Keywords

7.1 Introduction

The global mean surface (land and ocean) temperature has been increased by 0.87 °C (0.75 °C–0.99 °C) while mean land surface air temperature has increased by 1.53 °C (1.38 °C–1.68 °C) since 1850 to 2015 (IPCC 2019). Global warming is resulting in changes in the global climate system and almost all the natural and human systems in many countries. According to the intergovernmental panel on climate change (IPCC) special report 2019, global warming has resulted in an increased frequency, intensity and duration of heat-related events; change in precipitation patterns and greater frequency of some extreme events in most land regions. Yields of some crops (e.g., maize and wheat) in many lower-latitude regions are being affected negatively by observed climate changes. The main cause of global warming is a continuous increase in the atmospheric concentration of greenhouse gases (GHG) like CO2, CH4, N2O and fluorinated gases (F-gases: hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride) due to several anthropogenic activities. The total GHG emission has increased from 27 Gt CO2 eq/year in 1970 to 49 Gt CO2 eq/year in 2010 (IPCC2014a, b). Out of these greenhouse gases, CO2 is a major greenhouse gas responsible for about 72% of global warming followed by CH4 (20%), N2O (5%) and F-gases (3%) (Fig. 7.1).

Fig. 7.1
figure 1

The contribution of greenhouse gases in total global warming (IPCC 2014a, b)

Full size image

Despite several climate change mitigation measures, annual GHG emission is increasing on an average rate of 2.2% (1.0 Gt CO2 equivalent) per year from 2000 to 2010 compared to 1.3% (0.4 Gt CO2 equivalent) per year from 1970 to 2000 (IPCC 2014a, b). It is expected that future emission of GHG will cause warming and long-lasting changes in all components of the climate system. Limiting total human-induced warming to less than 2 °C relative to the period 1861–1880 is necessary to stop the devastating effect of climate change. Without any additional mitigation, the global mean surface temperature has been predicted to increase between 3.7 °C and 4.8 °C in 2100 as compared to pre-industrial levels. Limiting warming to less than 2 °C would require to keep cumulative CO2 emissions since 1870 to remain below about 2900 Gt CO2 (IPCC 2014a, b). However, about 1900 Gt CO2 had already been emitted up to 2011. Therefore, reduction in the atmospheric concentration of GHG is an urgent global need to arrest global warming and climate change.

Global warming can be reduced by the simultaneous effort of reducing CO2, CH4, N2O, and F-gases emission at the source and capturing already emitted atmospheric CO2 and storing in the long-lived pool. Therefore, identifying technologies that reduce anthropogenic GHG emission at the source as well as technologies that capture and store already emitted GHG become necessary to mitigate climate change. Several suggested strategies for the mitigation of climate change have been depicted in Fig. 7.2.

Fig. 7.2
figure 2

Strategies for climate change mitigation

Full size image

Among these, strategies that led to carbon sequestration in the soil as well as reduce CH4 and N2O emission should be considered as one of the most potent options. The conventional methods like agroforestry, afforestation and soil management are being promoted for soil carbon sequestration. However, increasing the levels of soil organic carbon by conventional agricultural management can take many years (Denman et al. 2007). Recently, interest has grown to sequester atmospheric carbon into biochar followed by its application to the soil. Soil application of biochar is being frequently reported as a potential option for climate change mitigation through carbon sequestration and other agricultural and environmental benefits. Application of biochar into the soil not only sequester carbon but also reduce emission of N2O and CH4 from soil, provide suitable option for management of agricultural and forestry wastes, enhancement of soil sustainability, reduction in fertilizer requirements, production of renewable energy and several other environmental benefits (Waters et al. 2011; Lehmann and Joseph 2009; Jeffery et al. 2011; Wang et al. 2016; Majumder et al. 2019). The reported potential benefits of biochar in environmental management have been depicted in Fig. 7.3. Therefore, the present chapter will discuss the role of biochar in carbon sequestration and greenhouse gas mitigation.

