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Transformation of nitrogen and carbon during composting of manure litter with different methods
Bioresource Technology 293 (2019) 122046
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Transformation of nitrogen and carbon during composting of manure litter with different methods Bingxin Tonga, Xuan Wangb, Shiqiang Wanga, Lin Mab, Wenqi Maa, a b
T
⁎
College of Resources and Environment Science, Hebei Agricultural University, Baoding 071000, China Center for Agricultural Resources Research, Institute of Genetic and Developmental Biology, The Chinese Academy of Sciences, Shijiazhuang 050021, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Composting methods Nitrogen Carbon Comprehensive assessment Greenhouse gases
In this study, to investigate the nitrogen and carbon characteristics throughout the composting process in different systems, four methods of composting including static treatment (ST), turning treatment (TT), forced aeration treatment (FAT), and forced aeration with acidification treatment (FAAT) were conducted. Organic matter degradation was improved in TT and FAT that accelerated the composting efficiency. The harmless time based on phytotoxicity was significantly shortened in FAT comparing with ST. However, nitrogen loss through ammonia volatilization increased 14.0%. Ammonia volatilization could be significantly decreased to 17.0% after acidification optimization with FAAT. Compared to FAT, the FAAT got an increased nitrous oxide production and decreased methane emission. Generally, the lowest global warming potential value (52.8 kg CO2-eq/t) was found in FAAT. Therefore, considering the environmental, fertilizer and toxicity indicators, the FAAT composting method is the most promising method, and has the potential to be promoted for implementation in practice.
1. Introduction Composting is globally recognized as one of the most suitable and effective methods for livestock waste treatment and is widely used in practice (Li et al., 2012). According to preliminary statistics, more than 17 Mt organic fertilizers were produced by composting every year in China (Jiang et al., 2016). However, a series of environmental problems caused by composting should not be ignored, especially ammonia (NH3) volatilization, nitrous oxide (N2O) and methane (CH4) emissions (Zeng et al., 2018). Therefore, it is very important to find an effective composting method that can coordinate the economic, environmental and resource demands, which is greatly depended on the transformation of carbon and nitrogen processes. As an aerobic biochemical process, composting is greatly influenced by ventilation methods associated with oxygen content that determine the transformation of carbon and nitrogen processes and the gaseous forms emitted (Chowdhury et al., 2014; He et al., 2017). Composting can be divided into many types based on the aeration modes (Jiang et al., 2016). The common composting methods include static, turning and forced aeration composting (Zeng et al., 2018). Static composting was broadly used in the past, but is gradually replaced by turning or forced aeration because of its long composting time (Van der Werf and Ormseth, 1997). ⁎
Turning or forced aeration could speed up the compost maturation process, but increase the NH3 emission and also have different impacts on greenhouse gas emissions from composting (He et al., 2017; Yuan et al., 2018). A study found that turning manure heaps can increase N2O emissions compared to the no-turning treatment, because the more efficient O2 supply of the turning treatments was suitable for N2O production (Jiang et al., 2015). However, some studies indicated that compared to static treatment, turning decreased N2O and CH4 emissions (He et al., 2017; Yuan et al., 2018). There have been inconsistent results regarding the effect of aeration strategy on GHG emissions from composting. A laboratory-scale forced aeration composting experiment showed that the higher aeration rates reduced the CH4 emission but increased the N2O emission (Jiang et al., 2011). Zeng et al. (2018) investigated the ventilation strategy of reducing aeration frequency at the initial stage and increasing aeration volume at the final stage of composting, which indicated the increases in CH4 emission and no impacts on N2O emission compared to the conventional aeration method. Various countermeasures aiming at reducing NH3 emission have been explored, e.g. adjusting the C/N ratio and moisture content (Jiang et al., 2016), aeration rate (He et al., 2017) and adding additives (Gu et al., 2018). Results from a meta-analysis showed that the addition of acid during composting was the greatest mitigation measure of NH3, which can significantly reduce the NH3 emission by 44.5% on average
Corresponding author at: 289 Lingyusi Street, Baoding 071000, China. E-mail address: [email protected] (W. Ma).
https://doi.org/10.1016/j.biortech.2019.122046 Received 17 June 2019; Received in revised form 19 August 2019; Accepted 20 August 2019 Available online 21 August 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
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2.2. Sampling
(Cao et al., 2019). However, the influence of acidification on GHG emissions was still not clear. For example, Pan et al. (2018) found that after the addition of citric acid the N2O emission nitrogen output reduced 51.26%, while with adding phosphoric acid the increasing proportion of N2O emissions was 31.89%. Meanwhile, when adding with citric acid and phosphoric acid to compost, the CH4 emission rate decreased significantly. In addition, Yang et al. (2015) and Mao et al. (2017) concluded inconsistent impacts of adding acid on N2O emissions during composting. Previous studies mainly focused on different gaseous mitigation strategies in single composting method. However, different composting methods were co-existence in practise such as, static heap, windrow composting with turning and forced aeration composting. There is a need to increase the understanding about the response to C and N transformation and gaseous emissions to various composting methods, with the aims to satisfy environmental, resources efficient and economic benefits during composting. Hence, our objectives of this study were to (i) understand the characteristics of C and N transformation processes during composting subjected to different methods, (ii) analyse the effect of different composting methods on nitrogen losses and GHG emissions, and (iii) clarify the comprehensive effects of different composting methods on composting product quality, nitrogen losses, GHG emissions and economic feasibility.
Three representative samples (approximately 400 g) were taken at day 0, 3, 7, 10, 14, 21, 28 and 35. Each fresh sample was divided into 2 parts. One part was stored at −20 °C and another part was air-dried. The air-dried sample were crushed and passed through 1 mm sieve for measuring physico-chemical properties (Wang et al., 2016b). A plexiglass chamber (1 m × 1 m × 0.2 m) was used for the collection of gaseous samples from the ST and TT treatments. During sampling, the box was put on the top of the surface of the composting piles and pressed into the pile to 15 cm depth, so that the top space was approximately 50 L. A vacuum pump was connected to the chamber to collect the gases from the pile at a flow rate of 90 m3 per hour controlled by a flowmeter that ensured 20 times volume of the top space (Kissel et al., 1977). There were two flowmeters connected to the pump. One was for measuring the ventilation rate through the composting materials surface and another one was used for adjusting the inlet ventilation rate into a conical flask with boric acid. The NH3 samples were measured 2 times daily and trapped with boric acid (2%) using a dynamic chamber method, then titrated with H2SO4 (Jiang et al., 2016). Greenhouse gases (N2O, CH4, and CO2) samples were collected with a static chamber method (Jiang et al., 2015). Sampling frequency was twice daily at one day intervals. The samples of N2O, CH4, and CO2 were analysed by gas chromatography (Agilent 7890A, America). Collection and measurement of the NH3 and GHG samples of the treatments FAT and FAAT was conducted following the methodologies in a previous study (Jiang et al., 2016).
2. Materials and methods 2.1. Feedstock of composting and experimental design The composting experiment was conducted at the Luancheng experimental station of the Chinese Academy of Sciences. Cow manure and maize straw were used as the composting feedstock. Cow manure collected from the Dingyuan dairy farm, and maize straw collected from a nearby village. Maize stalks were crushed to approximately 2 cm. The cow manure and maize straw were mixed at the ratio of 3:2 (w/w fresh weight basis), adjusting the moisture content to approximately 60% and the C/N ratio to nearly 30 of the initial materials (Jiang et al., 2011). The basic physical and chemical properties of the composting materials are shown in Table 1. There were 4 treatments in total: static treatment (ST), turning treatment (TT), forced aeration treatment (FAT), and forced aeration with acidification treatment (FAAT). The ST treatment simply involved packing the compost mixture into the stainless steel composting container (1.2 m × 1.2 m × 1.2 m). The TT treatment was supplied oxygen with turning. The pile was turned at the 3rd, 7th, 10th, 14th, 21st, 28th, and 35th day of composting (Wang et al., 2013). In the FAT treatment, the composting materials were mixed following the turning frequency of TT and an extra pump was used for continuous ventilation. The ventilation rate was 0.5 L/min/kg dry matter (Wang et al., 2016b). The FAAT treatment was a further modification of the FAT treatment. Sulphuric acid (H2SO4) was added to the initial materials before composting. The addition amount of H2SO4 was 10% of the initial molar quantity of nitrogen in the feedstock (Shi et al., 2011). The experimental period lasted 35 days, from April 14, 2017 to May 18, 2017.
