Development of the straw biochar returning concept in China
Biochar produced from straw has been shown to improve soil physicochemical properties. This review introduces the fundamental concepts, the broad applications, and underlying theory of straw biochar returning. Current developments in biochar industry and the production practices prevalent among enterprises in China are critiques. This review analyzes current knowledge gaps, challenges, and opportunities in the industrial application of straw biochar returning. Biochar standards, the quantitative and qualitative analysis methods for biochar, and high-value-added products that are based on biochar are critically examined with goal of providing recommendations for future studies. We propose production and modification of biochar that is application oriented to enhance its fitness for purpose as well as long-term and large-space–scale field study to better understand its impact on soil properties and ecotoxicology. Finally, we make prospects for the future development of SBR, including constructing a standard system about straw biochar returning and promoting self-discipline of biochar industry and the establishment of a biochar-based agricultural production model.
KeywordsStraw biochar returning (SBR) Biochar-based agricultural inputs Agriculture China
The “straw biochar returning (SBR)” concept was proposed by Professor Wenfu Chen of Shenyang Agricultural University in 2006 as a sustainable model for addressing the food, environment, and energy crisis. In recent years, there has been considerable advance in understanding the SBR theory and the development of application technologies through the support of a national and international drive for “green development”. This paper describes the origin, status, and prospects of SBR in China, and identifies current knowledge gaps and challenges.
1.1 The concept of straw biochar returning technology
Mishandling of crop straw can adversely affect food production capacity and may cause problematic biospheric changes such as land degradation and atmospheric pollution (Smil 1999). In 2015, around 900 million tons of crop straw were produced in China, of which nearly 20% was abandoned or burned (Ministry of Agriculture et al. 2016). Crop straw abandonment encourages reinoculation of the agricultural land by pests and diseases and may present challenges in land preparation. In contrast, burning is cost effective, less labor intensive, and may help in reduction of pests and diseases, but it often results in a loss of soil nutrients and atmospheric pollution (Zhang et al. 2016). Furthermore, the soil organic content (SOC) in most Chinese farmland is lower than the global average by at least 30% due to regional climatic and soil conditions as well as anthropogenic activities such as crop straw burning and extensive agricultural production (Song et al. 2019a, b). Hence, in China, the “National Agricultural Sustainable Development Plan (2015–2030)” identified that the conflict between the increasing demand for agricultural products and land degradation is the greatest threat to national food security (Ministry of Agriculture et al. 2015). Therefore, improvement of the quality of farmland and basic soil fertility remains a great priority.
Besides reducing the practice of burning crop straw, returning crop straw to farmland (also known as ‘slash-and-return’) has been shown to be a cost-effective technique for enhancing SOC sequestration (Yang et al. 2019). Previous studies showed that removing crop straw from farmland has resulted in a 10.9% decrease in SOC content, while returning increased the SOC by 43% (Li et al. 2016a, b). Hence, in 2016, the land area of straw returning reached 700 million mu (466,667 km2) in China, accounting for about 35% of the nationwide cultivated land (Information Office of Ministry of Agriculture 2017). In practice, pathogenic organisms, pests, nitrogen starvation, unsuitable climatic conditions, and other problems often limit the use of straw returning as a sustainable crop straw management strategy. However, results on the correlation between straw return and the rate of plant disease incidents have been mixed to date (Bonanomi et al. 2010). A previous study found a reduction in winter wheat soil-borne diseases in farmland amended with maize straws at rates below 7500 kg m−2, yet at 15000 kg m−2, there was a sharp increase in wheat common rot and sharp eyespot (Zhen et al. 2009). Furthermore, due to low returns, shortage of the large-horsepower agricultural machinery and extra labor input, many farmers are reluctant to return straw with the scanty government subsidies(Yin et al. 2016; Ying et al. 2018), thus, making burning a more attractive option.
Biochar is regarded as “black gold” for its prospective use for soil quality improvement, nutrient pollution mitigation, biomass waste minimization, and energy generation (Ilan 2013; McHenry 2011; Vaccari et al. 2011). In China, biochar has taken a pivotal role in circular agriculture, as it provides an avenue for straw utilization. In agriculture, it is an essential organic matter input raw material produced from straw and other crop-derived agro-wastes created by pyrolysis at 400–700 °C under no or limited oxygen supply (Chen et al. 2019). Crop straw is often preferred since it is abundant in agricultural production (Zhang et al. 2019) and is easier to decompose (Song X et al. 2019).