Fig. 7.3
figure 3

Environmental benefits of biochar application in to the soil

Full size image

7.2 Carbon Dioxide and its Impact on Global Warming

Among different greenhouse gases responsible for global warming, CO2 has the least global warming potential. Therefore, the global warming potential of other GHGs is determined with respect to CO2. It means CO2 absorbs less heat per molecule than the other greenhouse gases like methane and nitrous oxide. However, higher atmospheric concentration and a long life cycle in the atmosphere make CO2 the most responsible GHG for global warming. It contributes about 72% of the total global warming induced by anthropogenic activities (IPCC 2014a, b). Before the industrial revolution (the mid-1700s), the global average atmospheric concentration of CO2 was about 280 ppm and it has reached 407.4 ± 0.1 ppm in 2018 (Dlugokencky et al. 2019). The major sectors responsible for CO2 emission are fossil fuel combustion and industrial processes. CO2 emissions from fossil fuel combustion and industrial processes contributed about 78% of the total GHG emission increase from 1970 to 2010 (IPCC 2014a, b). Therefore, rising atmospheric CO2 concentration can be reduced by two processes (a) reducing emission by using energy-efficient technology; switching to renewable energy like biodiesel, ethanol, wind energy, solar power, reduced tillage, and efficient irrigation system and (b) capturing and storing emitted CO2 into the long-lived pool.

7.3 Role of Biochar in Carbon Sequestration

Plant assimilates CO2 by the process of photosynthesis and stores it into the above and below-ground biomass as well as supply liters to the soil for soil organic carbon buildup. This process of CO2 assimilation and storage by the plant into the long-lived reservoir of plant biomass and soil organic carbon is known as carbon sequestration. However, the duration of C-storage in biomass is highly variable. It varied from a few years in the annual crop to 5–60 years in the agroforestry system and a few hundred years in the forests. Soil organic carbon buildup by natural process is very slow and also prone to lose under intensive tillage practices and soil erosion. Therefore, land-use systems which lead to a large amount of carbon assimilation within a relatively short period and it’s storage for long period are suitable options for reducing the concentration of atmospheric CO2 and mitigating global warming and climate change.

Among different options of soil carbon sequestration, biochar application to soil is being considered as the most promising option for long term storage of carbon sequestered in biomass. It is reported that natural biochar sink in Australia is sequestering about 21 million tons of carbon dioxide annually (Graetz and Skjemstad 2003). Preliminary calculations suggests that if 2.5% of the world’s agricultural land produces biochar (ideally from wastes) for application into topsoil then atmospheric CO2 levels might be reduced to pre-AD 1752 level by 2050 (Jacquot 2008). According to Lehmann et al. 2006, globally, up to 12% of all anthropogenic land-use change emissions can be offset annually if slash-and-burn agriculture is replaced by slash-and-char systems. Further, it has been also suggested that, if other greenhouse gas emissions from soils after application of biochar are not elevated, and if emissions associated with production and transport of biochar and/or its feedstocks do not off-set the sequestered C, then the overall greenhouse effect will be abated by application of biochar into the soil (Roberts et al. 2010). This high potential of biochar for carbon sequestration is being advocated mainly due to its high carbon content and very stable form. The overall mechanism of carbon sequestration by biochar can be understood in Fig. 7.4.

Fig. 7.4
figure 4

Process of carbon sequestration by biochar

Full size image

The mechanism responsible for the higher potential of long term carbon storage by biochar application into the soil can be grouped in following properties of biochar

  1. (i)

    Biochar is a stable and rich form of carbon

  2. (ii)

    It leads to a relatively higher rate of carbon sequestration

  3. (iii)

    It leads to agricultural and forestry waste management

7.3.1 Biochar: A Stable and Rich Form of Carbon

There are two intrinsic properties of biochar that make it a very potential option for long term carbon sequestration in the soil. First is, its stability i.e. resistance to biotic and abiotic decay into the soil and second is relatively higher carbon content as compared to biomass. The estimated C-residence time of biochar in the soils has been reported to range between hundreds to thousands of years (Liang et al. 2008; Kuzyakov et al. 2009; Major et al. 2010; Zimmerman 2010). Wang et al. (2016) through a meta-analysis of 24 studies reported 108 days and 556 years mean residence time of labile and recalcitrant biochar resulting in only a small part of biochar bioavailable and remaining 97% contribute directly to long-term C sequestration in soil. The initial rate of biochar decomposition is relatively faster and gets decreased with time. The median rate of biochar decomposition in this meta-analysis was found to be 0.0046% day-1. The initial fast rate of decomposition is mainly due to the decomposition of the labile condensed fraction of biochar. This initial fast rate of decomposition disappears after 2 years and maintained at a very low level over prolonged periods (Kuzyakov et al. 2014; Fang et al. 2014).