2.3. Analytical methods Compost samples were analysed for moisture content (MC), pH, total organic carbon (TOC), total Kjeldahl nitrogen (TKN), NH4+-N, NO3−-N, and germination index (GI). The temperatures of the pile in the top, middle and bottom layer were measured using a metal thermometer two times daily (10:00 AM and 3:00 PM). The temperatures of the FAT and FAAT treatments were measured and recorded every 10 min automatically with a computer-controlled system. The samples were oven-dried at 105 °C to reach a constant weight for MC measurement (Li et al., 2012). The pH value was determined by diluting the sample with distilled water (1:10, w/v) and measuring using a pH meter (Starter 3C-Ohaus, America) (Li et al., 2012). Organic matter (OM) was determined by loss on ignition at 550 °C for 4 h using a muffle furnace (FR-1236, China). The conversion coefficient of organic matter and organic carbon was 1.724 (Li et al., 2012). The TKN was determined according to the manufacturer’s manual using an Analyzer (Hanon K9860, China) (TMECC, 2002). NH4+-N and NO3−-N were determined by diluting the sample with 2 mol/L KCl solution (1:10, w/ v) and measured with a continuous flow analyzer (SEAL AutoAnalyzer 3, Germany). The GI was measured by using the same diluted solution as for pH measurement, extracting 5 ml of the diluted solution into a culture dish with 9 cm filter paper at the bottom and placing 20 Chinese cabbage seeds into the culture dish and setting the dish in a 25 °C incubator for 48 h (Shi et al., 2011; Zucconi et al., 1981). All the aforementioned analyses were made with three replicates. The percentage of OM loss was calculated by applying Eq. (1) (Haug, 1993):
OMloss (%) = 100 ∗ (DM0 ∗ OM0 − DMT ∗ OMT )/(DM0 ∗ OM0 )
Table 1 The basic physical and chemical properties of the composting materials.a
Manure Maize straw a
Moisture (%)
TOC (%)
TN (%)
C/N
81.84 ± 0.33 26.76 ± 0.12
35.62 ± 0.14 47.66 ± 0.22
1.69 ± 0.02 1.39 ± 0.01
21.08 34.29
(1)
where DM0 was the dry matter weight of the original feedstock, OM0 was the organic matter content of the original feedstock, DMT was the dry matter weight at the end of composting, and OMT was the organic matter content of the last compost. The seed germination index formula was Eq. (2) (Zucconi et al., 1981):
Values are mean ± Standard deviation (n = 3). 2
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GI(%) = (Seed germination of compost sample ∗ Root length of compost sample) /(Seed germination of control ∗ Root length of control) ∗ 100
(2)
The ammonia emission N loss was Eq. (3) (Wang et al., 2016b):
NH3 - N emission loss (%) = Σ(AE i)/ITN ∗ 100, (i=1, 2 . .. n)
(3)
where AEi was the NH3-N emission amount of the n-th composting day, and ITN was the initial total nitrogen of the composting mixture. The N2O-N emission loss, CH4-C emission loss and CO2-C emission loss were calculated by applying Eq. (4), Eq. (5) and Eq. (6), respectively:
N2 O - N emission loss (%) = Σ(NE i)/ITN ∗ 100, (i= 1, 2 . .. n)
(4)
where NEi was the N2O emission N loss of the n-th composting day, and ITN was the initial total nitrogen of the composting mixture.
CH 4 - C emission loss (%) = Σ(ME i)/ITOC ∗ 100, (i = 1, 2 . .. n) (5) where MEi was the CH4 emission C loss of the n-th composting day, and ITOC was the initial total organic carbon of the composting mixture.
CO2 - C emission loss (%) = Σ(CE i)/ ITOC ∗ 100, (i = 1, 2 . .. n) (6) where CEi was the CO2 emission C loss amount of the n-th composting day, and ITOC was the initial total organic carbon of the composting mixture. For the global warming potential (GWP), 1 mol CH4 = 25 mol CO2eq, 1 mol N2O = 298 mol CO2-eq, and 1 mol NH3 = 3.86 mol CO2-eq (IPCC, 2014; Jiang et al., 2016). In the section of evolution of the initial nitrogen, the calculation formulation of organic nitrogen (Org-N) and others (Other-N) were Eqs. (7) and (8) (Jiang et al., 2016):
Org - N = TN −
NH+4
-N −
NO−3
-N
Other - N = Total N losses − NH3 loss − N2 O loss − sample loss
Fig. 1. Evolution of temperature (a) and pH (b) during composting. ST-static treatment, TT-turning treatment, FAT-forced aeration treatment, FAAT-forced aeration with acidification treatment. The error bars are standard deviation (n = 3).
significantly shorten duration of thermophilic phase was also founded in the continuous ventilation composting experiments (Jiang et al., 2016; Sun et al., 2019). The thermophilic phase in FAAT was obvious shorter than that in FAT and this was because of the addition of acid. Shi et al. (2011) also found that the treatments with addition of acid shortened the lasting time of high temperature period because acidification limited the activity of thermophilic microorganisms. However, little negative influence was found on the maturity of final compost product after 35 days composting period. The pH value sharply increased at the first 3 days and then kept a stable level for all the treatments during composting (Fig. 1b). The pH of FAT was nearly 8.6, which was significantly higher than that of other treatments (P < 0.05). The reasons were that forced aeration increased biodegradation of organic acids and mineralization of organic compounds (He et al., 2017). At last, the pH values of all the four treatments were < 7 (from 7.6 to 8.6). These results were in line with the previous study on adding additives to reduce N2O and NH3 emissions (Jiang et al., 2016), but in contrary to one previous study (last pH value was < 7) because of the feedstock of composting was only pig faeces (Tiquia and Tam, 1998). The pH value of each treatment ranked as FAT > TT > ST > FAAT, with the statistically significant differences among treatments (P < 0.05).
(7) (8)
In the section of evolution of the initial organic carbon, the others form carbon (Other-C) calculation was Eq. (9) (Jiang et al., 2016):
Other - C = Org - C − CO2 - C loss − CH 4 - C loss
(9)
2.4. Statistical analysis The data were analysed using one-way ANOVA. The least significant difference test was used to determine the significance of the difference in the mean values. All data were analysed using SPSS 17.0 for Windows. 3. Results and discussion 3.1. Temperature and pH The temperature in all the treatments increased sharply and exceeded 50 °C at the second day (Fig. 1a). For the ST, TT, FAT and FAAT treatments, the thermophilic phase (> 50 °C) lasted 34, 22, 8 and 6 days, respectively, which was long enough to satisfy the Chinese national standard for sanitation (GB 7959-2012, 2012). Compared to ST, the other three treatments had better aeration conditions that can supply sufficient oxygen for microbial and lead to a quick temperature increasing due to more dramatic organic matter degradation at the beginning. However, because of less easy degradable organic matter after thermophilic phase and more heat loss leaded by continued aeration, temperature of FAT and FAAT decreased earlier than other treatments (Jiang et al., 2013; Awasthi et al., 2016). A
3.2. Nitrogen characteristics of compost For the ST treatment, the TN concentration slightly increased during composting. However, for the TT, FAT and FAAT treatments, the TN content increased rapidly during the first 10 days and then remained at the same level (Fig. 2a). The TN content of the other three treatments was significantly greater than that in ST. This pattern indicated that measures such as turning, forced aeration and acidification could increase the TN content in composting products. These results are mainly 3
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Fig. 2. Changes of TN content (a), NH4+-N content (b), NO3−-N content (c), NH3 flux (d), N2O flux (e), Nitrogen loss (f) during composting. ST-static treatment, TTturning treatment, FAT-forced aeration treatment, FAAT-forced aeration with acidification treatment. The error bars are standard deviation (n = 3).
After the initial phase the OM and organic acid degraded greatly, and the content of NO3−-N in the compost increased with the onset of the high-temperature period. Another study indicated that the NO3−-N content increased sharply after the experiment started 4 weeks (the mesophilic and cooling phase) with pig manure and sawdust (Wang et al., 2016a). In the whole composting process the NO3−-N level of the ST and FAAT treatments was significantly higher than that of the other treatments, which was mainly because of the lower N loss ratio and conversion of NH4+-N to NO3−-N (Cao et al., 2019; Shi et al., 2011). At the end of composting, the highest content of NO3−-N was in the FAAT treatment, followed by the ST, FAT and TT treatment. The results showed that acidification could improve the NO3−-N content of composting products. Ammonia volatilization showed a trend of increasing first and then decreasing in all the treatments during the whole composting process (Fig. 2d), in line with the results of previous studies (e.g., Wang et al., 2013). The maximum NH3 emission rate reached at different days among the four treatments. The results of this study showed that NH3 emission peak appearance can be accelerated by forced ventilation and NH3 emission rate can be reduced by acidification, which is similar to the previous results (Cao et al., 2019; Jiang et al., 2011; SanchezMonedero et al., 2018; Shi et al., 2011). The addition of H2SO4 could reduce the NH3 emission rate comparing the FAT and FAAT treatments during composting, which is similar to the conclusions by Cao et al. (2019) and Shi et al. (2011). Comparison of FAT, TT and ST showed that forced aeration and turning increased the flux of NH3 emission. This result indicated that the rate of NH3 emission was related to the amount of aeration during composting (Gu et al., 2018; Pan et al.,
due to the weight reduction of the mass, caused by the OM degradation in the TT, FAT and FAAT treatments (Pan et al., 2018). Compared to ST, the TT and FAT treatments maintained higher TN concentration, with no significant difference between the latter two treatments. The FAAT treatment had a higher TN concentration than the other treatments because the addition of H2SO4 decreased the nitrogen loss (Cao et al., 2019; Shi et al., 2011). The content of NH4+-N firstly decreased and then tended to be stable of all the four treatments (Fig. 2b). Jiang et al. (2013) reported similar results, which primarily because the NH4+-N transformed into NH3. However, other research showed that the trend of NH4+-N concentration was different (increased at the beginning and then decreased) due to the conversion of organic nitrogen to NH4+-N via ammonification (Jiang et al., 2016; Pan et al., 2018). While, Gu et al. (2018) found that with the addition of sulphur (0.25% DW) the NH4+N content increased gradually. The NH4+-N content of ST was substantially higher than in other treatments because of the lower nitrogen loss and lower OM degradation (Jiang et al., 2015). The addition of H2SO4 increased the NH4+-N content as the certificate through comparison of the FAAT and FAT treatments, Cao et al.(2019) and Yuan et al. (2018) found the similar result, this was because of the addition of acid decreased the pH of the compost. Considering the NO3−-N content of all the treatments (Fig. 2c), the content decreased at the early stage and then showed an upward trend. Jiang et al. (2016) and Pan et al. (2018) found the similar results. The decline of NO3−-N content during the first days was due to N2O emission. This change could be explained as the conversion of NO3−-N by denitrifying bacteria to N2O (Gu et al., 2018; Szanto et al., 2007). 4
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TT > FATT > FAATT > ST. The NH4+-N content also showed a gradual decrease trend. The FAT treatment got the highest NH4+-N decrease ratio and followed by the TT, FAAT and ST. This indicated that forced aeration and turning would increase the NH4+-N losses during composting. The reason for the decrease of NH4+-N in TT was less than that of FAT may be because the TT treatment had more organic N degradation (Pan et al., 2018). The Fig. 3 also showed that NO3−-N occupied very little proportion in total nitrogen and all of its ratio was < 1% in four treatments. The greatest accumulation of N loss caused by NH3 emission was in the TT treatment, and the reason has explained in the 3.2 part of this paper. The rate of nitrogen loss caused by NH3 emission occupied the TN losses varied among the treatments, which was approximately 84.2% for TT, followed by FAT (83.8%), FAAT (81.7%) and ST (64.4%). The ST had the largest N2O emission (1.0%) and for TT, FAT and FAAT, this loss ratio was approximately 0.4%, 0.2% and 0.3%, respectively. There were some other unknown N losses ranging from 3.5 ~ 4.9% of the initial nitrogen. These perhaps caused by the loss of other nitrogen-containing gas (Jiang et al., 2016; Yuan et al., 2018).
