A global meta-analysis of greenhouse gas emissions and carbon and nitrogen losses during livestock manure composting: Influencing factors and mitigation strategies
Highlights
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Nitrogen content in feces significantly affects GHG emissions and nutrient losses of subsequent composting.
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Suitable conditions for emission reduction: C/N (22–28), moisture (60 %), pH (6), aeration rate (0.22 L·kg·min−1)
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Biochar and superphosphate had synergistic emission reduction on greenhouse gas.
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In-situ mitigation strategies can reduce GHGs ≤ 202 million tons of CO2-eq within China.
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N2O has the highest contribution rate to the greenhouse effect (74.22 %) in composting.
The management of manure composting contributes to alleviate the global greenhouse effect. To improve our understanding of this process, we conducted a meta-analysis of 371 observations from 87 published studies in 11 countries. The results showed that the difference in nitrogen content in feces significantly affected the greenhouse gas (GHG) emissions and nutrient losses of subsequent composting, with NH3-N, CO2-C, and CH4-C losses all increasing with its rise. Windrow pile composting (especially compared to trough composting) had lower GHG emissions and nutrient loss. C/N ratio, aeration rate, and pH value significantly affected NH3 emission, and a decrease in the latter two can reduce it by 31.8 % and 42.5 %, respectively. Decreasing the moisture content or increasing the turning frequency could decrease CH4 by 31.8 % and 62.6 %, respectively. The addition of biochar or
superphosphate had a synergistic emission reduction. The emission reduction of N2O and CH4 by biochar was more prominent (44 % and 43.6 %), while
superphosphate on NH3 (38.0 %) was better. And the latter was more suitable if added in 10–20 % of dry weight. Dicyandiamide was the only chemical additive (59.4 %) with better N2O emission reduction performance. Microbial agents with different functions had certain effects on NH3-N emission reduction, while the mature compost had a certain effect on N2O-N emissions (67.0 %). In general, N2O had the highest contribution to the greenhouse effect during composting (74.22 %).
Climate warming is caused mainly by an increase in greenhouse gas (GHG) emissions. Currently, reducing GHG emissions to combat climate change is a global consensus (Møller et al., 2022; Pardo et al., 2015).
With regard to global warming, carbon dioxide (CO2) is the largest contributing gas. However, non‑carbon dioxide (non-CO2) GHGs also contribute significantly to warming (Zhang et al., 2018). Methane (CH4) and nitrous oxide (N2O) are the second and third leading GHGs globally, with centennial warming potentials of 25–28 and 265–298 times that of CO2 (You et al., 2022), respectively. From the pre-industrial period to the 21st century, the concentration of CH4 in the atmosphere has more than doubled (Wuebbles and Hayhoe, 2002), while concentrations of N2O have also increased by 22 % (from 270 ppb to 330 ppb) (Yang et al., 2021). Anthropogenic contribution to CO2 is predominantly from secondary and tertiary industries, while for CH4 and N2O, agriculture is an important source of emissions, including management of soil nutrients, animal digestion, and manure management (Xiong et al., 2022; Zhang et al., 2021a).
Livestock manure management is a specific but pervasive problem in the agricultural sector. The routine storage of manure releases nearly 200 compounds, causing massive carbon (C) and nitrogen (N) losses, as well as environmental and public health problems (Zhang et al., 2021c; Zhao et al., 2020). Emissions of N2O and CH4 from livestock manure management contribute around 10 % of the total non-CO2 GHG emissions globally calculated as CO2 equivalents (Møller et al., 2022).
As part of the Paris Agreement, the United Nations Framework Convention on Climate Change regulates and accounts for the emissions of CH4 and N2O. And relevant countries or organizations have also set corresponding emission reduction targets. For example, compared to 1990, the EU's GHG emission reduction target is to reduce emissions by 55 % by 2030. To achieve these targets, agriculture (especially manure management processes) must contribute to this reduction (Møller et al., 2022). Furthermore, ammonia (NH3), the main form of N loss during manure management, is an important pollutant gas (Pardo et al., 2015). NH3emissions not only lead to the formation of particulate matter in the air (Zhang et al., 2021c; Liu and Zhang, 2011), but also directly or indirectly affect the GHG balance and climate change (Jiang et al., 2013; Szanto et al., 2007). Therefore, effective recycling of livestock manure and reducing the negative impact of the management chain on the environment is crucial.
Composting is one of the most economical and efficient forms of manure management, which can quickly mature livestock manure (Ba et al., 2020; Cao et al., 2019). Mature compost can also be used as a soil amendment or a source of long-term crop nutrients. This is of great significance for soil nutrient management and the sustainability of food production and environmental development (Li et al., 2021a). However, even if composting technology is used to treat livestock and poultry manure, the emission of large amounts of GHGs (CO2, CH4, N2O, and NH3) and the loss of C and N nutrients is still inevitable.
Over the past two decades, extensive research has been conducted on the patterns, influencing factors, and mitigation measures of GHG emissions during manure composting. GHG emissions are affected by several factors, such as the properties of raw materials, process types and control conditions, and exogenous additives (Shan et al., 2021; Liu et al., 2020; Luo et al., 2013). Simultaneously, numerous studies have reported the adjustment of factors related to GHG emissions and proposed multiple mitigation measures (Liu et al., 2020; Jiang et al., 2015; Fukumoto et al., 2011). Therefore, it is necessary to comprehensively analyze existing data to understand the current research status of factors and mitigation strategies for GHG emissions and carbon (C) and nitrogen (N) losses in livestock manure composting.
Some meta-analyses have further deepened our understanding of related issues by reviewing GHG emissions and C and N losses during composting (Zhang et al., 2021c; Ba et al., 2020; Zhao et al., 2020; Cao et al., 2019). However, firstly, previous studies did not focus on all animal feces, often only on a certain type of manure (Zhang et al., 2021c; Ba et al., 2020). But the composting process of animal manure has many similarities. Therefore, we hope to break through the barriers between GHG emissions from different manure composts through this study. Secondly, there is a lack of comprehensive analysis and elaboration on the factors and mitigation strategies that affect GHGs, including the properties of feces, mixed raw materials, process types and scales, process operating conditions, and exogenous additives (Cao et al., 2019). Thirdly, the results require further in-depth analysis. For example, studies have shown that GHG emissions vary with different types of manure (Yuan et al., 2021; Zhang et al., 2021c; Zhao et al., 2020); however, the properties of the types of manure has not been researched. Similarly, the impacts of different factors on GHG emissions were explored; however, the magnitude of the impact was not clearly defined. The additive technology that has been extensively explored is also considered to be related to the amount of addition. Finally, there is less research attempting to predict the current amount of GHG emissions and mitigation potential.
Therefore, a meta-analysis was conducted. Our meta-analysis is based on 371 observations from 87 published studies in 11 countries. The purpose is first to comprehensively understand the differences and influencing factors of GHG emissions and carbon and nitrogen losses between varieties of manure composting. Secondly, by systematically analyzing the correlation between GHG emissions and the properties of feces, process types and scales, mixed raw material properties, process operating conditions, and exogenous materials to propose suitable emission reduction conditions. Finally, the total GHG emissions, based on the annual production of 3.8 billion tons of livestock and poultry manure in China, are predicted as well as the maximum emission reduction that can be achieved through current mitigation strategies.
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