The primary reason for the higher stability of biochars in soils is their chemical recalcitrance i.e. resistant to microbial decomposition (Liang et al. 2008) which is due to the presence of aromatic structures. An increase in the aromaticity of organic matter leads to a more recalcitrant nature of the organic matter. Biochar is produced by the process of pyrolysis of biomass in an oxygen-limited environment at relatively low temperatures. During pyrolysis, the biomass undergoes devolatilization and the solid portion gets enriched in carbon. The H and O are preferably removed over C and the H/C and O/C ratios tend to decrease as biomass undergoes its transformation into biochar. This process results in very low H/C and O/C ratios in biochar as compared to the original biomass feedstock (Nsamba et al. 2015). The ratio of H/C and O/C is an indicator of the degree of aromaticity, carbonization, maturation and the stability of biochar (Lehmann and Joseph 2009). According to Krull et al. (2009), decreasing the H/C ratio in biochars indicates an increasing aromatic structure in the biochar. As pyrolysis temperature increases, the turbostratic layering inside of biochar increases in orderliness, the mass percentage of the fused aromatic C thereby increases, the produced biochar is thus often low in easily degradable C but high in recalcitrant C (Nguyen et al. 2010). The value of H/C ratios greater than 0.7 indicates a low biochar quality and pyrolysis deficiencies and the value of the O/C ratio greater than 0.4 indicates lower biochar stability (EBC 2012). The International Biochar Initiative (IBI) recommends a maximum value of 0.7 for the molar H/C ratio (Nsamba et al. 2015, Mary et al. 2016) to distinguish biochar from biomass that has not been or only somewhat thermo-chemically altered.

On the weight basis, biochar contains a high percentage of carbon as compared to original feedstock, however, it varied with the type of feedstock and pyrolysis condition. The carbon content has been reported to vary 29–50% in rice straw derived biochar and 70–85% in apple and oak tree branch at pyrolysis temperature ranging from 400–800 °C (Jindo et al. 2014). Billa et al. (2019) reported 57.59% carbon in rice husk derived biochar to 93.38% in Cassava residue derived biochar at the standard procedure for Biochar production. Lee et al. (2013) reported more than 84% carbon in biochar obtained from Bagasse, Cocopeat, Paddy straw, Palm kernel shell, Wood stem and Wood bark at constant pyrolysis temperature of 500 °C. Windeatt et al. (2014) reported 90.6, 88.6, 54.5, 93.9, 75.3, 83.2, 71.8 and 82.6% carbon in biochar obtained from Palm shell, Sugarcane bagasse, Rice husk, Coconut shell, Wheat straw, Cotton stalk, Olive pomace, and Coconut fiber. Purakayastha et al. (2015) reported 66% carbon in maize biochar, followed by 64% in pearl millet biochar, 64% in wheat biochar and 60% in rice biochar.

Charcol is also mineralized in soil otherwise the earth’s surface would be converted into charcoal within a period of time. Shneour (1966) successful oxidized artificial graphitic 14C to 14CO2 in the presence of soils with high microbial activity. Thermodynamically, the abiotic oxidation of elemental C to CO2 is a strongly exothermic reaction (∆H = −94,052 kJ). However, under environmental conditions, this process is extremely slow (Shneour 1966). Therefore, biochar is much more stable than uncharred organic matter i.e. biomass, and that this difference is the relevant measure for its ability to prevent carbon from being returned rapidly to the atmosphere.

7.3.2 Carbon Storage Potential of Biochar

Biochar is a rich source and a very stable form of carbon obtained from the pyrolysis of biomass. Biochar production itself do not sequester carbon from the atmosphere, however, it leads to the transformation of carbon sequestered in biomass into more stable form i.e. biochar as well as helps in enhancing soil organic carbon sequestration. It has been reported that conversion of biomass carbon to biochar leads to storage of about 50% of the initial carbon compared to the low amounts of carbon retained after burning (3%) and biological decomposition (less than 10–20% after 5–10 years) (Lehmann et al. 2006). Windeatt et al. (2014) reported 21.3–32.5% carbon retained in biochar obtained from pyrolysis of Palm shell, Sugarcane bagasse, Rice husk, Coconut shell, Wheat straw, Cotton stalk, Olive pomace and Coconut fiber at 600 °C. The balance of carbon stored in the biochar, as the mass fraction of the carbon remaining in biochar from the original feedstock carbon, was between 45% for olive pomace and 57% for coconut fiber. The average value of carbon stored from the original feedstock carbon was 51%. Therefore, the biochar itself represents a carbon stock that once added to soil tends to persist for a long time. Further, biochar additions can also interact with the native organic matter already present in soils, and either stimulate or reduce the rate of decomposition of the native soil organic matter (Paustian et al. 2019). Zhang et al. (2018) reported a 76.29% increase in SOC after 5 years of wheat straw biochar application in the soil at the rate of 40 t/ha. Both positive and negative effects on native soil organic matter decomposition following biochar addition have been reported (Song et al. 2016; Wang et al. 2016), but in most cases, these effects on the long-term soil C balance are small (Wang et al. 2016).

