 on day 42; the corresponding figures for the treatments were as follows: SB5, 372.0–939.6 mg kg 1 and 438.0 mg kg 1; SB10, 329.4–883.7 mg kg 1 and 426.2 mg kg 1; SB15, 323.8–688.1 mg kg 1 and 405.5 mg kg 1. The changes in concentrations of NH+4 and NO3 , also shown in Fig. 6, followed patterns of change similar to that shown by these two forms of nitrogen during aerobic composting. During the first

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Table 2 Characteristics of the mixtures during the composting process. PO34 (g kg

1

)

1

K+ (g kg

)

1

)

Cl (g kg

1

Na+ (g kg

)

1

Ca2+ (g kg

)

1

Mg2+ (g kg

Trials

Time (day)

DOC (g kg

)

SB0

0 7 14 21 28 35 42 63 84 105

5.37 ± 0.39 13.5 ± 1.11 6.25 ± 0.29 3.03 ± 0.31 3.56 ± 0.14 2.46 ± 0.10 1.26 ± 0.02 1.14 ± 0.06 1.42 ± 0.11 1.97 ± 0.10

0.64 ± 0.01 1.01 ± 0.07 0.59 ± 0.02 0.53 ± 0.05 0.62 ± 0.07 0.58 ± 0.01 0.54 ± 0.07 0.60 ± 0.05 0.65 ± 0.06 0.70 ± 0.06

4.50 ± 0.10 3.75 ± 0.25 3.87 ± 0.12 3.68 ± 0.13 4.12 ± 0.12 3.00 ± 0.10 3.50 ± 0.25 4.19 ± 0.06 5.25 ± 0.30 8.75 ± 0.50

2.03 ± 0.08 2.36 ± 0.20 2.23 ± 0.15 1.79 ± 0.13 1.81 ± 0.03 1.41 ± 0.03 1.58 ± 0.10 2.10 ± 0.05 3.33 ± 0.11 3.60 ± 0.23

0.50 ± 0.01 0.62 ± 0.05 0.50 ± 0.01 0.50 ± 0.01 0.50 ± 0.01 0.45 ± 0.01 0.45 ± 0.01 0.56 ± 0.01 0.81 ± 0.11 1.50 ± 0.15

0.22 ± 0.01 0.27 ± 0.01 0.18 ± 0.04 0.14 ± 0.06 0.13 ± 0.01 0.19 ± 0.01 0.16 ± 0.03 0.16 ± 0.01 0.23 ± 0.05 0.22 ± 0.01

0.22 ± 0.01 0.46 ± 0.01 0.25 ± 0.01 0.16 ± 0.01 0.23 ± 0.01 0.23 ± 0.04 0.23 ± 0.05 0.29 ± 0.02 0.40 ± 0.04 0.48 ± 0.05

SB05

0 7 14 21 28 35 42 63 84 105

4.65 ± 0.51 12.3 ± 0.46 5.20 ± 0.17 2.82 ± 0.35 2.88 ± 0.14 2.09 ± 0.06 1.16 ± 0.04 0.97 ± 0.12 1.10 ± 0.05 1.29 ± 0.03

0.77 ± 0.55 1.23 ± 0.12 0.73 ± 0.01 0.55 ± 0.04 0.69 ± 0.09 0.58 ± 0.09 0.57 ± 0.05 0.62 ± 0.04 0.70 ± 0.04 0.75 ± 0.05

5.62 ± 0.40 6.12 ± 0.37 5.06 ± 0.11 5.00 ± 0.25 4.93 ± 0.22 3.81 ± 0.18 4.00 ± 0.30 4.33 ± 0.08 6.50 ± 0.25 11.0 ± 0.20

3.17 ± 0.11 4.12 ± 0.49 3.20 ± 0.36 3.36 ± 0.42 2.75 ± 0.11 2.18 ± 0.31 2.13 ± 0.10 2.31 ± 0.16 4.51 ± 0.12 4.80 ± .21

