Biochar — An effective additive for improving quality and reducing ecological risk of compost: A global meta-analysis
Graphical abstract
Introduction
With the increasing of world population and urbanization, annual output of solid organic waste is increasing (Awasthi et al., 2018; Mirghorayshi et al., 2021). It is estimated that world's total output of solid organic waste will increase from 1.28 billion t year−1 in 2010 to more than 2.19 billion t year−1 in 2025 (Awasthi et al., 2018; Cao et al., 2019). Therefore, coping with the increasing huge amount of solid organic waste has become a great challenge faced by global community (Mirghorayshi et al., 2021; Springmann et al., 2018). Composting has been considered as an effective way to recycle organic waste due to its low cost, easy operation, and wide-ranging adaptation (Wang et al., 2018; Zhao et al., 2020). However, there are still some issues in the processes of compost production and soil application, such as the greenhouse gas emission and nitrogen loss during composting (Awasthi et al., 2017a; Chen et al., 2020; Guo et al., 2020a) and the introduction of harmful substances [e.g., the salt and heavy metals (HM)] to soils during compost application (Cui et al., 2021; Hao et al., 2019). This could counteract the benefits derived from compost soil application. Therefore, it is of great significance to solve the above problems and realize a high-efficient organic waste composting technology.
Previous studies have taken a lot of measures to solve the above problems, such as adding exogenous additives (e.g., biochar, zeolite, diatomite, and montmorillonite), covering film, or assembling a electric or magnetic field, etc. (Awasthi et al., 2017a; Cui et al., 2021; Hao et al., 2019; Ren et al., 2021; Tang et al., 2020; Wu et al., 2021; Xiong et al., 2021). Among these measures, biochar has been proved to be a promising compost additive due to its unique structure and outstanding physicochemical properties (Liu et al., 2020; Lu and Chen, 2020; Lu and Chen, 2018; Lu et al., 2020). It has multi-benefits to compost production, including improvement of physicochemical and nutrient properties, reduction of HM bioavailability, and removal of antibiotic resistance genes (Godlewska et al., 2017; Guo et al., 2020b; Khan et al., 2020; Sanchez-Monedero et al., 2018; Xiao et al., 2017). It also has positive potential to enhance environmental performance of organic compost production and application system (Jiang et al., 2021). However, in previous studies, the optimal biochar use conditions were generally determined only using a single indicator (e.g., greenhouse gas emissions, humification degree, or HM bioavailability) based on individual experimental results (Chen et al., 2017; He et al., 2018; Zhang et al., 2014). This may mislead the best utilization of biochar in composting. In addition, due to the high heterogeneity of biochar properties and composting conditions, the previous results were quite different or even opposite (Awasthi et al., 2017a; Dias et al., 2010). The influencing mechanisms of biochar on key physicochemical properties, transformations of nutrient, and HM bioavailability referring composting process and final compost product have been comprehensively discussed in some reviews (Godlewska et al., 2017; Guo et al., 2020b; Sanchez-Monedero et al., 2018; Xiao et al., 2017; Yin et al., 2021). However, only a few studies have been conducted to quantitatively investigate the responses of aimed compost indicators to biochar addition using the meta-analysis method (Cao et al., 2019; Zhang et al., 2021; Zhao et al., 2020). In previous meta-analysis studies, the authors were mainly aimed at the investigations of the responses of carbon or/and nitrogen cycle (e.g., carbon/nitrogen element loss and greenhouse gas emission) to biochar addition (Cao et al., 2019; Zhang et al., 2021; Zhao et al., 2020). The impacts of biochar on physicochemical properties, nutrient properties (e.g., TP and TK), and HM bioavailability of final compost product have not been examined. This greatly limits us to determine the optimal biochar use conditions (e.g., the addition rate, size, and feedstock type) in composting. Therefore, it is critical to conduct a systematic meta-analysis to quantitatively evaluate the quality (physicochemical and nutrient properties) and ecological risk of final compost product affected by biochar properties and composting conditions.
In the present study, we conducted a global meta-analysis based on 876 observations from 84 studies to quantitatively assess the overall effect sizes of biochar on compost indicators, including physicochemical and nutrient properties and ecological risk indicators. The grand mean responses of aimed compost indicators to biochar addition were further evaluated against the changes in biochar properties and composting conditions. Lastly, the influencing mechanisms of key biochar properties on the quality and ecological risk of compost product were discussed, and the best biochar use conditions were also suggested. This is the first study on the impacts of biochar on compost quality and ecological risk using the meta-analysis method. It can guide us to choose suitable kinds of biochar or develop engineered biochars with specific functionality to realize an optimal compost production mode. The referred compost quality and ecological risk indicators, biochar properties, and composting conditions are listed in Table 1.
Section snippets
Literature search and data collection
A systematic literature search was conducted of the Web of Science, Elsevier Science Direct, Springer Link, and Wiley Online Library databases using the keywords “biochar” and “compost”. The time horizon of data for inclusion of literature in the databases was up to February 2020. In order to include as many studies as possible, the information cited in the articles was examined.
Similar to previous studies (Cao et al., 2019; Dai et al., 2020; Zhao et al., 2020), the included studies in our
Statistical tests
The results of heterogeneity test, as given in Table S3 suggested that a significant Qb existed in most of the cases, because the data used in this meta-analysis was compiled from a variety of references. They also meant that most of the examined biochar properties and composting conditions could significantly affect response of compost indicator to biochar addition, which were further introduced in following sections. Moreover, as the results presented in Table S4, all the groups passed the
Biochar improves the physicochemical properties of compost
As the important indicators of compost quality, the physicochemical properties of pH, EC, and C/N, can reflect the salt toxicity and maturity of final compost product (Chen et al., 2017; Godlewska et al., 2017). The final compost product with higher pH may have unique advantages for improving acidified soils (Biederman and Harpole, 2013). The increased pH of biochar-amended compost could be ascribed to two reasons, including the relatively high pH of biochar itself and promotion of organic
Conclusions
This study confirmed that the addition of biochar can effectively improve quality of compost and reduce its ecological risk. This can be supported by the significant increases in pH (5.90%), contents of TN (9.50%) and TK (10.1%) and the significant decreases in EC (5.72%) and contents of bio-Cu (38.6%) and bio-Zn (22.9%) after biochar addition. However, these overall responses changed greatly among different categorized biochar groups, highlighting the undesired outcomes may occur if biochar
CRediT authorship contribution statement
Shunxi Zhou: Conceptualization, Methodology, Investigation, Writing – original draft. Fanlong Kong: Conceptualization, Funding acquisition, Resources, Writing – review & editing. Lun Lu: Conceptualization, Resources, Funding acquisition. Ping Wang: Conceptualization, Methodology, Funding acquisition, Resources, Writing – review & editing. Zhixiang Jiang: Supervision, Conceptualization, Methodology, Funding acquisition, Resources, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (41703084, 42076221, 22106136) and Demonstration and Guidance Program for Technology People-Benefit in Qingdao (20-3-4-6-nsh, 21-1-4-ny-1-nsh).
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