However, the net amount of carbon available for storage depends upon the percentage of biochar obtained from the original feedstock and carbon content of biochar. The carbon sequestration potential of the biochars, which is a measure of the amount of the original feedstock carbon that would be retained in biochar for long time periods upon addition to soil, is generally calculated by the following equation (Windeatt et al. 2014).

$$ CSB\ \left(\%\right)=\frac{\mathrm{M}\times \mathrm{Ch}\times {C}_{Ch}\times {R}_{50}}{\left(\mathrm{M}\times {C}_F\right)} $$

where, CSB is carbon sequestration potential of biochar (%), M is the mass of feedstock (g), Ch is the yield of biochar (%), CCh is the carbon content of the biochar, R50 is the recalcitrance index and CF is the carbon content of the feedstock.

$$ {R}_{50X}=\frac{T_{50x}}{T_{50\ graphite}} $$

where T50x and T50 graphite are the temperatures at which 50% of the biochar and graphite are oxidised respectively.

During pyrolysis both mass and carbon content of biomass feedstock decreases due to loss of volatile compounds. The biochar yield and carbon content have an inverse relation with the temperature of pyrolysis. With the increase in pyrolysis temperature, the biochar yield decreases while carbon content increases (Singh et al. 2017a, b, c; Singh et al. 2018; Tiwari et al. 2018; Jindo et al. 2014; Zhang et al. 2017). Increased heating temperature results in higher concentrations of fixed C, total C and stable-C in biochar, as well as higher heating value due to the increased release of volatile compounds (Crombie and Masek 2015). Further, chemical composition and carbon content of feedstock varied and therefore of biochar. It has been reported that biochar carbon content and biochar yield increase with the increase in lignin content of the feedstock (Sun et al. 2017). Biomass feedstock’s with relatively high ash contents produced relatively low fixed carbon biochars, which was attributed to the high ash content inhibiting the formation of aromatic carbon forms (Enders et al. 2012; Sun et al. 2017). The lowest carbon content was seen in rice husk biochar which has the highest ash content, conversely, the highest carbon content was seen in the coconut shell biochar which has the lowest ash content.

7.3.3 A Relatively Faster Rate of Carbon Sequestration

The carbon sequestration in biomass and soil by natural process takes a long time. Gupta et al. (2019) reported that sequestration of about 31.6 Mg C ha−1 in biomass and 3.32 Mg C ha−1 in soil by Hardwickia binata based silvopasture system took 30 years. Measured rates of soil C sequestration through the adoption of recommended management practices have been reported to range from 0.05 to 1.0 Mg C ha−1 year−1(Lal 2004). In India carbon sequestration potential of agroforestry systems has been estimated between 0.25–19.14 Mg C ha−1 year−1 in biomass and 0.003 to 3.98 Mg C ha−1 year−1 in soil (Dhyani et al. 2016). The period of biomass carbon sequestration by agroforestry depends on harvesting rotation. The harvesting period of different agroforestry tree species generally ranged from 5–60 years depending on their use (Chaturvedi et al. 2017) (Table 7.1). It means the agroforestry system stores carbon for 5–60 years and after harvesting of the tree, the stored biomass carbon gradually released to the atmosphere. However, the length of the C-locking period also depends on their use like timber, fuelwood and paper, and pulp. As compared to these systems, the application of biochar can lead to a faster rate of sequestration in the soil.

Table 7.1 Rotation period of major agroforestry trees species in India
Full size table

Most of the study advocates use of agricultural, animal and forest residue as feedstock for the production of biochar. These feedstocks are annually available in large amount and their conversion into biochar takes a relatively short period of time depending upon the capacity of the kiln to produce biomass. After conversion to biochar, they are directly applied to the soil. Application rates of 10, 25, 50 and 100 t ha−1 were all found to significantly increase crop productivity when compared to controls, which received no biochar addition (Jeffery et al. 2011). The greatest positive effects were seen in biochar application rates of 100 t ha−1 (39%) (Jeffery et al. 2011). Biochar carbon is a natural constituent of many soils and soil function is not generally impaired (and maybe enhanced) with the addition of large quantities (e.g., 100 t/ha or more) of biochar. Thus biochar application to the soil can lead to storage of about 10–100 Mg C ha−1 in the form of biochar carbon within 2 years. However, the rate of biochar based carbon sequestration further varies with the frequency of biochar application into the soil. As compared to other biological system, each quantity of biochar applied to soil gets continuously accumulated over time with very little loss or decay (Fig. 7.5). Due to its recalcitrance to decomposition in soil, single applications of biochar can provide beneficial effects over several growing seasons in the field (Steiner et al. 2007; Major et al. 2010).