0.50 ± 0.01 0.87 ± 0.05 0.68 ± 0.07 0.56 ± 0.06 0.56 ± 0.06 0.50 ± 0.01 0.50 ± 0.02 0.56 ± 0.04 0.87 ± 0.12 1.50 ± 0.20

0.50 ± 0.01 0.44 ± 0.01 0.22 ± 0.02 0.17 ± 0.10 0.19 ± 0.02 0.24 ± 0.02 0.16 ± 0.02 0.17 ± 0.02 0.24 ± 0.01 0.26 ± 0.02

0.73 ± 0.03 0.49 ± 0.01 0.27 ± 0.02 0.27 ± 0.03 0.35 ± 0.03 0.38 ± 0.09 0.27 ± 0.05 0.31 ± 0.02 0.42 ± 0.01 0.54 ± 0.04

SB10

0 7 14 21 28 35 42 63 84 105

4.11 ± 0.05 11.0 ± 0.49 4.26 ± 0.30 2.63 ± 0.31 2.40 ± 0.05 1.95 ± 0.08 1.14 ± 0.08 0.80 ± 0.25 0.96 ± 0.34 1.19 ± 0.02

0.90 ± 0.02 1.29 ± 0.20 0.80 ± 0.05 0.56 ± 0.04 0.44 ± 0.02 0.60 ± 0.09 0.58 ± 0.06 0.62 ± 0.05 0.73 ± 0.05 0.77 ± 0.06

6.00 ± 0.25 6.87 ± 0.37 5.62 ± 0.41 5.31 ± 0.31 5.00 ± 0.25 4.43 ± 0.21 4.81 ± 0.43 4.87 ± 0.37 7.38 ± 0.52 12.0 ± 0.30

3.80 ± 0.45 5.16 ± 0.10 4.03 ± 0.33 3.79 ± 0.45 3.31 ± 0.26 2.77 ± 0.23 2.86 ± 0.20 3.18 ± 0.41 5.25 ± 0.23 5.75 ± 0.31

0.50 ± 0.02 0.81 ± 0.12 0.56 ± 0.05 0.50 ± 0.01 0.62 ± 0.08 0.47 ± 0.09 0.50 ± 0.03 0.56 ± 0.04 0.94 ± 0.10 1.37 ± 0.12

0.52 ± 0.01 0.35 ± 0.01 0.26 ± 0.02 0.20 ± 0.04 0.20 ± 0.03 0.29 ± 0.03 0.16 ± 0.02 0.18 ± 0.03 0.24 ± 0.02 0.27 ± 0.02

0.76 ± 0.04 0.52 ± 0.01 0.34 ± 0.03 0.29 ± 0.04 0.33 ± 0.04 0.32 ± 0.08 0.29 ± 0.01 0.32 ± 0.02 0.44 ± 0.03 0.51 ± 0.02

SB15

0 7 14 21 28 35 42 63 84 105

3.89 ± 0.14 9.96 ± 0.15 3.70 ± 0.16 2.49 ± 0.15 2.20 ± 0.01 1.75 ± 0.10 0.84 ± 0.01 0.78 ± 0.34 0.88 ± 0.31 1.04 ± 0.17

1.05 ± 0.05 1.28 ± 0.11 0.83 ± 0.10 0.57 ± 0.05 0.62 ± 0.10 0.63 ± 0.09 0.58 ± 0.01 0.63 ± 0.04 0.75 ± 0.05 0.80 ± 0.05

6.87 ± 0.30 7.50 ± 0.50 7.06 ± 0.19 6.37 ± 0.35 5.75 ± 0.25 5.12 ± 0.37 5.56 ± 0.21 6.25 ± 0.25 7.44 ± 0.43 12.1 ± 0.37

3.99 ± 0.32 5.70 ± 0.31 5.55 ± 0.13 4.26 ± 0.01 4.23 ± 0.29 3.53 ± 0.13 3.41 ± 0.33 4.45 ± 0.39 5.90 ± 0.40 6.25 ± 0.32