2018). For TT, FAT and FAAT, NH3 emissions happened at the thermophilic stage and the emission patterns were similar to the findings of Jiang et al. (2011). The duration of NH3 emission is another factor contributed to total NH3 volatilization (Cao et al., 2019; Chowdhury et al., 2014). The duration of NH3 emission differed among the treatments. In FAAT and FAT, NH3 emission occurred mainly in the first 2 weeks, but for TT and ST occurred mainly in the first 3 weeks during composting. The N2O emissions trend of the ST treatment was different from other treatments (Fig. 2e). The N2O emission flux was high at the beginning of the composting process, but after the 7th day remained almost unchanged for the ST treatment. The reason for that was denitrification or nitrification had begun during the process of mixing the composting materials (Gu et al., 2018; Wang et al., 2017). In the later stage, due to the continuous high temperature (above 60 °C), the denitrification process was inhibited (Gu et al., 2018). However, He et al. (2017) and Pan et al. (2018) found that the N2O emissions peaks occurred at the thermophilic stage. The main reason for this difference was the nature of composting materials (Sanchez-Monedero et al., 2018). The N2O emission tendencies of the other three treatments were as follows: high emission rate happened at the early stage; the emission rate decreased sharply and maintained a low level during the hightemperature period; and the emission rate gradually increased at the late stage. At the early stage and high-temperature period, the reason for the emission patterns was described as the ST treatment. After the thermophilic phase, the N2O emission fluxes of the TT, FAT and FAAT treatments increased, which was because of the low temperature and high NO3−-N content of feedstock (Jiang et al., 2016; Zeng et al., 2018). Multiple comparisons indicated that N2O emission reduced with the higher aeration rate for the ST, TT and FAT treatments, similar to the results observed by others (Wang et al., 2017; Yang et al., 2013). With two-way comparisons of the four treatments, turning and forced aeration significantly reduced the emission of N2O, but acidification increased the emission of N2O. The nitrogen loss ratio was largely different among the treatments (Fig. 2f). The N loss ratio was: TT > FAT > FAAT > ST. These results showed that turning and forced aeration significantly increased the TN losses, while addition of acid reduced the losses (Pan et al., 2018). The NH3 emission was the main type of the N loss for all treatments (Cao et al., 2019; Gu et al., 2018; Szanto et al., 2007). For ST, TT, FAT, and FAAT, the percentage of NH3 emission N loss of the initial TN was 8.5%, 28.1%, 22.5%, and 17.0%, respectively. The NH3 emission patterns from this study are consistent with previous research (Szanto et al., 2007; Jiang et al., 2013). However, Kithome et al. (1999) found that the N loss caused by NH3 emission ranged from 47% to 62% of the initial N after 25 days of composting with manure, which was due to high initial TN content and low C/N ratio of the manure. Moreover, some other research showed the opposite result which only 2.4% ~ 3.9% of the initial TN lost in the form of NH3 because of the low C/N ratio of the feedstock (Szanto et al., 2007). While the N2O emission N loss was lower, the loss ratio was only approximately 1.0% (ST), 0.4% (TT), 0.2% (FAT), and 0.3% (FAAT), similar to the results of Yang et al. (2013). The N2O emission nitrogen loss was quite lower than NH3 emission nitrogen loss, which was only accounting for 0.7% to 7.6% of the total gaseous nitrogen loss, and the results were similar to the previous study (Yuan et al., 2018).
3.4. Carbonaceous gas emission The CH4 and CO2 emissions were the main routes of carbon loss during composting (Liu et al., 2017). The trend of CH4 emission of ST treatment showed an initial increase and then decrease (Fig. 4a). The emission rate tended to be moderate in the ST treatment after 24 days. Whereas the TT, FAT and FAAT treatment showed another trend (first decreased, then increased and decreased again). The highest CH4 emission flux happened during the thermophilic phase, the same as observed by others (Chang et al., 2019; Yuan et al., 2018). The reasons for the higher CH4 emission rate in ST than that in TT and FAT were because the internal environment of the pile body was more suitable for anaerobic action (Liu et al., 2017; Zeng et al., 2018). The FAAT had a slightly lower CH4 emission rate compared with FAT, which was because of the addition of acid (Pan et al., 2018). Yuan et al. (2018) and Pan et al. (2018) found that adding superphosphate and phosphoric acid can sharply mitigate the CH4 emissions. The ST had a higher emission rate compared to the other three treatments because of the relatively low oxygen concentration inside the compost materials (He et al., 2017). The trend of CO2 emission flux in each treatment was a fluctuation in the early stage and a tendency to be smooth after 21 days (Fig. 4b). There were two emission peaks of all the treatments, but the time of occurrence was not the same. Yuan et al. (2018) found the similar results (two peaks) in a sewage sludge composting with addition of phosphogypsum in a continuously aerated system. These two emission peaks may be caused by the change of the internal physicochemical properties in the pile (Chang et al., 2019). The explanation for the lowest CO2 emission of ST was the lack of O2 in the pile (Zeng et al., 2018). The results of C loss caused by CO2 showed that a certain amount of ventilation will increase the C loss during composting, which also observed by other studies (Cao et al., 2019; Jiang et al., 2013). The CO2 emissions mainly happened at the thermophilic phase of all the treatments, and the reason for this was the rapid degradation of OM (Jiang et al., 2016; Pan et al., 2018). The comparison of TT and FAT indicated that continuous aeration did not increase the CO2-C loss. But the TT had a greater CO2-C loss, which may be related to the longer period of high temperature (Zeng et al., 2018). The carbon loss in the form of CO2 was not significantly different between FAAT and FAT, which demonstrated that acidification did not affect the CO2 emission.
3.3. Evolution of the initial nitrogen As shown in Fig. 3, each form of nitrogen that accounted for the proportion of TN of the initial materials varied during the composting process. The N balance calculation methods were shown as Eqs. (7) and (8). At the beginning of the experiment the proportion of Org-N was approximately 82.0% for all the treatments. The Org-N content of each treatment showed a decreasing trend, while the amount of decrease in each treatment was different. The changes ratio of the treatments was:
3.5. Evolution of the initial organic carbon The carbon content of each form was converted to a percentage of the initial C content, and different types of C were displayed and analysed (Fig. 5). The carbon balance calculation formulation was as Eq. 5
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Fig. 3. Evolution of nitrogen in feedstock during composting. Others-undetermined nitrogen, N2O-nitrogen loss caused by N2O emission, NH3-nitrogen loss caused by NH3 emission, NO3−-N-nitrate nitrogen content, NH4+-N-ammonium nitrogen content, Org-N-organic nitrogen content. ST-static treatment, TT-turning treatment, FAT-forced aeration treatment, FAAT-forced aeration with acidification treatment.
time of thermophilic phase (Chowdhury et al., 2014; Liu et al., 2017). The organic C degradation mainly occurred in the thermophilic phase of TT, FAT and FAAT, more than 90% of C losses occurred during this period. For ST, the organic carbon degradation rate was basically maintained at the same level during composting. The results indicated that turning and forced aeration could stimulate the organic carbon degradation rate (Szanto et al., 2007; Jiang et al., 2013). The carbon loss caused by CO2-C was the majority and CH4-C loss was only a small part (Pan et al., 2018; Zeng et al., 2018). The CO2-C loss proportion of the total C loss was 72.7% (ST), 83.2% (TT), 83.9% (FAT) and 84.0% (FAAT), respectively. Carbon loss caused by CH4 emissions was different for all the treatments. The ST treatment had the greatest ratio of CH4-C emissions and the loss of CH4 accounted for approximately 14.4% of the TOC loss, which was significantly higher than that in TT (2.1%), FAT (1.3%) and FAAT (1.1%). The main reason for this pattern was because of the internal anaerobic environment in ST, which was more suitable for the reaction of CH4 production (Gu et al., 2018; He et al., 2017). The results of CH4-C loss showed that turning and forced aeration could significantly reduce CH4 emission, but acidification slightly decreased CH4 emission. 3.6. Maturity of compost Maturity is associated with plant growth potential or phytotoxicity (Zucconi et al., 1981), whereas stability is often related to the compost’s microbial activity (Sun et al., 2019). There are many indicators to evaluate the maturity of compost. In this study, the GI and C/N ratio were selected. It is generally believed that when the GI is greater than 90%, the compost is fully mature (Yuan et al., 2018; Zucconi et al., 1981). At the end of composting all the treatments reached the maturity standard except for the ST (GI 72.4%) (Table 2). For TT, FAT and FAAT, the maturity state was reached at the 14th, 10th and 10th day, respectively. This was because of the more ventilation rate of the two forced aeration treatments (Chang et al., 2019; He et al., 2017). The results indicated that turning and forced aeration had positive effect on the maturity, but the acidification had no effect. As shown in Table 2, the C/N ratio of all the treatments decreased
Fig. 4. Changes of CH4 (a) and CO2 (b) emission rates of all the treatments during composting. ST-static treatment, TT-turning treatment, FAT-forced aeration treatment, FAAT-forced aeration with acidification treatment.