Fig. 7.5
figure 5

Soil carbon storage after application of biochar and biomass in to the soil. (Modified from Wang et al. 2016)

Full size image

7.3.4 Easily Available Feedstock for Biochar Production

The feedstock suggested for biochar production should be sustainably available. Therefore unused agricultural crop residues are suggested as feedstock for biochar production. The commonly reported feedstock used for biochar production are rice straw, wheat straw, rice husk, coconut husk, coconut shell, cotton stalk, olive pomace, palm shell, rice husk, sugarcane bagasse, corn stover, cassava, corncob, coffee husk, groundnut husk, sawdust, poultry litter, paper pulp, wood chips, green waste, wood, peanut hull, pine chip (Jeffery et al. 2011; Windeatt et al. 2014; Billa et al. 2019). The world and Asian countries produced a large amount of crop residue annually thus provide sufficient feedstock for biochar production. In 2013, 5 billion tons of agricultural residues were produced worldwide out of which 47% was produced by Asia followed by America (29%), Europe (16%), Africa (6%) and Oceania (2%) (Cherubin et al. 2018). Most of this biomass is either burnt or discarded and some amount is used as animal feed, manure production, fuel, and soil incorporation.

India produced about 620 million ton crop residue in 2008–2009 out of which about 15.9% residue was burnt on the farm. Rice straw contributed 40% of the total residue burnt followed by wheat straw (22%) and sugarcane trash (20%) (Jain et al. 2014). In India, the disposal of such a large amount of crop residues is a major challenge. To clear the field rapidly and inexpensively and allow tillage practices to proceed unimpeded by residual crop material, the crop residues are burned in situ. Burning of crop residue eventually leads to unlocking of biomass sequestered CO2 into the atmosphere as well as resource wastage and atmospheric pollution. Burning of crop residues emitted 8.57 Mt. of CO, 141.15 Mt. of CO2, 0.037 Mt. of SOx, 0.23 Mt. of NOx, 0.12 Mt. of NH3 and 1.21 Mt. of particulate matter in 2008–2009 (Jain et al. 2014). The burning of rice straw in Punjab and Haryana causes severe air pollution into adjoining states like Delhi. The efficient utilization of agricultural residue is very important for sustainable agricultural production.

Production of biochar from these resources thus provides a sustainable option for efficient management of crop residue, animal waste, forest by-products vis a vis improvement of soil fertility, carbon sequestration and crop growth. Modern biomass pyrolysis technologies can use agricultural and forestry wastes (such as forest residues, mill residues, field crop residues or urban waste to sequester around 30 kg of carbon for each GJ of energy produced (Lehmann et al. 2006). If available crop and forest biomass are converted into biochar at the rate of 35% recovery and product with 70% carbon then about 1402 million tone biochar can be produced per year worldwide. Similarly in India 113.3 million tons of biochar can be produced from crop residue and 250 million tons from wood residue per year (Table 7.2).

Table 7.2 Available crop residue and potential of biochar production (million ton/ha/year)
Full size table

7.4 Factors Affecting Role of Biochar in Carbon Sequestration and Greenhouse Gas Mitigation

7.4.1 Biochar Yield and Carbon Content in Biochar

The amount of carbon sequestered in the soil through biochar depends upon biochar and carbon content in the biochar. Both of these are highly variable and depend upon the type of feedstock and pyrolysis condition (Tiwari et al. 2019a, b; Singh et al. 2019; Kour et al. 2019). Therefore, optimum pyrolysis condition that leads to higher biochar yield and carbon content will ensure the higher potential for carbon sequestration.

7.4.2 Rate and Frequency of Application

The soil carbon stored in the form of biochar is directly proportional to the amount and frequency of biochar applied to the soil. The higher the rate of biochar application, the higher the amount of carbon will be stored in the form of biochar in the soil. Further frequency of application i.e. annually or biannually or another time interval that finally leads to the accumulation of biochar in the soil is the main factor. Therefore the optimum rate of application and frequency which has a positive impact on crop yield and environment is necessary to determine.

7.4.3 Methods of Application and Soil Management

Soil applied biochar is also subjected to loss mainly in three different ways: (1) by erosion from the surface, (2) by abiotic and (3) biotic degradation. Biotic and abiotic factors have a limited effect on biochar loss while erosion through water and wind may lead to loss of biochar from applied soil. Therefore methods of application that led to minimum loss will ensure a higher rate of carbon sequestration. Subsurface application has been suggested to minimize the loss of biochar and enhance its storage.