The incorporation of biochar directly into the soil is currently not an economically viable technique, because the profit margins in crop production are low in China and the soil amendment process is labor intensive (Zhang et al. 2006). Hence, there is a need for reducing the costs of biochar production and application to encourage large-scale biochar adoption. Mixing biochar with chemical fertilizers to make biochar-based fertilizers can help reduce the costs of soil amendment due to the low cost of biochar compared to chemical fertilizers as well as enhance nutrient use efficiency due to their high cation exchange capacity (Yu et al. 2017; Prapagdee and Tawinteung 2017). Furthermore, considering the high diversity of soil physicochemical characteristics and soil quality needs in China, biochar-based fertilizers can be customized to meet the needs of each distinct farmland (Zygourakis 2017; Abiven et al. 2014.). Additional biochar value addition innovations and novel applications are still required to encourage biochar use in agriculture. For example, biochar-based soil amendments could be used in greenhouse cropping systems and its by-products as heat, tar, and pyro-liquids can be recovered, thus, improving the profitability of the production process.
2 Progress of SBR in China
Guided by the SBR concept, biochar engineering technology has rapidly developed in China in recent years. The first biochar research was funded by the Special Fund for Agro-Scientific Research in the Public Interest (201303095) supported by the Ministry of Agriculture in 2013. The aim of this project was to break through the key technologies of biochar preparation, and integrate and demonstrate the biochar application technologies in agriculture. This project had set up a low carbon agricultural technology system with biochar as the core, built a batch of demonstration projects, promoted the formation of the biochar-related standard system and provided important technical support for the development of the biochar industry. In 2017, “Research on biochar-based fertilizer and microbial fertilizer” was launched as one of the National Key Research and Development Projects of the 2016–2020 5-year plan. This program sought to improve the technical standard system of biochar-based fertilizer, develop a number of new biochar-based fertilizer products, integrate a batch of new technologies for biochar-based fertilizer production, promote the biochar industry development through large-scale demonstration, and achieve the goal of reducing fertilizer application and increasing fertilizer efficiency in a large area. With the gradual investment of government and private, biochar application in agriculture has become a multi-subject study area involving pyrolysis equipment manufacture, soil fertilizer science, plant nutrition and agronomy.
The “slash-and-char” practice, although unintentionally using biochar for soil fertilization, is a common traditional agricultural practice worldwide. Due to lack of advanced farm machinery, early communities resorted to cutting and burning straw as way of minimizing amount and handling costs of agro-waste. Meanwhile, SBR began in the early part of the twenty-first century with the establishment of the first SBR company in Liaoning Province, China. In 2016, this company was listed in the new over-the-counter market, marking a milestone in biochar industrialization. According to our network survey, at least 110 companies were engaged in biochar production and application in 2017 in China, of which nearly all of them were small or micro-businesses. The actual number of biochar enterprises is likely to be much smaller, because some of the companies could not distinguish between biochar and other similar materials such as charcoal, and some producers of BBQ charcoal just wanted to share the spotlight of biochar. In June 2017, the Biochar Industry Technology Innovation Strategic Alliance of China (BITISAC) was established, indicating the formation of an industrial community.
The potential application in agriculture and other industries of biochar has gained the attention of the Chinese government. In 2014, “Biochar and Comprehensive Utilization Technology of Agro-Forestry Waste” was listed in the Technical Catalog of Key Energy Saving and Emission Reduction Technologies in Liaoning Province (second batch) (Department of Science and Technology of Liaoning Province et al. 2014) and became a candidate reserve technology approved by the government. Also, in 2014, “Coproduction of Char, Gas, Oil Based on Biomass Pyrolysis and Agricultural Application Technology of Straw Biochar” was included in the National List of Key Low Carbon Technology (NDRC 2014). In January 2015, the standard of “Biochar-Based Fertilizer (DB21∕T 2398-2015)” came into operation in Liaoning Province. This standard specified the requirements for test methods, inspection rules, packaging, transport and storage of biochar-based fertilizer. In April 2017, the agricultural standard of “Biochar-Based Fertilizer (NY/T 3041-2016)” was officially disseminated and implemented with the Ministry of Agriculture listing SBR as one of the ten major modes of comprehensive utilization of straw resources in China (General Office of the Ministry of Agriculture 2017). At the end of November 2017, the State Energy Administration and the Ministry of Environmental Protection issued a notice that supported biomass carbonization and production of biochar-based fertilizer as new supplementary energy production models (National Energy Administration and Ministry of Environmental Protection 2017). In 2019, the National Technology Extension Service Center of the Ministry of Agriculture and Rural Affairs proposed a large-scale demonstration of utility of biochar-based fertilizer in farmlands from eight provinces in China covering at least 4 million mu (2667 hm2).
