Biochar for sustainable soil management

Abstract

Soil degradation represents a serious threat to the global environment and the United Nations Sustainable Development Goals (SDGs; Amundson et al., 2015; FAO, 2011; UNCCD, 2017). Sustainable soil management is called for by a wide range of stakeholders (Bryan et al., 2018; FAO, 2015; Hou et al., 2020). In recent years, biochar application has been proposed as a green and innovative solution for achieving multiple goals towards sustainable soil use and management, including carbon sequestration, heavy metal immobilization, and soil health enhancement (Hou, 2020; Lehmann et al., 2011). Biochar is made from the pyrolysis of biomass under an oxygen-limited environment. The concept was brought up only about a decade ago, but its actual usage can date back to pre-Columbian Amazonians (Wang, Ok, et al., 2020). Since the emergence of the concept in 2006, the number of publications regarding the usage of biochar in soil has increased exponentially (see Figure 1), with a search in Web of Science Core Collections returning 2,756 results for 2019 alone, and average increasing rate at 29% over the past 5 years. It is also interesting to note that biochar application in soil is increasingly linked to sustainable development and sustainability. Pertaining literature increased by 108% from 2018 to 2019. However, the sustainability considerations seem to have originated from fragmented considerations of specific disciplinary angles. The more holistic concept of sustainable soil management has rarely been mentioned in these studies. To fill this gap, our editorial team at the journal of Soil Use and Management is commissioning a virtual special issue (VSI) with the topic of ‘Biochar and Sustainable Soil Management’. So far we have collected 15 papers, covering topics including (a) new trends in biochar research; (b) the effect of biochar on greenhouse gas emission and soil carbon sequestration; (c) the effect of biochar addition on nutrient retention and crop yield; (d) the use of biochar in remediation of heavy metal contaminated soil and land reclamation; and (e) the effect of biochar on physicochemical and biological properties of soil. We intend to keep updating this VSI and continuously collect high-quality biochar research papers that fall under the umbrella of sustainable soil management. Soil pollution by various heavy metals and metalloids is widely distributed (FAO, 2018; Hou, O'Connor, et al., 2020), resulting in harms to the general public and causing disproportionate health issues to disadvantaged groups (O'Connor et al., 2020; Zhang et al., 2020). Biochar has been found to be effective in immobilizing heavy metals including Cd, Pb, etc (Jing et al., 2020; O'Connor et al., 2018). Various modification strategies have been explored to strengthen the immobilization capability of biochar made from a variety of feedstock (Wang et al., 2020; Zhang, Hou, et al., 2020). Besides the remediation of heavy metal contaminated soil, biochar has also been explored to address various types of degraded land. Biochar was used to facilitate the rehabilitation of coal mine spoils (Ghosh et al., 2020). Researchers have also used biochar to improve the growth of phytoremediation plants in As- and Pb-contaminated soil at a Ag-Pb mine (Lebrun et al., 2020). In the Yellow River Delta of China, researchers used biochar to remediate saline soil (Xiao & Meng, 2020). Although the general effectiveness of biochar in immobilizing heavy metals and restoring degraded land has been well documented and proven in field trials, its long-term effectiveness is still of debate. Once biochar is placed in the field, it is exposed to various environmental stressors including UV irradiation from sunlight, variation in temperature, moisture, freeze–thaw conditions, acid attack, and biological degradation. These ageing mechanisms can cause cracking, dissolution, oxidation and fragmentation in the material itself and result in remobilization of heavy metals (Hou, Wang, et al., 2020; Wang, O'Connor, et al., 2020; Zhao, O'Connor, et al., 2020). Therefore, sustainable soil management will require biochar material itself to be more long-lasting and sustainable. Biochar is made from biomass that contains a wide range of nutrient elements, such as nitrogen, phosphorus, sulphur and potassium. During pyrolysis and/or weathering processes, these elements can be converted into inorganic forms that are more bioavailable. A myriad of research has focused on the use of