Biochar’s stability and effect on the content, composition and turnover of soil organic carbon
Graphical abstract
Introduction
Soil organic carbon (SOC) constitutes approximately 70% of the global terrestrial C pool and is estimated to be on the order of 1500 Pg C in the top meter of soil (Batjes, 1996, Jobbágy and Jackson, 2000). SOC is vital for microbial, plant and animal life because it acts as a key C source for the soil microbes, retains essential nutrients and is critical for maintaining soil fertility and long-term agricultural sustainability (Baldock and Skjemstad, 2000, Leifeld et al., 2005). Moreover, SOC acts as both a source and a sink of atmospheric CO2, and plays an important role in regulating global climate change (Paustian et al., 1997, Trumbore and Czimczik, 2008). Additionally, SOC interacts strongly with environmental pollutants (Li et al., 2010, Sun et al., 2010, Xing and Pignatello, 1997), and any shift in the structural composition could affect the extent to which pollutants are sequestered in soils. In the context of these critical functions and the large pool size of SOC, the processes potentially changing its amount and composition need to be understood.
Biochar is an emerging carbonaceous material derived from renewable resources (e.g., agricultural crop residues and livestock wastes). It is a relatively recently coined scientific term. Many researchers use a range of terms (e.g., black carbon, char, and charcoal) while referring to biochar. Although there is semantic overlap among these terms, they are differentiated from each other according to production conditions and/or objectives of their main applications (Table 1). The advisory committee of the International Biochar Initiative (IBI) has standardized the definition of biochar as ‘‘a solid material obtained from the thermochemical conversion of biomass in an oxygen-limited environment’’ (IBI, 2012). The intrinsic physicochemical characteristics of biochar vary greatly with the feedstock source (e.g., wood, crop and animal manure) and pyrolysis condition (e.g., heating treatment temperature (HTTs), dry or wet pyrolysis, heating rate and residence time) (Aller, 2016, Han et al., 2014, Keiluweit et al., 2010, Lian and Xing, 2017, Merzari et al., 2019, Wang et al., 2016b, Xie et al., 2015). In general, animal manure-based biochar contained the highest ash content, followed by crop- and wood-derived biochar (Wang et al., 2016b); biochar pyrolyzed under low HTTs (≤400 oC) had higher labile OC (e.g., low molecular weight aliphatic compounds) and oxygen (O) content but showed lower pH, porosity, aromaticity, ash and OC content than biochar produced at high HTTs (HTT > 400 °C) (Han et al., 2014, Keiluweit et al., 2010); the longer residence time at a given HTT allows for a more completely charred biochar with less labile OC (Zhang et al., 2015a).
To date, the potential of biochar in terms of both agronomic and environmental benefits has been extensively highlighted in many studies (Grutzmacher et al., 2018, Joseph et al., 2009, Sarauer et al., 2019, Sohi et al., 2010). These benefits have covered issues such as mitigation of global warming, waste management, production of bioenergy, crop productivity and soil health (Atkinson et al., 2010, Laird, 2008, Mathews, 2008, Woolf et al., 2010, Xia et al., 2019). The application of biochar to soils is increasingly attracting the attention from policy makers in the United States, Australia, Europe, Japan, and some developing countries. The extensive incorporation of biochar into soils would inevitably affect the amount and composition of SOC. Following biochar addition, biochar-derived OC can mix with SOC. Depending on production conditions of biochar and soil conditions (e.g., clay content and soil temperature), approximately 80–97% OC of biochar has been reported to be unmineralized to CO2 for hundreds to thousands of years (Bruun et al., 2014, Farrell et al., 2013, Keith et al., 2011, Luo et al., 2011, Naisse et al., 2015, Nguyen et al., 2014, Sigua et al., 2016, Singh et al., 2012). This stable OC fraction of biochar would directly increase the amount of OC and alter the composition of soil OC though physical mixing. In addition, the shift in structure of biochar caused by microbial oxidation has been observed, although most of the OC fractions of biochar are not mineralized to CO2 for a long time (Heitkötter and Marschner, 2015, Mukherjee et al., 2014, Rechberger et al., 2017, Wang et al., 2017b), showing that biochar can be involved in the biogeochemical processes in soils. Mukherjee et al. (2014) compared the physicochemical characteristics of newly made and 15-month-field-aged biochars (wood-derived biochars produced at 250, 400, and 650 °C). They found the increasing amount of O-containing functional groups of biochars, including substituted aryl, carboxyl and carbonyl C and observed the colonization of biochar surface by microbes by using scanning electronic microscopy (SEM) images. Furthermore, biochar can affect the physical (e.g., porosity), chemical (e.g., soil pH) and biological properties (e.g., microbial activity and community) of soil that are critical for soil biogeochemical processes (e.g., oxidation/degradation) regulating OC turnover (Gul et al., 2015, Hart and Luckai, 2013, Kookana et al., 2011, Rousk et al., 2009, Sun and Lu, 2014) and indirectly change the OC storage and compositions.