7.4.4 The Negative Effect of Biochar Application

Besides its potential agricultural benefits, biochar may contain inherent contaminants, either introduced by its feedstock (e.g., heavy metals), or co-produced during (improper) pyrolysis (e.g., polycyclic aromatic hydrocarbons) (Hilber et al. 2017). However, the link between biochar’s inherent contaminants and toxicity to soil meso and macrofauna remains unclear, with data being often contradictory and influenced by feedstock and pyrolysis conditions.

7.4.5 Farmer Adoption

The ultimate user of biochar for soil application is farmers. Therefore, the rate of farmer adoption is one of the most influential factors for carbon sequestration in the soil through biochar. The latest development leads to a reduction in the production cost (−10 to 30 USD t−1); however, the use of biochar in commercial agriculture remains scarce (Maroušek et al. 2019). While many beneficial reports on biochar trials exist in the scientific literature, laboratory and institute field trial, the practice of applying it to the soil in the commercial farm had just began, and no widely accepted guidelines currently exist (Majro et al. 2010). Further, biochar has not been officially advocated as a measure for carbon sequestration by any regulatory regime or program. Some of the organization like international biochar initiative is working on developing protocol and package and practices of biochar application in agricultural soil. Dickinson et al. (2015) reported that the net present value of biochar application to soils was positive in a sub-Saharan African context but negative in a Northwestern European context, due to a combination of greater production costs and more modest yield benefits in the latter scenario. Therefore, without an understanding of farmers’ roles as the main stakeholders in the generation and use of this innovation, the use of biochar is unlikely to be effective.

7.5 Role of Biochar in Mitigation of Methane Emission

7.5.1 Methane Emission and Its Impact on Global Warming

Methane (CH4) is the second potential greenhouse gas after CO2 which contributes about 15% of total anthropogenically induced global warming and climate change. Its global warming potential is 28 times higher than CO2. Its concentration in the atmosphere has increased from 715 ppb in the pre-industrial era to 1863 ppb in 2018. Since 2007–2015, its concentration is increasing at the rate of +6.9 ± 2.7 ppb year−1. Therefore it can be said that the contribution of methane in climate change is increasing as compared to CO2 (when its slowdown is seen in the last 3 years) (Saunois et al. 2016). Methane is produced under anaerobic conditions (submerged soil) through the anaerobic digestion of organic matter by the action of methanogenic bacteria (Fig. 7.6). Around 70–80% of methane emission is of biological origin and remaining from natural sources. Table 7.3 describes the source and sink of methane emission from soil. Natural sources like swamps, marshes, ocean, forest soil, termites, etc. cause 20–30% of methane emission, while around 70–80% of emission is anthropogenic. Enteric fermentation from ruminants and conventional puddled rice cultivation practices contributes around 20–40% of its emission.

Fig. 7.6
figure 6

Methanogenesis process of methane emission from soil

Full size image
Table 7.3 Sources and sink of methane emission from soil
Full size table

7.5.2 Biochar and Methane Emission

Many studies have shown that soil is responsible for around 15–30% of methane emission and most of the anthropogenic emission comes from paddy cultivation (12%) (Xiao et al. 2018; Malyan et al. 2016). Therefore most of the study has been done for the reduction of methane emission from the paddy field. There are several proposed options for reducing methane emission from paddy field. Some of the potential options are direct-seeded rice (Gupta et al. 2016a and 2016b), intermittent wetting and drying (Gupta et al. 2015), system of rice intensification; use of neem oil coated urea (NOCU) (Gupta et al. 2016a) and use of Azolla as biofertilizers (Malyan et al. 2019), etc. Recently it has been reported that soil application of biochar also offers great potential for the reduction of methane.