(9). For all the treatments, organic C decreased obviously. However, the changes in all the treatments were not the same. The organic C losses ratio of TT, FAAT, FAT and ST was 58.4%, 50.3%, 51.2% and 28.9%, respectively. These results were related to the supply of O2 and lasting
6
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Fig. 5. Evolution of CH4, CO2, Organic-C and Others during composting. ST-static treatment, TT-turning treatment, FAT-forced aeration treatment, FAAT-forced aeration with acidification treatment. Others-undetermined carbon, CH4-C-carbon losses caused by CH4, CO2-C-carbon losses caused by CO2, Org-C-total organic carbon.
gradually during composting which was because of the rate of C decomposition greater than N loss (Chang et al., 2019; Szanto et al., 2007). The C/N ratio of ST, TT, FAT and FAAT was 22.5, 18.6, 16.7 and 15.6, respectively. Mathur et al. (1993) believed that the compost achieves maturity criterion when the C/N ratio is < 20. Using this threshold, the compost of TT, FAT and FAAT reached the mature state but that of ST did not.
3.7. Comprehensive assessment Environmental indicators (GWP caused by N loss and C loss), fertilizer indexes (N content) and toxicity indicators (GI and harmlessness time) were chosen to make a comprehensive assessment of all the treatments (Fig. 6). The GWP value of ST, TT, FAT and FAAT was 119.1, 73.3, 54.1 and 52.8 kg CO2-eq/t, respectively. For the fertilizer indexes, the N content of the compost ranked from low to high as ST, TT, FAT and FAAT. About the toxicity indicators, the harmlessness time of treatment FAT and FAAT was the shortest and the treatments reached maturity state except for ST treatment based on GI. Fig. 6 showed that there were trade-off among various objectives of the composting methods. If the composting space and time were sufficient and the
Fig. 6. Integrated assessment metric of all the treatments. ST-static treatment, TT-turning treatment, FAT-forced aeration treatment, FAAT-forced aeration with acidification treatment.
environment influence was not considered, the static composting method was recommended. However, if the production efficiency, compost quality and environment influence were simultaneously considered during composting, the forced aeration with acidification method would be chosen. To achieve the multiple goals of agricultural
Table 2 Germination index and C/N ratio changes during the composting period.a GI (%) Time (day)
ST
0 3 7 10 14 21 28 35
52.3 49.2 49.0 48.0 58.7 72.2 72.3 72.8
a
± ± ± ± ± ± ± ±
5.87 4.88 4.29 10.03 5.37 9.56 9.77 13.89
C/N ratio
TT
FAT
FAAT
ST
69.1 ± 4.39 69.8 ± 7.31 56.9 ± 10.25 64.7 ± 4.32 96.6 ± 14.34 91.4 ± 14.32 116.0 ± 11.91 100.1 ± 14.35
51.7 ± 4.71 57.9 ± 8.18 69.5 ± 9.20 138.3 ± 12.52 116.5 ± 9.80 137.7 ± 9.43 121.1 ± 10.29 109.6 ± 5.87
0.3 ± 0.07 61.1 ± 7.28 73.8 ± 10.04 92.8 ± 11.69 141.3 ± 13.49 98.0 ± 9.30 91.0 ± 10.33 92.7 ± 9.63
30.0 28.9 28.2 27.2 27.0 26.7 26.1 25.5
Values are mean ± Standard deviation (n = 3). 7
TT
± ± ± ± ± ± ± ±
0.17 0.19 0.24 0.25 0.12 0.27 0.10 0.17
30.0 28.3 24.3 21.6 19.6 19.2 19.0 18.6
FAT
± ± ± ± ± ± ± ±
0.15 0.17 0.21 0.23 0.11 0.25 0.09 0.15
30.1 25.0 21.3 19.1 18.1 17.5 16.9 16.7
FAAT
± ± ± ± ± ± ± ±
0.14 0.16 0.20 0.22 0.11 0.23 0.09 0.14
30.0 24.7 20.6 18.2 17.1 16.5 16.3 15.6
± ± ± ± ± ± ± ±
0.15 0.15 0.19 0.21 0.10 0.22 0.08 0.14
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sustainable development in China, treatment of livestock manure through the composting subjected to forced aeration combined with acidification can be the most suitable measures for practical production.
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Effect of C/N ratio, aeration rate and moisture content on ammonia and greenhouse gas emission during the composting. J. Environ. Sci. (China) 23, 1754–1760. Jiang, T., Schuchardt, F., Li, G., Guo, R., Luo, Y., 2013. Gaseous emission during the composting of pig feces from Chinese Ganqinfen system. Chemosphere 90, 1545–1551. Jiang, T., Li, G., Tang, Q., Ma, X., Wang, G., Schuchardt, F., 2015. Effects of aeration method and aeration rate on greenhouse gas emissions during composting of pig feces in pilot scale. J. Environ. Sci. (China) 31, 124–132. Jiang, T., Ma, X., Tang, Q., Yang, J., Li, G., Schuchardt, F., 2016. Combined use of nitrification inhibitor and struvite crystallization to reduce the NH3 and N2O emissions during composting. Bioresour. Technol. 217, 210–218. Kissel, D.E., Brewer, H.L., Arkin, G.F., 1977. Design and test of a field sampler for ammonia volatilization1. Soil. Sci. Soc. Am. J. 41, 1133–1138. Kithome, M., Paul, J.W., Bomke, A.A., 1999. 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Comparative evaluation of the use of acidic additives on sewage sludge composting quality improvement, nitrogen conservation, and greenhouse gas reduction. Bioresour. Technol. 270, 467–475. Sanchez-Monedero, M.A., Cayuela, M.L., Roig, A., Jindo, K., Mondini, C., Bolan, N., 2018. Role of biochar as an additive in organic waste composting. Bioresour. Technol. 247, 1155–1164. Shi, C., Wang, J., Li, G., Jiang, T., Wei, H., Ma, Z., 2011. Control of different chemical additives on nitrogen loss during composting of pig manure. J. Agro-Environ. Sci. 30, 1001–1006. Sun, Q., Chen, J., Wei, Y., Zhao, Y., Wei, Z., Zhang, H., Gao, X., Wu, J., Xie, X., 2019. Effect of semi-continuous replacements of compost materials after inoculation on the performance of heat preservation of low temperature composting. Bioresour. Technol. 279, 50–56. Szanto, G.L., Hamelers, H.M., Rulkens, W.H., Veeken, A.H.M., 2007. NH3, N2O and CH4 emissions during passively aerated composting of straw-rich pig manure. Bioresour. Technol. 98, 2659–2670. Tiquia, S.M., Tam, N.F.Y., 1998. Composting of spent pig litter in turned and forced aerated piles. Environ. Pollut. 99, 329–337. TMECC (Test Methods for the Examination of Composts and Composting), 2002. In: Thompson, W., Leege, P., Millner, P., Watson, M.E. The US Composting Council, US Government Printing Office, < http://tmecc.org/tmecc/index.html > . Van der Werf, P., Ormseth, J., 1997. Measuring process parameters at an enclosed composting facility. Biocycle 38, 58–61. Wang, M., Awasthi, M.K., Wang, Q., Chen, H., Rex, X., Zhao, J., Li, R., Zhang, Z., 2017. Comparison of additives amendment for mitigation of greenhouse gases and ammonia emission during sewage sludge co-composting based on correlation analysis. Bioresour. Technol 243, 520–527. Wang, X., Selvam, A., Chan, M., Wong, J.W.C., 2013. Nitrogen conservation and acidity control during food wastes composting through struvite formation. Bioresour. Technol. 147, 17–22. Wang, X., Selvam, A., Wong, J.W.C., 2016b. Influence of lime on struvite formation and nitrogen conservation during food waste composting. Bioresour. Technol. 217, 227–232. Wang, Q., Wang, Z., Awasthi, M.K., Jiang, Y., Li, R., Ren, X., Zhao, J., Shen, F., Wang, M., Zhang, Z., 2016a. Evaluation of medical stone amendment for the reduction of nitrogen loss and bioavailability of heavy metals during pig manure composting. Bioresour. Technol. 220, 297–304. Yang, F., Li, G., Yang, Q., Luo, W., 2013. Effect of bulking agents on maturity and gaseous emissions during kitchen waste composting. Chemosphere 93, 1393–1399. Yang, F., Li, G., Shi, H., Wang, Y., 2015. Effects of phosphogypsum and superphosphate on compost maturity and gaseous emissions during kitchen waste composting. Waste Manage. (Oxford) 36, 70–76. Yuan, J., Li, Y., Chen, S., Li, D., Tang, H., David, C., Li, S., Li, W., Li, G., 2018. Effects of phosphogypsum, superphosphate, and dicyandiamide on gaseous emission and compost quality during sewage sludge composting. Bioresour. Technol. 270, 368–376. Zeng, J., Yin, H., Shen, X., Liu, N., Ge, J., Han, L., Huang, G., 2018. Effect of aeration interval on oxygen consumption and GHG emission during pig manure composting. Bioresour. Technol. 250, 214–220. Zucconi, F., Forte, M., Monac, A., Beritodi, M., 1981. Biological evaluation of compost maturity. Biocycle 22, 27–29.
