Biochar — An effective additive for improving quality and reducing ecological risk of compost: A global meta-analysis

Highlights

• Biochar can effectively improve quality and reduce ecological risks of compost.

• pH is a key indicator determining quality and ecological risks of compost.

• Increase in compost pH is a key mechanism for the decreased HM ecological risk.

• Application of straw biochar with an addition rate of 10–15% is highly recommended.

Abstract

Biochar is considered as a promising additive with multi-benefits to compost production. However, how the biochar properties and composting conditions affect the composting process and quality and ecological risk of compost is still unclear. In the present study, we conducted a global meta-analysis based on 876 observations from 84 studies. Overall, regardless of biochar properties and composting conditions, biochar addition could significantly increase the pH (5.90%), germination index (26.6%), contents of nitrate nitrogen (56.6%), total nitrogen (9.50%), and total potassium (10.1%), and degree of polymerization (29.4%) while decrease the electrical conductivity (−5.70%), contents of ammonium nitrogen (−33.7%), bioavailable zinc (−22.9%), and bioavailable copper (−38.6%), and emissions of ammonia (−44.2%), nitrous oxide (−68.4%), and methane (−61.7%). Other compost indicators, including the carbon to nitrogen ratio and total phosphorus content, were found to be insignificantly affected by biochar addition. The responses of tested compost indicators affected by the biochar properties and composting conditions were further explored, based on which the addition of straw biochars at a rate of 10–15% was recommended due to its greater potential to improve quality of compost and reduce its ecological risk. Combining the results of linear regression analysis and structural equation model, the increase in compost pH caused by biochar addition was identified as the key mechanism for the increased nutrient content and decreased heavy metal bioavailability. These results could guide us to choose suitable kinds of biochar or develop engineered biochars with specific functionality to realize an optimal compost production mode.

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.