Aside from impacts on soil C storage, several studies suggest that biochar amendments may decrease soil CH4 emissions, which would further contribute to greenhouse gas mitigation. Liu et al. (2011) tested the emission rate in rice soil with the incorporation of two biochar i.e. bamboo chips and straw char with control. A reduction of 51.1% and 91.2% of methane in bamboo chips and straw char biochar was found compared to control plots. The reduction in methane emission was attributed to either inhibition of methanogenic bacteria or enhancement in methylotrophic bacteria. Similarly, amending the soil with rice husk biochar @ of 2 and 4% (weight biochar/weight soil) showed 45.2 and 54.9% reduction in methane emission compared to control (Pratiwi and Shinogi 2016). When soil amended with biochar and control (no biochar) were studied in elevated CO2 and temperature conditions then result showed that cumulative methane emission was much lower with 112.2 mg kg−1 compared to 185.4 mg kg−1dry weight soil per season in control (Han et al. 2016). The reduction of methane emission under biochar amended soil was attributed to enhanced methanotrophic activity (especially methanotrophic pmoA gene) which favors rhizospheric activity for methanotrophs. This result confirmed that applying rice straw biochar in soil not only reduces methane activity but at the same time enhances rice productivity (Han et al. 2016). Similarly, many studies show that the application of biochar results in reduced methane emission. Like Rondon et al. (2005) found reduced emission from Brachiaria humidicola and soybean plots treated with biochar @ 15 g kg−1 soil and 30 g kg−1 soil respectively. Similarly, in 2006, Rondon et al. found a reduction in methane emission in tropical soil with the application of wood-based biochar @ 20 t ha−1. Therefore it can be said that methane emission rate varies with soil type, biochar physicochemical properties and agronomic management including water management practices (Zhang et al. 2010).

7.5.3 Factors Affecting Methane Emission from Biochar Applied Soil

Methane flux in the soil is affected by biochar in two ways (i) adsorption of methane on biochar surface (ii) enhanced methanotrophs activity via increased aeration (Jeffery et al. 2016). In the soil, methane is oxidized under aerobic conditions by the action of methanotrophs which takes methane as substrate and converts it into carbon dioxide and water. Additionally, many other things matter like soil physical and chemical properties, biomass type and process of biochar preparation (pyrolysis temperature) also affect the methane emission process. In general, adding biochar helps in enhancing soil fertility which increases methanogenic activity (Feng et al. 2012). The relation of biochar and methane emission is via methanotrophs group of microorganisms. Methanotrophs are gram-negative bacteria whose growth is stimulated through biochar application which ultimately acts as a sink for methane emission. Secondly, biochar increases the porosity of soil which allows new microbial hotspot to develop in this small porous structure where microbes retain methane for their metabolic activity and hence decrease methane emission (Feng et al. 2012). Several biochars and soil factors affect methane emission from soil. Some of them are discussed below:

  1. (i)

    Water Management: Biochar when added in the puddled field under saturated condition increases methane sink or reduces source emission which means either methanotrophic process enhances or methanogenesis reduced. In the rice field, methane emission is mostly via three ways i.e. diffusion, ebullition, and plant-mediated transport. Plant mediated transport through aerenchyma is the major pathway. The application of biochar increases methane oxidation at anoxic/oxic surface of the plant. Contrary to this, in upland soils where the oxic environment persists, the application of biochar can enhance methane emission as they will provide more substrate for methanogens. Thus the application of biochar in flooded soil is more beneficial than its application in non-flooded soil in reducing methane emission and overall carbon footprint (Jeffery et al. 2016).

  2. (ii)

    pH: Methane production or oxidation is microbe mediated process where both methanogens and methanotrophs work under a certain optimum conducive environment. Optimum pH for both is 6–8. Most of the biochar is basic (pH > 7) in nature which provides a liming effect to the soil. As methanotrophs are more sensitive to aluminium toxicity at lower pH, hence increasing pH via biochar applications helps in enhancing their activity and thus more methane gets sunk (Jeffery et al. 2011).

  3. (iii)

    Pyrolysis temperature: Biochar produced via high pyrolysis temperature has a higher methane sink due to reduced H: Corganic ratio. Less H: Corganic ratio means more aromaticity. Additionally, it was found that higher temperature pyrolyzed biochar have lesser labile compounds on their surface which provide less area for microbial methanogens for reduction and thus less methane emission (Bruun et al. 2011).

  4. (iv)

    Feedstocks: In general feedstock composition does not show much difference in methane flux except for biochar produced from sewage sludge. Biochar made from sewage sludge showed enhanced methane sink especially on acidic soil (Cayuela et al. 2014).

  5. (v)

    Branauer, Emmett, and Teller (BET) surface area: pyrolysis temperature and BET surface area of biochar are positively linked. This indicates that in these types of biochar adsorption of produced methane on its surface occurs which reduces its emission (Jeffery et al. 2016).