The use of biochar-based organic fertilizer and biochar-based organic–inorganic compound fertilizers has been increasing, since the policy of using organic alternatives to reduce the amount of chemical fertilizers was proposed (Schulz and Glaser 2012). The annual production of biochar-based fertilizer in northeast China is nearly 200,000 tons. Several biochar-based fertilizers have been developed to date that are suitable for different types of crops and soils. Typically, biochar-based fertilizers decrease the chemical fertilizer input by 10%.
Biochar is often used for replenishing organic matter in farmland as well as in bioremediation of contaminated soils. Current research focused on biochar soil amendment in soils with low organic carbon such as acidified red soil (Dai et al. 2014), sandy soil (Uzoma et al. 2011), and saline–alkali soil (Wang and Xu 2013). Furthermore, biochar has a high concentration of carbon, which has been shown to be a low-cost biosorbent for metals and organic contaminants (Gwenzi et al. 2017). Hence, research interest on environmental behavior of heavy metals or organic compounds in biochar-amended soils has grown significantly in the past decade (Hofer 2011; Lu et al. 2017; Yang and Jiang 2014). Figure 2 illustrates the biochar-based product development under the guidance of SBR using rice production as an example.
3 Problems in SBR
3.1 Biochar standards and quantification methodology
The taxonomy and morphology of biochar, properties critical for soil amendment, are often overlooked in current official functional definitions adopted for biochar. In SBR, the definition of biochar remains very broad and is inadequate for their qualitative and quantitative assessment in different matrices and compositions. Animal waste and manure are generally accepted as raw materials of biochar by the European Biochar Certificate Version 4.8 (EBC 2013). On the other hand, ash from biomass power generation is also considered biochar by the International Biochar Initiative (IBI) certification (IBI 2015). The testing procedure adopted by both EBC and IBI determine the carbonization extent and ecological safety of the biochar but cannot distinguish straw biochar from biochar derived from other biomass. It is also difficult to separate biochar from peat or lignite. The agricultural standard “Biochar-based fertilizer (NY/T 3041-2016)” of China provides a method to quantify and identify biochar in the fertilizer via carbon content analysis and electron microscope. However, this method can only work when there are no biochar analogs.
Apart from “Biochar-based fertilizer (NY/T 3041-2016),” there are no other biochar-related standards available. Unfortunately, imprecise standards may restrict the development and application of biochar-based products. For example, biochar amendment has been shown to cause microbial community shifts (Chen et al. 2017; Zhang et al. 2018; Zhu et al. 2017), yet there are no standards or regulations on the maximum allowable effects on microorganisms. Furthermore, without standards, the effectiveness of novel biochar products will be difficult to assess. Hence, there is an urgent need for the development of national and international standards that address the fitness of purpose of the biochar products as well as regulate their human and environmental health safety.
3.2 Biochar production and deep processing
The physicochemical characteristics of biochar are often influenced by the pyrolysis conditions such as the highest treatment temperature (HTT) and residence time (Kim et al. 2012; Peng et al. 2011; Ronsse et al. 2013). Pyrolysis conditions often fluctuate within the kiln, thus, it is challenging to produce biochar with consistent and homogenous characteristics. Manufacturer often claim that HTT and residence time are carefully controlled and regulated, and this is extremely difficult to do in practice. As a result, application-oriented biochar design and production remain largely difficult to achieve.
Pyrolysis conditions are often influenced by the type of the kiln and the particle size of the feedstock. Rotary kilns are often preferred for the pyrolysis process than semi-closed kilns and batch kilns. They allow a more precise process control, are more efficient, and require less labor. However, the pretreatment costs of the raw biomass are higher in rotary kilns, because they require the raw biomass to be of consistent size. In semi-closed kilns, preventing biomass winding and adhesion is important for HTT control. Hence, agitators are often used when charring rice husk or other materials with a relatively small particle size. Unfortunately, agitators are not so effective when charring straw.
