Research articleAcidification of manure reduces gaseous emissions and nutrient losses from subsequent composting process
Graphical abstract
Introduction
In China, 6.7 Tg of ammonia (NH3) is estimated to arise from the management of livestock manure, equivalent to 49% of the total NH3 emission from agriculture (Bai et al., 2016). Of this, livestock housing and manure storage represent approximately 73%, highlighting the need for improvements in current manure management practices (Bai et al., 2017). Manure acidification is a promising abatement strategy, potentially reducing emission by up to 93% (Hou et al., 2015; Ti et al., 2019). However, previous studies have not assessed the impacts of the types of manure and acids on NH3 mitigation.
It is well established that acidification is effective at reducing NH3 emission, however, when acidified manure from housing or storage is used as a feedstock for composting, it may affect the efficiency of the composting process and the quality of the final product. Some studies have reported inhibition of microbial activity and organic matter biodegradation with acidification (Tran et al., 2015; Wu et al., 2019), while others have not observed this (Pan et al., 2018; Tong et al., 2019). This inconsistency among observations could be attributed to differences in the degree of acidification. To test this hypothesis, the effects of the degree of acidification (pH range 5–8) at storage on the composting process and the quality of the final product were investigated.
Emissions of gases such as NH3, nitrous oxide (N2O) and methane (CH4) impact on air quality and contribute to climate change. The use of acidified manure as a feedstock for composting could decrease compost pH and conserve NH3 as NH4+, improving the fertilizer value of the compost. However, studies with soils have shown that: i) an acidic pH inhibits N2O reduction (Liu et al., 2010); ii) the conserved NH4+ could provide a substrate for nitrification and the production of N2O (Zeng et al., 2013); and iii) the resulting NO3− production may also inhibit the reduction of N2O to N2 (Blackmer and Bremner, 1978). Based on this existing knowledge, acidification of manure used as composting feedstock may enhance N2O emission. Several recent findings support this hypothesis (Pan et al., 2018; Tong et al., 2019), however, to date there has been no assessment of the impact of the degree of acidification at storage on N2O emission from subsequent composting. Previous studies have also confirmed the inhibitory effects of acidification on CH4 emission (Pan et al., 2018; Hao et al., 2005), but again, there has been no systematic evaluation of the degree of acidification on CH4 emission.
Composting is a common practice, particularly in developing countries, for recycling nutrients (mainly nitrogen, N and phosphorus, P) to land. The N and P content of the end product is an important indicator of compost quality, and the specific forms that they are present in are associated with plant availability and environmental risk. It is generally accepted that acidification of manure as composting feedstock will conserve N as NH4+. However, published data regarding compost P forms are limited, and information on the effects of the degree of acidification on P transformations is lacking. Phosphorus is present in various forms in compost but mainly as inorganic P (Pi) (Wei et al., 2015). This can be characterized as water-extractable P (an indicator of potential runoff, Kleinman et al., 2007), labile P (i.e. plant-available), Fe-/Al-bound phosphates, Ca-bound phosphates and residue P using sequential chemical extraction (Dou et al., 2000). Research in soils has shown that the forms of P present depend on soil pH (Batjes, 2011), and include a large variety of compounds with accompanying elements such as Ca, Fe and Al that are characterized by varying solubility. The solubility of Ca phosphates decreases with increasing soil pH, whereas the solubility of Fe-/Al-bound P decreases with decreasing soil pH (Brod et al., 2015).
Acidification of manure is a proposed treatment for the purpose of reducing NH3 emission from storage. The control of the emission of one compound might enhance the emission of the same compound or of another compound at a downstream stage of the manure management chain (Kai et al., 2008). Therefore, it has been proposed that introducing a technology should be evaluated using a systematic perspective.
In this study, the effects of the degree of acidification on NH3 emission from manure storage were quantified using data mining. A subsequent composting experiment was conducted using acidified manure from storage with pH in the range 5–8 as feedstock to assess the effects on: i) the composting process and quality of the products; ii) NH3, N2O and CH4 emissions; iii) N and P transformations and balances. The objective of this study was to explore the optimum acidified pH for manure from a systematic perspective.
Section snippets
Source of data
Data used in this study were extracted from peer-reviewed literature from before July 13, 2019, using Google Scholar. In this literature search, the following specific keywords were combined: manure, ammonia emission, acidification, storage. The information cited in the articles was examined to collect additional metadata.
Study selection criteria
The following criteria were used to select data for the assessment: (i) both a control and acidification treatment are included; (ii) data of the mitigation of NH3 from manure
Effects of manure acidification at storage on ammonia emission: data mining
The observed mitigation of NH3 emission from livestock manure was summarized according to manure types and acid types to assess the systematic effect (Fig. 1). Sources of the study (31 papers with 142 paired comparisons) for these specific combinations are given in Appendix A.
Previous studies have clearly demonstrated the efficacy of pH reduction in the mitigation of NH3 emission from storage. From this data mining, acidification of livestock manure at storage reduced NH3 emission by an average
Conclusion
This work combined existing literature data with new experimental results to illustrate the benefits of manure acidification. Data mining results showed that acidification of livestock manure at storage is effective at reducing NH3 emission (70%), whilst the type of acid used is an important consideration. Acidification of manure (pH6) used as a feedstock for composting significantly reduced NH3 (21.3%), N2O (17.6%) and CH4 (20%) emissions and GWP (9.6%); however, N2O emission was enhanced by
CRediT authorship contribution statement
Yubo Cao: Data curation, Formal analysis, Methodology, Investigation, Validation, Writing - original draft, Writing - review & editing. Xuan Wang: Supervision, Formal analysis, Funding acquisition, Validation, Writing - review & editing. Ling Liu: Methodology, Validation. Gerard L. Velthof: Validation, Writing - review & editing. Tom Misselbrook: Validation, Writing - review & editing. Zhaohai Bai: Validation, Funding acquisition. Lin Ma: Writing - review & editing, Validation, Funding
Declaration of competing interest
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgments
We thankfully acknowledge the financial support from the National Key Research and Development Program of China [2018YFC0213300]; Science and Technology Program of Hebei [18226607D]; the Research Equipment Developing Program of Chinese Academy of Sciences [YJKYYQ20170066]; Science and Technology Service Network Initiative [KFJ-STS-ZDTP-053]; Distinguished Young Scientists Project of Natural Science Foundation of Hebei [D2017503023]; Hebei Dairy Cattle Innovation Team of Modern Agroindustry
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These authors contributed equally.