7.6 Role of Biochar in Mitigation of Nitrous Oxide Emission

7.6.1 Nitrous Oxide Emission and Its Impact on Global Warming

Nitrous oxide (N2O) is a potent greenhouse gas (GHG) with 114 years of atmospheric residence time and 265 times higher global warming potential (GWP) than that of CO2 (IPCC 2014a, b). It contributes about 5% of the total human-induced global warming. The atmospheric concentration of N2O has reached 350 ppb since the pre-industrial (1750) level (285 ppb). It has steadily increased at a rate of 0.73 ± 0.03 ppb year−1 over the last three decades and accounts for approximately 5% of the total greenhouse effect (IPCC 2014a, b). From 1961 to 2010 the global N2O emission from agriculture has increased by about 3 times from 1.44 Tg to 4.25 Tg (Fagodiya et al. 2017). Besides this, it is also responsible for the destruction of the stratospheric ozone as once it transported to the stratosphere it releases the ozone depleting-chemicals through chlorine or nitrogen oxide catalyzed processes (Ravishankara et al. 2009). A doubling of atmospheric N2O would cause a 10% decrease in the ozone layer that would increase ultraviolet radiation reaching the earth’s surface by about 20%. Agriculture, fossil fuel combustion and industrial process, biomass burning, atmospheric deposition, and human sewage are the major anthropogenic sources of the N2O emission (Fig. 7.7). Among the anthropogenic sources, agriculture is the largest source which accounted for 67% of total anthropogenic N2O emission. Out of which 42% is direct emission from nitrogenous fertilizers and manure management; and 25% are indirect emissions from the runoff and leaching of fertilizers (IPCC 2013).

Fig. 7.7
figure 7

Various sources of nitrous oxide emission

Full size image

7.6.2 Biochar and Nitrous Oxide Emission

There are several proposed options for reducing N2O emissions from soil. Some of the potential options are the use of nitrification inhibitors (NIs) and neem oil coated urea (NOCU) (Gupta et al. 2016a, b; Fagodiya et al. 2019) and use of Azolla as biofertilizers (Malyan et al. 2019). Besides increasing soil C storage and reducing CH4 emissions, biochar application in soil has reported reducing N2O emission which would further contribute to greenhouse gas mitigation. Recently it has been reported that soil application of biochar also offers great potential for the reduction of N2O emission from soil mainly due to high porosity, pH, and specific surface area of biochar. Rondon et al. (2005) reported first time in a greenhouse experiment that after biochar application the N2O emissions were decreased by 50–80%. Since then, biochar gets popularity as a potential option for N2O mitigation from agricultural soils and since then several studies have been conducted using biochar. Some of these studies are synthesized in Table 7.4. A recent meta-analysis of publication from various field and laboratory study by Borchard et al. (2019) has reported the 38% average reduction of N2O emissions while Verhoeven et al. (2017) has reported average reductions of 9–12% while an earlier global assessment (Cayuela et al. 2014) suggested greater average reductions of almost 50%, compared to none biochar amended soils.

Table 7.4 Some results of reduction in N2O emission after biochar application in to the soil
Full size table

There are several properties of biochar which are helpful in the reduction of N2O emissions from agricultural soils. Some of these are porosity, pH, acidic and basic functional groups, specific surface area, and redox properties which are mainly varying with the type of feedstock used for biochar and pyrolysis conditions (Grutzmacher et al. 2018; Cayuela et al. 2014). Once biochar applied to soils, the interaction between the soil and biochar can alter the soil pH, oxygen level, and microbial composition and activity which turn into a reduction in N2O emission (Edwards et al. 2018; Harter et al. 2014). Biochar application reduced the substrate concentration (NO3−, NH4+) for nitrification and denitrification reactions which limits the microbial activity and reduced N2O emissions (Fig. 7.8) (Zheng et al. 2013). Besides this, it may enhance the soil pH and N2O reductase concentration into the soils which lead to the final conversion of N2O into N2 and thereby reduced N2O emission (Harter et al. 2014).

Fig. 7.8
figure 8

Mechanism of N2O emission from soil

Full size image

7.7 Conclusions

Biochar is a rich form of stable carbon and a suitable option for mitigating climate change through long term carbon storage and reduction in emission of GHG like N2O and CH4. Aside from carbon storage, biochar also provides a sustainable solution for managing the large volume of crop residue thus saving them from burning and air pollution. Production of renewable energy and heat during biochar production, increase in nutrient and water use efficiency after soil application further help in mitigating climate change through reduction in emission of CO2 from fossil fuel combustion. However, yield, characteristics, and rate of biochar application significantly vary in the different study and it depends upon feedstock quality and pyrolysis temperature. Further, most of the reported studies are confined to laboratory or institute level field trial and adoption of biochar for application in agricultural soil by farmers is still lacking. There is a need to develop region and feedstock specific guidelines and policies for biochar production and application in the field to exploit potential benefits for enhancing soil quality and mitigating climate change.