3.8. Applications and future research recommendations Different composting methods were comprehensively evaluated in terms of harmless time, fertilizer characteristics and environmental effects. These results are of great significance for guiding the composting practice, especially in China where generated 4 billion tons manure each year. Forced aeration composting method with acid significantly improved manure treatment efficiency and shortened maturity time, it also conserved more N and released less CO2-eq. Meanwhile, to further understanding the mechanism of N and C transformation, more effort should be done on the following potential field: (1) the dynamic changes of microbial population in composting process under different composting methods; (2) determining N (C) fate by the isotope labelling methods because some unknown N (C) losses were existed during the evolution of N (C) in composting process; and (3) expanding the scale of the experiment and carrying out pilot or larger scale experiments to test the feasibility and effectiveness of the advanced measures in practice, based on the first results from our experiments. 4. Conclusions Except for the static treatment, the other treatments all reached full maturity. Turning and forced aeration sped up the composting process and improved efficiency, but also increased the C and N losses. All of ventilation modes reduced the emissions of N2O and CH4, while acidification increased N2O emissions and decreased CH4 emissions. Forced aeration with acid composting method got the best compost quality, lowest N losses and GWP value. Overall, considering of the environment, agronomy value, phytotoxicity and efficiency indicators, the method forced aeration with acidification was most suitable for composting practice production. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 31272247), the Program of International S&T Cooperation, Ministry of Science and Technology, China (No. 2015DFG91990), Science and Technology Program of Hebei (No. 18226607D and 19227305D) and Hebei Dairy Cattle Innovation Team of Modern Agroindustry Technology Research System (No. HBCT2018120206). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.122046. References Awasthi, M.K., Wang, Q., Rena, X., Zhao, J., Huang, H., Awasthi, S.K., Lahori, A.H., Li, R., Zhang, Z., 2016. Role of biochar amendment in mitigation of nitrogen loss and greenhouse gas emission during sewage sludge composting. Bioresour. Technol. 219, 270–280. Cao, Y., Wang, X., Bai, Z., Chadwick, D., Misselbrook, T., Sommer, S.G., Qin, W., Ma, L., 2019. Mitigation of ammonia, nitrous oxide and methane emissions during solid waste composting with different additives: a meta-analysis. J. Clean. Prod. 235, 626–635. Chang, R., Li, Y., Chen, Q., Guo, Q., Jia, J., 2019. Comparing the effects of three in situ methods on nitrogen loss control, temperature dynamics and maturity during composting of agricultural wastes with a stage of temperatures over 70 °C. J. Environ. Manage. 230, 119–127. Chowdhury, M.A., Neergaard, A.D., Jensen, L.S., 2014. Potential of aeration flow rate and biochar addition to reduce greenhouse gas and ammonia emissions during manure composting. Chemosphere 97, 16–25.
8
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Transformation of nitrogen and carbon during composting of manure litter with different methods Bingxin Tonga, Xuan Wangb, Shiqiang Wanga, Lin Mab, Wenqi Maa, a b
T
⁎
College of Resources and Environment Science, Hebei Agricultural University, Baoding 071000, China Center for Agricultural Resources Research, Institute of Genetic and Developmental Biology, The Chinese Academy of Sciences, Shijiazhuang 050021, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Composting methods Nitrogen Carbon Comprehensive assessment Greenhouse gases
In this study, to investigate the nitrogen and carbon characteristics throughout the composting process in different systems, four methods of composting including static treatment (ST), turning treatment (TT), forced aeration treatment (FAT), and forced aeration with acidification treatment (FAAT) were conducted. Organic matter degradation was improved in TT and FAT that accelerated the composting efficiency. The harmless time based on phytotoxicity was significantly shortened in FAT comparing with ST. However, nitrogen loss through ammonia volatilization increased 14.0%. Ammonia volatilization could be significantly decreased to 17.0% after acidification optimization with FAAT. Compared to FAT, the FAAT got an increased nitrous oxide production and decreased methane emission. Generally, the lowest global warming potential value (52.8 kg CO2-eq/t) was found in FAAT. Therefore, considering the environmental, fertilizer and toxicity indicators, the FAAT composting method is the most promising method, and has the potential to be promoted for implementation in practice.
1. Introduction Composting is globally recognized as one of the most suitable and effective methods for livestock waste treatment and is widely used in practice (Li et al., 2012). According to preliminary statistics, more than 17 Mt organic fertilizers were produced by composting every year in China (Jiang et al., 2016). However, a series of environmental problems caused by composting should not be ignored, especially ammonia (NH3) volatilization, nitrous oxide (N2O) and methane (CH4) emissions (Zeng et al., 2018). Therefore, it is very important to find an effective composting method that can coordinate the economic, environmental and resource demands, which is greatly depended on the transformation of carbon and nitrogen processes. As an aerobic biochemical process, composting is greatly influenced by ventilation methods associated with oxygen content that determine the transformation of carbon and nitrogen processes and the gaseous forms emitted (Chowdhury et al., 2014; He et al., 2017). Composting can be divided into many types based on the aeration modes (Jiang et al., 2016). The common composting methods include static, turning and forced aeration composting (Zeng et al., 2018). Static composting was broadly used in the past, but is gradually replaced by turning or forced aeration because of its long composting time (Van der Werf and Ormseth, 1997). ⁎
Turning or forced aeration could speed up the compost maturation process, but increase the NH3 emission and also have different impacts on greenhouse gas emissions from composting (He et al., 2017; Yuan et al., 2018). A study found that turning manure heaps can increase N2O emissions compared to the no-turning treatment, because the more efficient O2 supply of the turning treatments was suitable for N2O production (Jiang et al., 2015). However, some studies indicated that compared to static treatment, turning decreased N2O and CH4 emissions (He et al., 2017; Yuan et al., 2018). There have been inconsistent results regarding the effect of aeration strategy on GHG emissions from composting. A laboratory-scale forced aeration composting experiment showed that the higher aeration rates reduced the CH4 emission but increased the N2O emission (Jiang et al., 2011). Zeng et al. (2018) investigated the ventilation strategy of reducing aeration frequency at the initial stage and increasing aeration volume at the final stage of composting, which indicated the increases in CH4 emission and no impacts on N2O emission compared to the conventional aeration method. Various countermeasures aiming at reducing NH3 emission have been explored, e.g. adjusting the C/N ratio and moisture content (Jiang et al., 2016), aeration rate (He et al., 2017) and adding additives (Gu et al., 2018). Results from a meta-analysis showed that the addition of acid during composting was the greatest mitigation measure of NH3, which can significantly reduce the NH3 emission by 44.5% on average
Corresponding author at: 289 Lingyusi Street, Baoding 071000, China. E-mail address: [email protected] (W. Ma).
https://doi.org/10.1016/j.biortech.2019.122046 Received 17 June 2019; Received in revised form 19 August 2019; Accepted 20 August 2019 Available online 21 August 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.
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2.2. Sampling
(Cao et al., 2019). However, the influence of acidification on GHG emissions was still not clear. For example, Pan et al. (2018) found that after the addition of citric acid the N2O emission nitrogen output reduced 51.26%, while with adding phosphoric acid the increasing proportion of N2O emissions was 31.89%. Meanwhile, when adding with citric acid and phosphoric acid to compost, the CH4 emission rate decreased significantly. In addition, Yang et al. (2015) and Mao et al. (2017) concluded inconsistent impacts of adding acid on N2O emissions during composting. Previous studies mainly focused on different gaseous mitigation strategies in single composting method. However, different composting methods were co-existence in practise such as, static heap, windrow composting with turning and forced aeration composting. There is a need to increase the understanding about the response to C and N transformation and gaseous emissions to various composting methods, with the aims to satisfy environmental, resources efficient and economic benefits during composting. Hence, our objectives of this study were to (i) understand the characteristics of C and N transformation processes during composting subjected to different methods, (ii) analyse the effect of different composting methods on nitrogen losses and GHG emissions, and (iii) clarify the comprehensive effects of different composting methods on composting product quality, nitrogen losses, GHG emissions and economic feasibility.
Three representative samples (approximately 400 g) were taken at day 0, 3, 7, 10, 14, 21, 28 and 35. Each fresh sample was divided into 2 parts. One part was stored at −20 °C and another part was air-dried. The air-dried sample were crushed and passed through 1 mm sieve for measuring physico-chemical properties (Wang et al., 2016b). A plexiglass chamber (1 m × 1 m × 0.2 m) was used for the collection of gaseous samples from the ST and TT treatments. During sampling, the box was put on the top of the surface of the composting piles and pressed into the pile to 15 cm depth, so that the top space was approximately 50 L. A vacuum pump was connected to the chamber to collect the gases from the pile at a flow rate of 90 m3 per hour controlled by a flowmeter that ensured 20 times volume of the top space (Kissel et al., 1977). There were two flowmeters connected to the pump. One was for measuring the ventilation rate through the composting materials surface and another one was used for adjusting the inlet ventilation rate into a conical flask with boric acid. The NH3 samples were measured 2 times daily and trapped with boric acid (2%) using a dynamic chamber method, then titrated with H2SO4 (Jiang et al., 2016). Greenhouse gases (N2O, CH4, and CO2) samples were collected with a static chamber method (Jiang et al., 2015). Sampling frequency was twice daily at one day intervals. The samples of N2O, CH4, and CO2 were analysed by gas chromatography (Agilent 7890A, America). Collection and measurement of the NH3 and GHG samples of the treatments FAT and FAAT was conducted following the methodologies in a previous study (Jiang et al., 2016).