Dust, tar, and labile organic acid are the main possible pollutants of biochar production on site (Feng et al. 2018; Kuśmierz et al. 2016). Low-cost and effective ways for tar collection and labile organic acid capture need to be developed. These have the potential to provide added value as by-products of biochar production.
3.3 Value-added biochar-based agro-inputs
3.4 Liquidity of benefits of carbon sequestration and greenhouse gas reduction
Adoption of biochar in soil amendment is primarily due to its high potential for carbon sequestration and greenhouse gas (GHG) reduction (Liu et al. 2011; Wang et al. 2018). Current studies focused on analysis and estimation of carbon footprint in large-scale and in field experiments (Hertwich and Peters 2009; Gan et al. 2011). However, a systematic analysis of the carbon footprint in the whole life cycle of SBR is still insufficient. Furthermore, current methods for measuring biochar sequestration are not well recognized. Due to the lack of standard measurement methods, assigning carbon credits to biochar enterprises remains difficult despite the establishment of a national carbon trading scheme under the impetus of the low carbon economy in China. This greatly reduces the motivation for investment in biochar production, thus, restricting industrial development. In addition, it is difficult for local and national governments to introduce industrial promotion policies due to the lack of reliable reference data.
4 Recommendations for further research
4.1 Application-oriented biochar production and modification
Tailoring biochar for different applications can be achieved by altering the physicochemical characteristics of the biochar (Abiven et al. 2014). Biochar pH is an important characteristic in soil amelioration; it can be adjusted by controlling the HTT in the kiln (Fidel et al. 2017a). For example, production of wastewater sludge biochar at low temperatures yielded a highly acidic biochar, while higher temperatures yielded alkaline biochar (Hossain et al. 2011). Besides influencing biochar pH, HTT plays a critical role in removal of volatile organic compounds. Ahmad et al. (2012) found biochar production from soybean stover and peanut shells at 700 °C removed trichloroethylene (TCE) at a rate comparable to activated carbon. Furthermore, biochar yield and reactivity are often affected by particle size, lignin, and inorganic matter contents. Demirbas (2004) found that high temperature and smaller particle size decreased biochar yield by increasing the heating rate. In contrast, higher lignin content in olive husks resulted in a higher biochar yield compared with corn cobs. Higher ash content in corn cobs decreased the biochar reactivity during gasification compared to olive husk.
Biochar design and directional preparation are theoretically feasible using an accurate pyrolysis model. Although the pyrolysis process has been extensively studied, precise control on a routine pyrolysis process is challenging because of the dramatic variations in the feedstock characteristics (e.g., crop type, origin, water content and elemental composition). Furthermore, it is also difficult to guarantee the consistency of biochar even when using the same biomass raw material due to process variations. However, fast or flash pyrolysis has been shown to enhance biochar consistency. To better understand the biochar production for development of accurate process models, there is need for creation of a large database and to conduct a corresponding meta-analysis. Furthermore, development of robust biochar kilns with high biomass compatibility should remain a priority.
Biochar modification using chemical agents before pyrolysis or physicochemical methods (such as soak and load and second heating) can improve pore size development and increase surface activity, thus, greatly improving the adsorption capability of the biochar (Kołodyńska et al. 2017; Ma et al. 2014; Wu et al. 2017). Hence, biochar modification is currently a hot topic in biochar technology, and more pilot tests and field experiments are expected before large-scale application begins. In the SBR system, biochar modification needs more economic consideration, because despite being theoretically feasible, the procedure may increase costs which may not be adequately compensated for by the corresponding increase in crop yield.
4.2 Biochar standardization
Although IBI and EBC issued biochar certification methods, there are currently no clearly stated standards for the qualitative assessment of biochar. The “Standardized Product Definition and Product Testing Guidelines for Biochar (version 2.1)” (IBI 2015) gives clear indicators for toxicity evaluation and only uses two indices to evaluate the basic properties of biochar, namely organic carbon content (Corg ≥ 10%) and H/C molar ratio (H/Corg ≤ 0.7). The biochar certification of EBC version 4.8 (EBC 2013) added three indexes (i.e., the total carbon content (C ≥ 50%), bio-carbon minerals (BCM < 50%), and O/C molar ratio (O/C ≤ 0.4)). Unfortunately, IBI and EBC overlook the type of biomass raw materials, which is the decisive factor influencing the physical and chemical properties of biochar. To better assess biochar, certification standards may need further upgrading to include critical parameters such as type of biomass, and how to distinguish and accurately quantify the carbon components in biochar.