0.50 ± 0.02 0.81 ± 0.12 0.56 ± 0.05 0.50 ± 0.01 0.62 ± 0.10 0.50 ± 0.05 0.56 ± 0.06 0.56 ± 0.03 0.87 ± 0.12 1.25 ± 0.20

0.57 ± 0.01 0.44 ± 0.01 0.28 ± 0.03 0.23 ± 0.04 0.23 ± 0.03 0.37 ± 0.04 0.18 ± 0.02 0.21 ± 0.01 0.25 ± 0.01 0.29 ± 0.03

0.71 ± 0.01 0.54 ± 0.01 0.39 ± 0.02 0.29 ± 0.03 0.38 ± 0.05 0.34 ± 0.05 0.30 ± 0.02 0.33 ± 0.04 0.44 ± 0.03 0.52 ± 0.02

1

)

9.5

8.0 7.0

5.0

pH value

-1

-1

SUV254/DOC (L mg m )

9.0

6.0

4.0

8.5

8.0

3.0 SB0 SB05 SB10 SB15

2.0 1.0

SB0 SB05 SB10 SB15

7.5

7.0 0

0.0 0

7

14

21

28

35

42

49

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Composting days

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Composting days Fig. 4. Changes in pH during composting.

Fig. 3. Changes in SUV254/DOC ratios during composting.

7 days, the content of NH+4 peaked, the values being generally in agreement with those obtained by others (Huang et al., 2004; Sánchez-García et al., 2015; Zhu, 2007). In the present work, the highest pH values did not coincide with peak NH+4 concentrations

observed during the thermophilic phase. However, our results match those obtained by Huang et al. (2004), who investigated changes in biological properties of the compost made from pig manure mixed with sawdust and found that the production of organic and inorganic acids lowered the concentration of NH+4 and showed no immediate effect on pH.

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in SB5, 148.0–166.3 in SB10, and 117.0–134.7 in SB15. In comparison with the concentration in the control, SB10 decreased by 10% and SB15 by 27%, whereas SB5 was not significantly different. Biochar has the potential to reduce the volatilization of ammonia during composting mostly because of three factors. (1) The greater sorption capacity of biochar (because of its larger surface area) leads to more NH+4 being absorbed, which means less NH+4 is available during composting (Hua et al., 2009; Malin´ska et al., 2014). In the present study, addition of biochar significantly reduced the emissions of ammonia during the first week. (2) Biochar creates a favorable micro-environment for nitrifying bacteria that convert ammonia to nitrate, which means that biochar-treated compost is richer in nitrogen (Zhang and Sun, 2014). (3) Biochar absorbs larger quantities of ammonia because of the greater mineralization of organic matter during composting (Dias et al., 2010; Malin´ska et al., 2014). Nitrate content, however, was almost negligible; began to increase owing to nitrification only after 42 days; and was lower in SB0, probably because biochar has a higher NO3 adsorption capacity (Mizuta et al., 2004). Compared to SB0, the other treatments had lower NH+4 contents and higher NO3 contents, indicating that biochar had led to a more favorable micro-environment for the nitrifying bacteria. Nitrification index (NH+4/NO3 ratio) was used as a measure of the maturity of compost (Das et al., 2011): a value below 0.5 suggests fully mature compost; that above 0.5 up to 3.0, mature compost; and that above 3.0, immature compost. In the present study, the values after composting for 42 days and 63 days, respectively, were as follows: 4.6 and 0.9 in the control (SB0), 4.1 and 0.5 in SB5, 2.9 and 0.3 in SB10, and 2.4 and 0.3 in SB15. These changes were much greater than the values reported earlier (Das et al., 2011; Zhang and Sun, 2014) and indicate that the final product of the treatments with 10–15% biochar was the most mature. Phosphorus is the second limiting nutrient after nitrogen in most soils used for crop production. As shown in Table 2, the trend

4.0

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Composting days Fig. 5. Changes in electrical conductivity during composting.