biochar as a nutrient enhancer or alternative nutrient supplier. Moreover, biochar can retain certain nutrients, thus reducing nutrient loss via leaching or gaseous emission. A recent meta-analysis showed that biochar alone does not increase crop yield; however, when combined with inorganic fertilizers, biochar could increase crop yield by 15% compared with applying inorganic fertilizers alone (Ye et al., 2020). Biochar can also alter nutrient interaction, rendering a possibility of nutrient optimization. For instance, biochar produced at 350°C, derived from coffee straw and eucalyptus bark, was found to optimize phosphorus fertilizer application (Fonseca et al., 2020). Biochar holds much promise because this material can be potentially produced by a decentralized simple set-up in one's backyard or farm field (Maroušek et al., 2019), similar to what ancient people have done. Research progress on this front can benefit millions of smallholder farmers (Cui et al., 2018; Hazell et al., 2007). Healthy soil and sustainable agricultural practice promote biodiversity (Kremen & Merenlender, 2018; Mader et al., 2002), which further enhances essential ecological services (Isbell et al., 2011). Biochar can alter the physicochemical properties of soil in multiple ways, thus improving soil health. For example, biochar can result in improvements in soil aggregate distribution, water holding capacity and soil compaction. Biochar has been found to be effective in alleviating soil loss because of erosion. When combined with NPK fertilizers, biochar was found to be particularly effective in decreasing soil erosion because of runoff (Bashagaluke et al., 2019), owing to improved hydro-physical properties (Abdo, 2020). Waste walnut wood biochar significantly reduced wind erosion (Pajouhesh et al., 2020). However, similar to concerns on the longevity of biochar's effect in immobilizing heavy metals, there are also concerns on biochar's effect on improving soil health. A long-term experiment conducted in Northern China suggested that the effect of biochar on bulk density, soil pH, water moisture and crop yield all diminished to insignificant amounts over a 5-year period (Dong et al., 2019). It is necessary to obtain a better understanding of the factors affecting the duration of biochar's effect, and design optimized application strategies accordingly. Soil represents the largest terrestrial carbon pool (Le Quere et al., 2018). Soil carbon storage is affected by farm management strategies (Chowaniak et al., 2020; Govindasamy et al., 2020; Singh & Whelan, 2020) and soil microbial processes can also result in the emission of N2O (Davidson, 2009; Hu et al., 2020; Wang et al., 2020), a greenhouse gas with 298 times the atmospheric heat-trapping ability of CO2 (IPCC, 2014). Biochar application increases soil organic content in soil, resulting in carbon sequestration (Zhao, Lin, et al., 2020). Biochar addition could alleviate N2O emission induced by straw return (Li et al., 2020); however, the biochar dosage needs to be optimized because high biochar dosage was found to significantly reduce nitrogen retention and nitrogen usage by crops (Ma et al., 2020). It is still unclear whether or not biochar can play a critical role in the current global battle against climate change. Nevertheless, the synergistic effect of biochar on carbon sequestration and climate change mitigation makes it an attractive option. With a new American President entering the White House and returning to climate talks, climate action is expected to accelerate and biochar can probably play a much larger role than its current status. Sustainable soil use and management practice are critical for meeting the United Nations' SDGs over the next decade (FAO, 2016). Biochar can play a big role in promoting sustainable soil management. A large body of scientific literature on biochar is emerging in recent years, especially regarding novel pyrolysis and modification strategies to obtain engineered biochar of superior performance (see Figure 2). However, the actual application of biochar in fields is still limited, mainly driven by research demonstrations. In order to render commercial viability to biochar application, it is imperative to take into consideration the full life cycle of biochar usage, including production, distribution, longevity, as well as cost and public acceptance. Many challenges remain; yet, there is evidence to be optimistic about this black gold and its usage in promoting sustainable soil management.