2. Materials and methods 2.1. Feedstock of composting and experimental design The composting experiment was conducted at the Luancheng experimental station of the Chinese Academy of Sciences. Cow manure and maize straw were used as the composting feedstock. Cow manure collected from the Dingyuan dairy farm, and maize straw collected from a nearby village. Maize stalks were crushed to approximately 2 cm. The cow manure and maize straw were mixed at the ratio of 3:2 (w/w fresh weight basis), adjusting the moisture content to approximately 60% and the C/N ratio to nearly 30 of the initial materials (Jiang et al., 2011). The basic physical and chemical properties of the composting materials are shown in Table 1. There were 4 treatments in total: static treatment (ST), turning treatment (TT), forced aeration treatment (FAT), and forced aeration with acidification treatment (FAAT). The ST treatment simply involved packing the compost mixture into the stainless steel composting container (1.2 m × 1.2 m × 1.2 m). The TT treatment was supplied oxygen with turning. The pile was turned at the 3rd, 7th, 10th, 14th, 21st, 28th, and 35th day of composting (Wang et al., 2013). In the FAT treatment, the composting materials were mixed following the turning frequency of TT and an extra pump was used for continuous ventilation. The ventilation rate was 0.5 L/min/kg dry matter (Wang et al., 2016b). The FAAT treatment was a further modification of the FAT treatment. Sulphuric acid (H2SO4) was added to the initial materials before composting. The addition amount of H2SO4 was 10% of the initial molar quantity of nitrogen in the feedstock (Shi et al., 2011). The experimental period lasted 35 days, from April 14, 2017 to May 18, 2017.
2.3. Analytical methods Compost samples were analysed for moisture content (MC), pH, total organic carbon (TOC), total Kjeldahl nitrogen (TKN), NH4+-N, NO3−-N, and germination index (GI). The temperatures of the pile in the top, middle and bottom layer were measured using a metal thermometer two times daily (10:00 AM and 3:00 PM). The temperatures of the FAT and FAAT treatments were measured and recorded every 10 min automatically with a computer-controlled system. The samples were oven-dried at 105 °C to reach a constant weight for MC measurement (Li et al., 2012). The pH value was determined by diluting the sample with distilled water (1:10, w/v) and measuring using a pH meter (Starter 3C-Ohaus, America) (Li et al., 2012). Organic matter (OM) was determined by loss on ignition at 550 °C for 4 h using a muffle furnace (FR-1236, China). The conversion coefficient of organic matter and organic carbon was 1.724 (Li et al., 2012). The TKN was determined according to the manufacturer’s manual using an Analyzer (Hanon K9860, China) (TMECC, 2002). NH4+-N and NO3−-N were determined by diluting the sample with 2 mol/L KCl solution (1:10, w/ v) and measured with a continuous flow analyzer (SEAL AutoAnalyzer 3, Germany). The GI was measured by using the same diluted solution as for pH measurement, extracting 5 ml of the diluted solution into a culture dish with 9 cm filter paper at the bottom and placing 20 Chinese cabbage seeds into the culture dish and setting the dish in a 25 °C incubator for 48 h (Shi et al., 2011; Zucconi et al., 1981). All the aforementioned analyses were made with three replicates. The percentage of OM loss was calculated by applying Eq. (1) (Haug, 1993):
OMloss (%) = 100 ∗ (DM0 ∗ OM0 − DMT ∗ OMT )/(DM0 ∗ OM0 )
Table 1 The basic physical and chemical properties of the composting materials.a
Manure Maize straw a
Moisture (%)
TOC (%)
TN (%)
C/N
81.84 ± 0.33 26.76 ± 0.12
35.62 ± 0.14 47.66 ± 0.22
1.69 ± 0.02 1.39 ± 0.01
21.08 34.29
(1)
where DM0 was the dry matter weight of the original feedstock, OM0 was the organic matter content of the original feedstock, DMT was the dry matter weight at the end of composting, and OMT was the organic matter content of the last compost. The seed germination index formula was Eq. (2) (Zucconi et al., 1981):
Values are mean ± Standard deviation (n = 3). 2
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GI(%) = (Seed germination of compost sample ∗ Root length of compost sample) /(Seed germination of control ∗ Root length of control) ∗ 100
(2)
The ammonia emission N loss was Eq. (3) (Wang et al., 2016b):
NH3 - N emission loss (%) = Σ(AE i)/ITN ∗ 100, (i=1, 2 . .. n)
(3)
where AEi was the NH3-N emission amount of the n-th composting day, and ITN was the initial total nitrogen of the composting mixture. The N2O-N emission loss, CH4-C emission loss and CO2-C emission loss were calculated by applying Eq. (4), Eq. (5) and Eq. (6), respectively:
N2 O - N emission loss (%) = Σ(NE i)/ITN ∗ 100, (i= 1, 2 . .. n)
(4)
where NEi was the N2O emission N loss of the n-th composting day, and ITN was the initial total nitrogen of the composting mixture.
CH 4 - C emission loss (%) = Σ(ME i)/ITOC ∗ 100, (i = 1, 2 . .. n) (5) where MEi was the CH4 emission C loss of the n-th composting day, and ITOC was the initial total organic carbon of the composting mixture.
CO2 - C emission loss (%) = Σ(CE i)/ ITOC ∗ 100, (i = 1, 2 . .. n) (6) where CEi was the CO2 emission C loss amount of the n-th composting day, and ITOC was the initial total organic carbon of the composting mixture. For the global warming potential (GWP), 1 mol CH4 = 25 mol CO2eq, 1 mol N2O = 298 mol CO2-eq, and 1 mol NH3 = 3.86 mol CO2-eq (IPCC, 2014; Jiang et al., 2016). In the section of evolution of the initial nitrogen, the calculation formulation of organic nitrogen (Org-N) and others (Other-N) were Eqs. (7) and (8) (Jiang et al., 2016):
Org - N = TN −
NH+4
-N −
NO−3
-N
Other - N = Total N losses − NH3 loss − N2 O loss − sample loss
Fig. 1. Evolution of temperature (a) and pH (b) during composting. ST-static treatment, TT-turning treatment, FAT-forced aeration treatment, FAAT-forced aeration with acidification treatment. The error bars are standard deviation (n = 3).
significantly shorten duration of thermophilic phase was also founded in the continuous ventilation composting experiments (Jiang et al., 2016; Sun et al., 2019). The thermophilic phase in FAAT was obvious shorter than that in FAT and this was because of the addition of acid. Shi et al. (2011) also found that the treatments with addition of acid shortened the lasting time of high temperature period because acidification limited the activity of thermophilic microorganisms. However, little negative influence was found on the maturity of final compost product after 35 days composting period. The pH value sharply increased at the first 3 days and then kept a stable level for all the treatments during composting (Fig. 1b). The pH of FAT was nearly 8.6, which was significantly higher than that of other treatments (P < 0.05). The reasons were that forced aeration increased biodegradation of organic acids and mineralization of organic compounds (He et al., 2017). At last, the pH values of all the four treatments were < 7 (from 7.6 to 8.6). These results were in line with the previous study on adding additives to reduce N2O and NH3 emissions (Jiang et al., 2016), but in contrary to one previous study (last pH value was < 7) because of the feedstock of composting was only pig faeces (Tiquia and Tam, 1998). The pH value of each treatment ranked as FAT > TT > ST > FAAT, with the statistically significant differences among treatments (P < 0.05).
(7) (8)
In the section of evolution of the initial organic carbon, the others form carbon (Other-C) calculation was Eq. (9) (Jiang et al., 2016):
Other - C = Org - C − CO2 - C loss − CH 4 - C loss
(9)
2.4. Statistical analysis The data were analysed using one-way ANOVA. The least significant difference test was used to determine the significance of the difference in the mean values. All data were analysed using SPSS 17.0 for Windows. 3. Results and discussion 3.1. Temperature and pH The temperature in all the treatments increased sharply and exceeded 50 °C at the second day (Fig. 1a). For the ST, TT, FAT and FAAT treatments, the thermophilic phase (> 50 °C) lasted 34, 22, 8 and 6 days, respectively, which was long enough to satisfy the Chinese national standard for sanitation (GB 7959-2012, 2012). Compared to ST, the other three treatments had better aeration conditions that can supply sufficient oxygen for microbial and lead to a quick temperature increasing due to more dramatic organic matter degradation at the beginning. However, because of less easy degradable organic matter after thermophilic phase and more heat loss leaded by continued aeration, temperature of FAT and FAAT decreased earlier than other treatments (Jiang et al., 2013; Awasthi et al., 2016). A
3.2. Nitrogen characteristics of compost For the ST treatment, the TN concentration slightly increased during composting. However, for the TT, FAT and FAAT treatments, the TN content increased rapidly during the first 10 days and then remained at the same level (Fig. 2a). The TN content of the other three treatments was significantly greater than that in ST. This pattern indicated that measures such as turning, forced aeration and acidification could increase the TN content in composting products. These results are mainly 3
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Fig. 2. Changes of TN content (a), NH4+-N content (b), NO3−-N content (c), NH3 flux (d), N2O flux (e), Nitrogen loss (f) during composting. ST-static treatment, TTturning treatment, FAT-forced aeration treatment, FAAT-forced aeration with acidification treatment. The error bars are standard deviation (n = 3).