Thus far, the available peer-reviewed scientific literature about biochar application in soils has mainly dealt with its properties and applications in soil pollution remediation, soil fertility and C sequestration (Ameloot et al., 2013, Atkinson et al., 2010, Gurwick et al., 2013, Lehmann et al., 2011, Manyà, 2012, Stavi and Lal, 2013, Tan et al., 2015). However, few review articles systematically describe the effect of biochar application on the amount, composition and turnover of SOC pool. By using “biochar; soil organic carbon / soil organic matter” as searching phrase on the Web of Science, the feedback shows that there are only seven reviews published during the period from 01/01/2000 to 12/21/2019.
In the present review, we conducted an exhaustive search of international literature to overview and analyze the published studies related to the effect of biochar on the SOC pool. The direct input of biochar-OC to the SOC pool, and indirect effects on SOC by affecting soil physicochemical (porosity, aggregation, pH and cation exchange capacity (CEC)) and biological properties were elaborated in detail. In addition, since humic substance (HS) was traditionally treated as a proxy for SOC and there is a relatively high availability of studies on biochar’s effect on content and composition of HS, the impact of biochar on the content and composition of HS fraction was underscored. For each section, knowledge gaps and future prospects have been identified.
Section snippets
Stability of biochar in soils
The OC of biochar contained at least two pools, namely, labile and stable OC (Cross and Sohi, 2011, Ennis et al., 2012, Leng et al., 2019, Mandal et al., 2016, Zimmerman, 2010). Labile and stable OC were referred to as the OC with low and high ability to resist mineralization, respectively. Currently, many studies have been performed to determine the stability or mineralization of biochar in soils (Fang et al., 2014, Luo et al., 2011). As summarized in Fig. 1a, the mineralized amount of biochar
Input of stable OC fraction from biochar to soil
The most direct effect of biochar on the amount and composition of SOC was the input or release of OC from biochar to soils and mixture with SOC pool. Yin et al. (2014) investigated the effect of biochar on the labile and total OC pools of soils and observed that although the content of dissolved OC (DOC) in soils decreased from 193.27 to 152.95 mg/kg by adding biochar after an incubation of 112 days, total OC content increased significantly from 6.94 to 10.25 g/kg. They explained that the
Impact of biochar addition on SOC by affecting soil physico-chemical and biological properties
Ample evidence from the scientific literature suggests that the addition of biochar into soils can modify many soil physico-chemical and biological properties such as soil porosity, aggregation, pH, CEC, hydrophobicity (water repellency) and microbial activity (Buss et al., 2018, Gul et al., 2015, Lehmann et al., 2011, Luo et al., 2013). These properties play an important role in the turnover of SOC in soils, and thus any changes in these properties will indirectly affect the SOC (John et al.,
Effect of biochar addition on the amount and composition of HSs in soil
HSs are heterogeneous complexes consisting of large macromolecules with functional groups formed by biochemical reactions. Relying on simple extraction with alkalis and acids, HS could be operationally categorized into “base-soluble/acid-insoluble HA, acid/base soluble FA, and insoluble humin (HM)”. As the major component of SOC, HSs represent >50% of SOC in soils (Kononova, 1966) and over 70% of SOC in unlithified sediments (Durand and Nicaise, 1980). The variation in its amount and
Future research directions
Extensive studies have suggested the profound effect of biochar addition on the amount, composition and turnover of SOC. However, as indicated throughout this review, some research gaps and issues still need to be further resolved: 1) Stable OC of biochar was partly contributed by the OC components which were associated with soil minerals. In addition, interaction with soil minerals was a key soil process, by which biochar affected soil aggregation. However, biochar-soil mineral interaction
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.
Acknowledgment
This research was supported by the Innovative Research Group of the National Natural Science Foundation of China (No. 51721093), National Key R&D Program of China (2017YFA0605001), Special Program of China Postdoctoral Science Foundation (2019TQ0070), Program of China Postdoctoral Science Foundation (2019 M660195), National Natural Science Foundation--Outstanding Youth Foundation (41522303), National Natural Science Foundation of China (41473087), the Fundamental Research Funds for the Central
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