4.3 Interactions between organic carbon contained in biochar and other pools
The terrestrial carbon pool is a source and sink of atmospheric CO2 and plays an important role in the process of global climate change. There is about 2500 Gt of carbon in soil (Lal 2008), which is close to 80% of the carbon pool in terrestrial ecosystems, and about 3.1 times the total amount of carbon in the atmosphere (Oelkers 2008). A small increase or decline in the soil carbon pool may significantly affect carbon emissions, and in turn affect global climate change. Compared with natural soil carbon pool such as in grasslands and forests, the farmland soil carbon pool is easily affected by human activities and has great potential in carbon sequestration; it has, therefore, been a primary focus in climate change mitigation by the international community (Burney et al. 2010). At present, biochar, because of its notable stability, is expected to be a long-term carbon pool, but its impact on native carbon in farmland ecosystems remains unknown.
Currently, researchers are focusing on understanding the interactions between biochar and soil organic carbon. However, it is worth noting that carbon in biochar and soil exists in organic and inorganic form. Organic and inorganic carbon are often in a dynamic exchange that is influenced by local environmental factors (Gong et al. 2016.). Condensed aromatic carbons are the predominant organic carbon form present in biochars and they are responsible for their high chemical and biological recalcitrance in soils. They are produced during pyrolysis by conversion of 20–50% biopolymer carbon found in raw biomass (Fidel et al. 2017b). Besides the recalcitrant organic carbon pool, biochars often contain a labile organic carbon that comprises volatile organic compounds and incompletely pyrolyzed biopolymers as well as an inorganic carbon pool, which comprises carbonates. Previous studies demonstrated that in biochar-amended soils, labile organic carbon and the inorganic carbon pools significantly contribute to short-term CO2 emissions (Dong et al. 2019). However, the labile carbon pool is relatively small, and is often stabilized by the larger recalcitrant condensed aromatic carbons pool. Pyrolysis temperature plays a critical role in determining the proportion of labile and recalcitrant carbon pool in the soil organic matter. A previous study found that pyrolysis at high temperatures resulted in recalcitrant carbon pool dominating sewage sludge biochar, while labile carbon pool dominated at low temperatures (de Figueiredo et al. 2019). Furthermore, increasing the pyrolysis temperature has been shown to decrease the effect of type of feedstock on the amount of labile carbon pool in biochar. Therefore, besides soil organic carbon, attention should also be paid to the dynamics of the detailed carbon pool and carbon sequestration. For SBR, it is necessary to clearly understand the nature of organic carbon contained in biochar, and its interaction with other carbon pools after being added to the soil (Liu et al. 2016; Wang et al. 2016).
4.4 Measurement of carbon sequestration and greenhouse gas reduction of biochar application
Some studies show that biochar has greater chemical, thermal, and biological stability after pyrolysis than raw biomass materials such as straw (Glaser et al. 2002); the half-life of biochar in soil is about 1400 years, and the average residence time is about 2000 years (Yakov et al. 2009). Biochar amendment often reduces mineralization of soil organic carbon (Liang et al. 2010), the emissions of CH4 (Feng et al. 2012) and N2O (Wang et al. 2011), and increases the biological yield of crops (Kimetu et al. 2008; Rondon et al. 2007; Zhang et al. 2010). The above effects contribute to carbon sequestration and greenhouse gas emission reductions. Calculation of the contribution of SBR to carbon sequestration and GHG emission reductions is still a long way off, as is including SBR as part of a carbon trading system.
