Combined use of nitrification inhibitor and struvite crystallization to reduce the NH3 and N2O emissions during composting
Graphical abstract
CK: control; C0: 15% Mg and P salts; C1: 15% Mg and P salts + 2.5% DCD; C2: 15% Mg and P salts + 5.0% DCD; C3: 15% Mg and P salts + 7.5% DCD; C4: 15% Mg and P salts + 10.0% DCD. GHG: greenhouse gas. Global warming potential calculation: 1 mol NH3 = 3.86 mol CO2-eq, 1 mol N2O = 298 mol CO2-eq.
Introduction
Composting of animal manure is a widely used and effective technology, however harmful gasses, such as ammonia (NH3), and nitrous oxide (N2O), are emitted during the process as secondary pollution. N2O for example is a significant greenhouse gas (GHG) with global warming potential 298 times higher than that of carbon dioxide (CO2) (IPCC, 2007) and is considered to be an important factor in ozone depletion (Ravishankara et al., 2009). Ammonia has been shown to be a significant, and increasing, component of airborne fine particulate matter (PM2.5) in northern China (Li et al., 2013), from 2003 to 2012 its proportion has increased from 7.5% to 12%. Unlike nitrate and sulphate pollution, ammonium in the atmosphere is largely generated from agricultural activities (Shen et al., 2011).
During composting, N2O can be produced through both nitrification and de-nitrification processes. One formation mechanism is through incomplete nitrification of NH3 which is oxidized to hydroxylamine (NH2OH) by ammonia mono-oxygenase (AMO), which can be further oxidized to nitroxyl (NOH) by hydroxylamine oxidoreductase which produces N2O after polymerization and dehydration (Canfield et al., 2010). During denitrification, nitrite is reduced to NO by nitrite reductase, which is further reduced by nitric oxide reductase to N2O (Moenne-Loccoz and Fee, 2010). As a result of these processes, 0.4–9.9% of total nitrogen (TN) is emitted as N2O during the composting of animal manure (Tsutsui et al., 2015). The N2O emission rate is affected by the feedstocks C/N ratio, aeration conditions, and turning frequency (Jiang et al., 2011). When compost is well operated, the global warming potential of N2O can be reduced to about 11–18 CO2-eq t−1, but this still accounts for 35–74% of total GHG emission (Jiang et al., 2013). NH3 emission accounts for 9–32% of initial total nitrogen (Fukumoto et al., 2011, Jiang et al., 2011). The emission rate can be affected by C/N ratio, aeration condition, moisture content, porosity, pile density, and so on (El Kader et al., 2007, Jiang et al., 2011).
Stuvite crystallization (SCP) is one of the most effective methods of mitigating NH3 emission in waste water treatment and in recent years has been used in composting to reduce NH3 loss and to increase the compost quality (Fukumoto et al., 2011, Wang et al., 2013, Chan et al., 2016). When the application ratio of phosphate and magnesium salts is 1:1, the first reaction is:
Subsequently, the following reaction occurs under alkaline conditions and the struvite was formatted
The best H3PO4 and MgO application rate is about 10–20% of total nitrogen (mole/mole) (Jeong and Hwang, 2005), and with appropriate application rate, SCP can decrease NH3 emission by 40–84% (Zhang and Lau, 2007, Ren et al., 2010).
Dicyandiamide (DCD, C2H4N4) is a well-known nitrification inhibitor that has been studied for over 90 years (Kelliher et al., 2008). DCD works by reducing the amoA gene in ammonia oxidizing bacteria (AOB) especially at high nitrogen application rates (Dai et al., 2013). Slowing nitrification results in decreased N2O production and emission rates, and reduced nitrate concentrations in soil decreases the potential for N2O production from denitrification (Kelliher et al., 2008). DCD operates in a bacteriostatic mode and does not kill soil bacteria but rather inhibits or reduces their activity. O’Callaghan et al. (2010) reported that AOB are significantly affected by DCD in which reduces population size and activity, while having little impact on the overall soil bacterial activity. DCD has been widely used in agriculture due to its low cost, minimal volatility, and solubility in water (Tian et al., 2015).
It has been documented that DCD is effective at decreasing N2O emissions from fields treated with mineral fertilizer or urine. Depending on the crop system and climate, the N2O emission rate was reduced by 17–90% (Kelliher et al., 2008, Dai et al., 2013, Cahalan et al., 2015, Wang et al., 2015).
While literature relating to DCD is extensive, only few published studies examine the use of DCD during composting, especially in combination with the SCP process. The purpose of the present study is to evaluate this combination of nitrification inhibitor and stuvite crystallization on NH3 and N2O emission during composting and to determine the most effective application rate and application time.
Section snippets
Raw materials and composting installation
Pig feces and corn stalk were used as raw materials in this research. Pig feces were taken from a pig fattening farm located in Beijing. Corn stalk was obtained from Shangzhuang research station of China Agricultural University. To achieve the appropriate moisture content and C/N ratio, pig feces and corn stalks were mixed at a ratio of 7:1 (wet weight). Compositions of the raw materials and the mixture are shown in Table 1.
In order to simulate the forced aeration system, trials were carried
Temperature and oxygen concentration evolution
The temperature of all treatments increases sharply at the beginning of composting and exceeded 65 °C after 3 days (Fig. 2A). The thermophilic phase lasted around 2 weeks which is long enough to satisfy the Chinese national standard GB 7989-87 for sanitation. After the exhaustion of easily degradable carbon, the thermophilic phase ends and the temperatures decrease gradually reaching atmospheric temperature after the third week. When compare to the windrow system the thermophilic phase in this
Conclusion
Results of this study suggested that the SCP and DCD could be combined used during the composting without any phytotoxin. Combination use could reduce the NH3 and N2O by 45–53% and 76–78%, respectively. The DCD degraded fast during the thermophilic phase, as the high temperature and high free ammonia content could inhibit the nitrifiers, the DCD is suggested to be applied in the maturing stage (<45 °C). Nitrification inhibition works well when the DCD content is higher than 50 mg kg−1 that means
Acknowledgements
Financial support for these investigations was provided by the National Natural Science Foundation of China (No. 41201282). In addition, this research was part of the Chinese National Science and Technology Support Program (2012BAD14B01/18) and Leshan Normal University Scientific Program (Z1159/15ZA0271).
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