After the initial phase the OM and organic acid degraded greatly, and the content of NO3−-N in the compost increased with the onset of the high-temperature period. Another study indicated that the NO3−-N content increased sharply after the experiment started 4 weeks (the mesophilic and cooling phase) with pig manure and sawdust (Wang et al., 2016a). In the whole composting process the NO3−-N level of the ST and FAAT treatments was significantly higher than that of the other treatments, which was mainly because of the lower N loss ratio and conversion of NH4+-N to NO3−-N (Cao et al., 2019; Shi et al., 2011). At the end of composting, the highest content of NO3−-N was in the FAAT treatment, followed by the ST, FAT and TT treatment. The results showed that acidification could improve the NO3−-N content of composting products. Ammonia volatilization showed a trend of increasing first and then decreasing in all the treatments during the whole composting process (Fig. 2d), in line with the results of previous studies (e.g., Wang et al., 2013). The maximum NH3 emission rate reached at different days among the four treatments. The results of this study showed that NH3 emission peak appearance can be accelerated by forced ventilation and NH3 emission rate can be reduced by acidification, which is similar to the previous results (Cao et al., 2019; Jiang et al., 2011; SanchezMonedero et al., 2018; Shi et al., 2011). The addition of H2SO4 could reduce the NH3 emission rate comparing the FAT and FAAT treatments during composting, which is similar to the conclusions by Cao et al. (2019) and Shi et al. (2011). Comparison of FAT, TT and ST showed that forced aeration and turning increased the flux of NH3 emission. This result indicated that the rate of NH3 emission was related to the amount of aeration during composting (Gu et al., 2018; Pan et al.,
due to the weight reduction of the mass, caused by the OM degradation in the TT, FAT and FAAT treatments (Pan et al., 2018). Compared to ST, the TT and FAT treatments maintained higher TN concentration, with no significant difference between the latter two treatments. The FAAT treatment had a higher TN concentration than the other treatments because the addition of H2SO4 decreased the nitrogen loss (Cao et al., 2019; Shi et al., 2011). The content of NH4+-N firstly decreased and then tended to be stable of all the four treatments (Fig. 2b). Jiang et al. (2013) reported similar results, which primarily because the NH4+-N transformed into NH3. However, other research showed that the trend of NH4+-N concentration was different (increased at the beginning and then decreased) due to the conversion of organic nitrogen to NH4+-N via ammonification (Jiang et al., 2016; Pan et al., 2018). While, Gu et al. (2018) found that with the addition of sulphur (0.25% DW) the NH4+N content increased gradually. The NH4+-N content of ST was substantially higher than in other treatments because of the lower nitrogen loss and lower OM degradation (Jiang et al., 2015). The addition of H2SO4 increased the NH4+-N content as the certificate through comparison of the FAAT and FAT treatments, Cao et al.(2019) and Yuan et al. (2018) found the similar result, this was because of the addition of acid decreased the pH of the compost. Considering the NO3−-N content of all the treatments (Fig. 2c), the content decreased at the early stage and then showed an upward trend. Jiang et al. (2016) and Pan et al. (2018) found the similar results. The decline of NO3−-N content during the first days was due to N2O emission. This change could be explained as the conversion of NO3−-N by denitrifying bacteria to N2O (Gu et al., 2018; Szanto et al., 2007). 4
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TT > FATT > FAATT > ST. The NH4+-N content also showed a gradual decrease trend. The FAT treatment got the highest NH4+-N decrease ratio and followed by the TT, FAAT and ST. This indicated that forced aeration and turning would increase the NH4+-N losses during composting. The reason for the decrease of NH4+-N in TT was less than that of FAT may be because the TT treatment had more organic N degradation (Pan et al., 2018). The Fig. 3 also showed that NO3−-N occupied very little proportion in total nitrogen and all of its ratio was < 1% in four treatments. The greatest accumulation of N loss caused by NH3 emission was in the TT treatment, and the reason has explained in the 3.2 part of this paper. The rate of nitrogen loss caused by NH3 emission occupied the TN losses varied among the treatments, which was approximately 84.2% for TT, followed by FAT (83.8%), FAAT (81.7%) and ST (64.4%). The ST had the largest N2O emission (1.0%) and for TT, FAT and FAAT, this loss ratio was approximately 0.4%, 0.2% and 0.3%, respectively. There were some other unknown N losses ranging from 3.5 ~ 4.9% of the initial nitrogen. These perhaps caused by the loss of other nitrogen-containing gas (Jiang et al., 2016; Yuan et al., 2018).
2018). For TT, FAT and FAAT, NH3 emissions happened at the thermophilic stage and the emission patterns were similar to the findings of Jiang et al. (2011). The duration of NH3 emission is another factor contributed to total NH3 volatilization (Cao et al., 2019; Chowdhury et al., 2014). The duration of NH3 emission differed among the treatments. In FAAT and FAT, NH3 emission occurred mainly in the first 2 weeks, but for TT and ST occurred mainly in the first 3 weeks during composting. The N2O emissions trend of the ST treatment was different from other treatments (Fig. 2e). The N2O emission flux was high at the beginning of the composting process, but after the 7th day remained almost unchanged for the ST treatment. The reason for that was denitrification or nitrification had begun during the process of mixing the composting materials (Gu et al., 2018; Wang et al., 2017). In the later stage, due to the continuous high temperature (above 60 °C), the denitrification process was inhibited (Gu et al., 2018). However, He et al. (2017) and Pan et al. (2018) found that the N2O emissions peaks occurred at the thermophilic stage. The main reason for this difference was the nature of composting materials (Sanchez-Monedero et al., 2018). The N2O emission tendencies of the other three treatments were as follows: high emission rate happened at the early stage; the emission rate decreased sharply and maintained a low level during the hightemperature period; and the emission rate gradually increased at the late stage. At the early stage and high-temperature period, the reason for the emission patterns was described as the ST treatment. After the thermophilic phase, the N2O emission fluxes of the TT, FAT and FAAT treatments increased, which was because of the low temperature and high NO3−-N content of feedstock (Jiang et al., 2016; Zeng et al., 2018). Multiple comparisons indicated that N2O emission reduced with the higher aeration rate for the ST, TT and FAT treatments, similar to the results observed by others (Wang et al., 2017; Yang et al., 2013). With two-way comparisons of the four treatments, turning and forced aeration significantly reduced the emission of N2O, but acidification increased the emission of N2O. The nitrogen loss ratio was largely different among the treatments (Fig. 2f). The N loss ratio was: TT > FAT > FAAT > ST. These results showed that turning and forced aeration significantly increased the TN losses, while addition of acid reduced the losses (Pan et al., 2018). The NH3 emission was the main type of the N loss for all treatments (Cao et al., 2019; Gu et al., 2018; Szanto et al., 2007). For ST, TT, FAT, and FAAT, the percentage of NH3 emission N loss of the initial TN was 8.5%, 28.1%, 22.5%, and 17.0%, respectively. The NH3 emission patterns from this study are consistent with previous research (Szanto et al., 2007; Jiang et al., 2013). However, Kithome et al. (1999) found that the N loss caused by NH3 emission ranged from 47% to 62% of the initial N after 25 days of composting with manure, which was due to high initial TN content and low C/N ratio of the manure. Moreover, some other research showed the opposite result which only 2.4% ~ 3.9% of the initial TN lost in the form of NH3 because of the low C/N ratio of the feedstock (Szanto et al., 2007). While the N2O emission N loss was lower, the loss ratio was only approximately 1.0% (ST), 0.4% (TT), 0.2% (FAT), and 0.3% (FAAT), similar to the results of Yang et al. (2013). The N2O emission nitrogen loss was quite lower than NH3 emission nitrogen loss, which was only accounting for 0.7% to 7.6% of the total gaseous nitrogen loss, and the results were similar to the previous study (Yuan et al., 2018).
3.4. Carbonaceous gas emission The CH4 and CO2 emissions were the main routes of carbon loss during composting (Liu et al., 2017). The trend of CH4 emission of ST treatment showed an initial increase and then decrease (Fig. 4a). The emission rate tended to be moderate in the ST treatment after 24 days. Whereas the TT, FAT and FAAT treatment showed another trend (first decreased, then increased and decreased again). The highest CH4 emission flux happened during the thermophilic phase, the same as observed by others (Chang et al., 2019; Yuan et al., 2018). The reasons for the higher CH4 emission rate in ST than that in TT and FAT were because the internal environment of the pile body was more suitable for anaerobic action (Liu et al., 2017; Zeng et al., 2018). The FAAT had a slightly lower CH4 emission rate compared with FAT, which was because of the addition of acid (Pan et al., 2018). Yuan et al. (2018) and Pan et al. (2018) found that adding superphosphate and phosphoric acid can sharply mitigate the CH4 emissions. The ST had a higher emission rate compared to the other three treatments because of the relatively low oxygen concentration inside the compost materials (He et al., 2017). The trend of CO2 emission flux in each treatment was a fluctuation in the early stage and a tendency to be smooth after 21 days (Fig. 4b). There were two emission peaks of all the treatments, but the time of occurrence was not the same. Yuan et al. (2018) found the similar results (two peaks) in a sewage sludge composting with addition of phosphogypsum in a continuously aerated system. These two emission peaks may be caused by the change of the internal physicochemical properties in the pile (Chang et al., 2019). The explanation for the lowest CO2 emission of ST was the lack of O2 in the pile (Zeng et al., 2018). The results of C loss caused by CO2 showed that a certain amount of ventilation will increase the C loss during composting, which also observed by other studies (Cao et al., 2019; Jiang et al., 2013). The CO2 emissions mainly happened at the thermophilic phase of all the treatments, and the reason for this was the rapid degradation of OM (Jiang et al., 2016; Pan et al., 2018). The comparison of TT and FAT indicated that continuous aeration did not increase the CO2-C loss. But the TT had a greater CO2-C loss, which may be related to the longer period of high temperature (Zeng et al., 2018). The carbon loss in the form of CO2 was not significantly different between FAAT and FAT, which demonstrated that acidification did not affect the CO2 emission.