4.5 Life-cycle evaluations at large- and long-term scale
Large-scale temporal and spatial studies provide the data required for setting soil management and conservation goals. From an ecological point of view, short-term field trials only reflect the transient dynamics of certain processes and the long-term effects are most likely different (Clutton-Brock and Sheldon 2010; Thrush et al. 1999; Tilman 1989). Hence, long-term studies such as the Broadbalk Wheat Experiment at the Rothamsted Experiment Station offer critical data on the theoretical development of soil science and ecology as well as valuable information on local agricultural production (Heckrath et al. 1995; Moss et al. 2004). Furthermore, there is need for long-term ecological evaluation, since biochar is highly recalcitrant (Burrell et al. 2016; Li et al. 2016a, 2016b; Singh and Cowie 2014). However, most of the “long-term” experiments had serious limitations; they lasted less than 5 years, were conducted in pot experiments rather than field studies, and in some studies, biochar was not used specifically for soil amendment. Besides scientific research, economical evaluation of SBR on a relatively large scale (e.g., at a national or regional scale) is also needed. It is important to carry out comprehensive life-cycle analysis at a national or regional scale beginning from straw collection, charring, biochar processing, transportation, crop cultivation, and other further steps. Such a study will adequately account for regional and national differences in SBR practices, for example, different communities use different methods in straw collection which might have varying costs. Hence, the ecological and environmental benefits should be considered and calculated together with the economic benefits.
5 Recommendations and prospects
5.1 Implementation of the “Plan of Doubling the Agricultural Soil Carbon pool” and promotion of SBR
High-quality cultivated soil has always been a scarce resource. The organic carbon content of the cultivated soil surface layer in China is much lower than the global average; on average the soil organic matter content is about 2%, and it is even lower in middle- to low-yielding fields. Therefore, there is a lot of room to increase the soil carbon pool in China (Chen et al. 2009; Pan and Zhao 2005; Xie 2004). It is suggested that SBR should be used as a supplementary means of returning straw directly to the field. Trials should be conducted first in the middle- and low-yielding fields with the key objective being to enrich the soil organic matter to a content exceeding 3% or start a “Plan of Doubling the Agricultural Soil Carbon pool”.
5.2 Setting up an SBR experimental region and incorporating biochar into the national carbon trading system
The biochar industry strongly contributes to the common good by facilitating soil carbon sequestration and GHG emission reduction. A national carbon trading system may help bring more attention to the role of biochar in the farmland carbon sink. During the formulation and promulgation of a carbon emission trading scheme, biochar-derived carbon sinks should be included in the quota management and CO2 emission enterprises should be allowed to offset the emission reduction task by purchasing biochar carbon sinks. Hence, there is an urgent need for accelerating the preparation of biochar-related agricultural carbon sequestration or GHG reduction methodology and standards.
5.3 Constructing a standard system about SBR and promoting self-discipline of biochar industry
At present, most biochar enterprises in China are very small, poorly funded, poorly managed, and lack scientific and technological support. Some of the enterprises cannot objectively and accurately explain the concept of SBR and the advantages of biochar products. This impedes progress in production and adaption of biochar technology, because intentional or unintentional false propaganda may mislead consumers as well as confuse them. Therefore, a standard system aiming at the whole industrial chain needs to be created urgently. Associations or alliances related to SBR and biochar should be established to promote self-discipline, to link researchers and entrepreneurs, and to improve the biochar industry.
5.4 Expanding biochar or SBR to biochar-based agriculture
Although significant progress has been made in SBR in China, large-scale application of biochar is still difficult because of the technological, financial, and policy challenges. Improvement in the social economy led food demand trends to change from sufficient food supply to high-quality food supply (including a surge in demand for natural tastes and flavors). In this context, biochar producers should change their final product from biochar-based agro-inputs to biochar-based agricultural products, thus, changing their target consumer from farmers to all those who need safe food. After all, introducing the concept of biochar-based agricultural products might be much easier than persuading farmers to use biochar-based fertilizer, since it is closer for consumers to understand as it aligns to current health food market standards. Therefore, biochar-based agriculture should be highlighted to promote the development of a biochar-based fertilizer market.
We gratefully acknowledge the support provided by the: Earmarked Fund for “Modern Agro-industry Technology Research System” (CARS-01-46); National Key R & D Program “Research and Development of Biochar-Based Fertilizer and Microbial Fertilizer” (2017YFD0200800); Innovative Talents Promotion Plan of Ministry of Science and Technology (2017RA2211); Liaoning Revitalization Talents Program (No. XLYC1802094); and Shenyang Support Plan for Young & Middle-aged Scientific and Technological Innovation Talents (RC180204).
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