The content of DON peaked and then decreased gradually as the content of DOC increased. The increase in NH+4 concentration was due to simultaneous ammonification, increased temperature and pH, and the mineralization of organic nitrogen compounds. Neither DON content nor NH+4 concentration had any effect on the content of solid N, which increased in all the vessels as a consequence of the net loss of dry matter in terms of carbon dioxide and of the overall decrease in weight during composting (Dias et al., 2010; Huang et al., 2004). After an initial increase, the amounts of NH+4 decreased, largely owing to the loss through volatilization and nitrification: nitrifying bacteria are not thermophilic and cannot thrive at high temperatures. The final concentrations of NH+4 (mg kg 1) after 35–42 days of composting were as follows: 147.9–184.8 in SB0, 142.0–172.3

2500 SB0

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Composting days Fig. 6. Changes in the content of dissolved organic nitrogen and concentrations of NH+4 and NO3 in water-extractable solutions during composting.

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in the concentration of water-soluble PO34 was similar to that in the concentration of NH+4: the values began increasing from the start, peaked after 7 days, and then kept decreasing until the end. The loss of water-soluble PO34 was probably owing to the mineralization of organic phosphorus and consumption by microorganisms (Biederman and Harpole, 2013). The concentrations of PO34 increased significantly as a result of adding biochar, being 22– 28% higher compared to the control after composting for 7 days. The concentration of water-extractable K+ and Cl in the trials with added biochar demonstrated changes to the EC values throughout the composting process. Both of K+ contents and Cl contents increased until day 7, decreased until day 35, and then increased again until the end of the composting process. After composting for 35–42 days, the concentration (g kg 1) of K+ was 5.6 in SB15, 4.8 in SB10, 4.0 in SB15, and 3.5 in SB0. The addition of biochar increased the concentration of water-soluble K+ significantly, by 14–59% compared to the control. The concentration of Cl (g kg 1) was 1.6 in SB0, 3.4 in SB15, 2.8 in SB10, and 2.1 in SB5, the overall increase over the control being 35–116%. A growing crop takes up cations from the soil and needs to adsorb a similar quantity of anions to remain neutral, Cl being the preferred anion. The wheat-straw biochar contained low levels of Ca2+ and Mg2+, which are both metal ions and only slightly soluble in water when biochar is pyrolyzed at temperatures above 500 °C. The concentrations of Ca2+ and Mg2+ showed little change during the composting process, although their contents in the treatments were higher compared to the control, indicating that biochar checks the losses of Ca2+ and Mg2+ during composting. Another point in favor of biochar was that the concentrations of Na+ were not higher compared to the control, because excessive levels of Na+ could lead to poor soil structure (Busoms et al., 2015). Straw biochar’s own characteristics and it provides a habitat for microorganisms, some of which can increase the availability of oxygen and water (Laird et al., 2010). Hence, straw biochar hastens organic matter degradation when it was composted with pig manure and straw. The wheat-straw biochar was rich in nutrients, including PO34 , Cl , and K+, and it led to richer compost. Furthermore, the negative surface charge increased the capacity for exchange with cations in the compost and allowed the retention of nutrients, including K (Liang et al., 2006) and P (Laird et al., 2010). All these factors might have synergistic effects in improving the soil characteristics and crop development after the following application into soil. However, the straw biochar did not alter the soil nutrient environment as much as chemical fertilizers can; in other words, addition of biochar is superior to chemical fertilizers in increasing the concentrations of P and K in plant tissue (Biederman and Harpole, 2013). 4. Conclusions Adding biochar derived from wheat straw to the substrate for composting is a potentially sustainable way of managing agricultural waste. Adding straw biochar to pig manure significantly increased the uptake of P, K, Ca, and Mg; led to faster degradation of organic matter; and accelerated stabilization and detoxification. The optimal proportions of biochar were 10–15% (w/w) under the conditions of the present experiment, in which composting was carried out for 42 days. Lastly, enriching biochar with chemical fertilizers can turn it into a slow-release fertilizer and thereby enhance the benefits derived from the added nutrients. Acknowledgement The authors are grateful to the Shanghai International Science & Technology Cooperation Fund (Grant Number: 13590700800).

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