3.3. Evolution of the initial nitrogen As shown in Fig. 3, each form of nitrogen that accounted for the proportion of TN of the initial materials varied during the composting process. The N balance calculation methods were shown as Eqs. (7) and (8). At the beginning of the experiment the proportion of Org-N was approximately 82.0% for all the treatments. The Org-N content of each treatment showed a decreasing trend, while the amount of decrease in each treatment was different. The changes ratio of the treatments was:
3.5. Evolution of the initial organic carbon The carbon content of each form was converted to a percentage of the initial C content, and different types of C were displayed and analysed (Fig. 5). The carbon balance calculation formulation was as Eq. 5
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Fig. 3. Evolution of nitrogen in feedstock during composting. Others-undetermined nitrogen, N2O-nitrogen loss caused by N2O emission, NH3-nitrogen loss caused by NH3 emission, NO3−-N-nitrate nitrogen content, NH4+-N-ammonium nitrogen content, Org-N-organic nitrogen content. ST-static treatment, TT-turning treatment, FAT-forced aeration treatment, FAAT-forced aeration with acidification treatment.
time of thermophilic phase (Chowdhury et al., 2014; Liu et al., 2017). The organic C degradation mainly occurred in the thermophilic phase of TT, FAT and FAAT, more than 90% of C losses occurred during this period. For ST, the organic carbon degradation rate was basically maintained at the same level during composting. The results indicated that turning and forced aeration could stimulate the organic carbon degradation rate (Szanto et al., 2007; Jiang et al., 2013). The carbon loss caused by CO2-C was the majority and CH4-C loss was only a small part (Pan et al., 2018; Zeng et al., 2018). The CO2-C loss proportion of the total C loss was 72.7% (ST), 83.2% (TT), 83.9% (FAT) and 84.0% (FAAT), respectively. Carbon loss caused by CH4 emissions was different for all the treatments. The ST treatment had the greatest ratio of CH4-C emissions and the loss of CH4 accounted for approximately 14.4% of the TOC loss, which was significantly higher than that in TT (2.1%), FAT (1.3%) and FAAT (1.1%). The main reason for this pattern was because of the internal anaerobic environment in ST, which was more suitable for the reaction of CH4 production (Gu et al., 2018; He et al., 2017). The results of CH4-C loss showed that turning and forced aeration could significantly reduce CH4 emission, but acidification slightly decreased CH4 emission. 3.6. Maturity of compost Maturity is associated with plant growth potential or phytotoxicity (Zucconi et al., 1981), whereas stability is often related to the compost’s microbial activity (Sun et al., 2019). There are many indicators to evaluate the maturity of compost. In this study, the GI and C/N ratio were selected. It is generally believed that when the GI is greater than 90%, the compost is fully mature (Yuan et al., 2018; Zucconi et al., 1981). At the end of composting all the treatments reached the maturity standard except for the ST (GI 72.4%) (Table 2). For TT, FAT and FAAT, the maturity state was reached at the 14th, 10th and 10th day, respectively. This was because of the more ventilation rate of the two forced aeration treatments (Chang et al., 2019; He et al., 2017). The results indicated that turning and forced aeration had positive effect on the maturity, but the acidification had no effect. As shown in Table 2, the C/N ratio of all the treatments decreased
Fig. 4. Changes of CH4 (a) and CO2 (b) emission rates of all the treatments during composting. ST-static treatment, TT-turning treatment, FAT-forced aeration treatment, FAAT-forced aeration with acidification treatment.
(9). For all the treatments, organic C decreased obviously. However, the changes in all the treatments were not the same. The organic C losses ratio of TT, FAAT, FAT and ST was 58.4%, 50.3%, 51.2% and 28.9%, respectively. These results were related to the supply of O2 and lasting
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Fig. 5. Evolution of CH4, CO2, Organic-C and Others during composting. ST-static treatment, TT-turning treatment, FAT-forced aeration treatment, FAAT-forced aeration with acidification treatment. Others-undetermined carbon, CH4-C-carbon losses caused by CH4, CO2-C-carbon losses caused by CO2, Org-C-total organic carbon.
gradually during composting which was because of the rate of C decomposition greater than N loss (Chang et al., 2019; Szanto et al., 2007). The C/N ratio of ST, TT, FAT and FAAT was 22.5, 18.6, 16.7 and 15.6, respectively. Mathur et al. (1993) believed that the compost achieves maturity criterion when the C/N ratio is < 20. Using this threshold, the compost of TT, FAT and FAAT reached the mature state but that of ST did not.
3.7. Comprehensive assessment Environmental indicators (GWP caused by N loss and C loss), fertilizer indexes (N content) and toxicity indicators (GI and harmlessness time) were chosen to make a comprehensive assessment of all the treatments (Fig. 6). The GWP value of ST, TT, FAT and FAAT was 119.1, 73.3, 54.1 and 52.8 kg CO2-eq/t, respectively. For the fertilizer indexes, the N content of the compost ranked from low to high as ST, TT, FAT and FAAT. About the toxicity indicators, the harmlessness time of treatment FAT and FAAT was the shortest and the treatments reached maturity state except for ST treatment based on GI. Fig. 6 showed that there were trade-off among various objectives of the composting methods. If the composting space and time were sufficient and the
Fig. 6. Integrated assessment metric of all the treatments. ST-static treatment, TT-turning treatment, FAT-forced aeration treatment, FAAT-forced aeration with acidification treatment.
environment influence was not considered, the static composting method was recommended. However, if the production efficiency, compost quality and environment influence were simultaneously considered during composting, the forced aeration with acidification method would be chosen. To achieve the multiple goals of agricultural
Table 2 Germination index and C/N ratio changes during the composting period.a GI (%) Time (day)
ST
0 3 7 10 14 21 28 35
52.3 49.2 49.0 48.0 58.7 72.2 72.3 72.8
a
± ± ± ± ± ± ± ±
5.87 4.88 4.29 10.03 5.37 9.56 9.77 13.89
C/N ratio
TT
FAT
FAAT
ST
69.1 ± 4.39 69.8 ± 7.31 56.9 ± 10.25 64.7 ± 4.32 96.6 ± 14.34 91.4 ± 14.32 116.0 ± 11.91 100.1 ± 14.35
51.7 ± 4.71 57.9 ± 8.18 69.5 ± 9.20 138.3 ± 12.52 116.5 ± 9.80 137.7 ± 9.43 121.1 ± 10.29 109.6 ± 5.87
0.3 ± 0.07 61.1 ± 7.28 73.8 ± 10.04 92.8 ± 11.69 141.3 ± 13.49 98.0 ± 9.30 91.0 ± 10.33 92.7 ± 9.63
30.0 28.9 28.2 27.2 27.0 26.7 26.1 25.5
Values are mean ± Standard deviation (n = 3). 7
TT
± ± ± ± ± ± ± ±
0.17 0.19 0.24 0.25 0.12 0.27 0.10 0.17
30.0 28.3 24.3 21.6 19.6 19.2 19.0 18.6
FAT
± ± ± ± ± ± ± ±
0.15 0.17 0.21 0.23 0.11 0.25 0.09 0.15
30.1 25.0 21.3 19.1 18.1 17.5 16.9 16.7
FAAT
± ± ± ± ± ± ± ±
0.14 0.16 0.20 0.22 0.11 0.23 0.09 0.14
30.0 24.7 20.6 18.2 17.1 16.5 16.3 15.6
± ± ± ± ± ± ± ±
0.15 0.15 0.19 0.21 0.10 0.22 0.08 0.14
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sustainable development in China, treatment of livestock manure through the composting subjected to forced aeration combined with acidification can be the most suitable measures for practical production.
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3.8. Applications and future research recommendations Different composting methods were comprehensively evaluated in terms of harmless time, fertilizer characteristics and environmental effects. These results are of great significance for guiding the composting practice, especially in China where generated 4 billion tons manure each year. Forced aeration composting method with acid significantly improved manure treatment efficiency and shortened maturity time, it also conserved more N and released less CO2-eq. Meanwhile, to further understanding the mechanism of N and C transformation, more effort should be done on the following potential field: (1) the dynamic changes of microbial population in composting process under different composting methods; (2) determining N (C) fate by the isotope labelling methods because some unknown N (C) losses were existed during the evolution of N (C) in composting process; and (3) expanding the scale of the experiment and carrying out pilot or larger scale experiments to test the feasibility and effectiveness of the advanced measures in practice, based on the first results from our experiments. 4. Conclusions Except for the static treatment, the other treatments all reached full maturity. Turning and forced aeration sped up the composting process and improved efficiency, but also increased the C and N losses. All of ventilation modes reduced the emissions of N2O and CH4, while acidification increased N2O emissions and decreased CH4 emissions. Forced aeration with acid composting method got the best compost quality, lowest N losses and GWP value. Overall, considering of the environment, agronomy value, phytotoxicity and efficiency indicators, the method forced aeration with acidification was most suitable for composting practice production. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 31272247), the Program of International S&T Cooperation, Ministry of Science and Technology, China (No. 2015DFG91990), Science and Technology Program of Hebei (No. 18226607D and 19227305D) and Hebei Dairy Cattle Innovation Team of Modern Agroindustry Technology Research System (No. HBCT2018120206). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.122046. References Awasthi, M.K., Wang, Q., Rena, X., Zhao, J., Huang, H., Awasthi, S.K., Lahori, A.H., Li, R., Zhang, Z., 2016. Role of biochar amendment in mitigation of nitrogen loss and greenhouse gas emission during sewage sludge composting. Bioresour. Technol. 219, 270–280. Cao, Y., Wang, X., Bai, Z., Chadwick, D., Misselbrook, T., Sommer, S.G., Qin, W., Ma, L., 2019. Mitigation of ammonia, nitrous oxide and methane emissions during solid waste composting with different additives: a meta-analysis. J. Clean. Prod. 235, 626–635. Chang, R., Li, Y., Chen, Q., Guo, Q., Jia, J., 2019. Comparing the effects of three in situ methods on nitrogen loss control, temperature dynamics and maturity during composting of agricultural wastes with a stage of temperatures over 70 °C. J. Environ. Manage. 230, 119–127. Chowdhury, M.A., Neergaard, A.D., Jensen, L.S., 2014. Potential of aeration flow rate and biochar addition to reduce greenhouse gas and ammonia emissions during manure composting. Chemosphere 97, 16–25.
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