In rice-producing countries, the abundance of rice residue must be sustainably managed to avoid environmental problems. Moreover, repeated usage of inorganic fertilizers has caused rapid soil deterioration in these arable lands. Converting rice husk residue into rice husk biochar (RHB) and reapplying it back to paddy fields as a soil amendment can be a sustainable approach in rice production. Due to its adsorptive capability, nutrient retention capacity and high silica content, RHB can improve soil fertility and improving the effectiveness of fertilizer when mixed together due to its nutrient retention capacity and high silica content. High silica content in rice husk biochar provides better nutrient retention, turgidity, and structure for plant. The objectives of this review article are to compile studies related to rice husk biochar properties and factors affecting it and application of rice husk biochar in Asian agriculture.
1. Introduction
The degradation of arable land and increased agriculture waste generation has become a major global issue. This is especially true for Asia, where agriculture remains a key component of its economy and provides a livelihood to billions. Policy makers and researchers are struggling to find innovative ways to address these issues, as the demand for agricultural products will only grow in Asia due to its booming population growth and increased economic development (Food and Agriculture Organization, 2017). Farmers depend on inorganic fertilizers to counter soil nutrient issues; but this causes long-term damage. This is due to repeated and imbalanced application of fertilizer causes organic matter mineralization, reduction in soil carbon stocks and increased soil acidification (Arif et al., 2017, Alves et al., 2019, Oladele et al., 2019).
Rice is a staple food in most regions of Asia, being home to more than 90% of rice production and consumption. Asian countries that are major rice producers include China, India, Indonesia, Bangladesh, Vietnam, Thailand, Myanmar, Japan, Philippines, Republic of Korea and Pakistan (International Rice Research Institute, 2020). India supplies most of the world’s rice, having exported 3.7 million tons of basmati rice and 8.27 million tons of non-basmati rice between 2014 and 2015 alone (Bandumula, 2018). Yet, China remains the leader in rice production, accounting for 28% of the world’s rice production, followed by India (22%), Indonesia (10%), Bangladesh (7%), Vietnam (6%), Thailand (5%), Myanmar (4%) Philippines (2.5%), Japan (1.5%), Cambodia (1.3%) and Pakistan (1%) (Bandumula, 2018). As of 2012, Asian countries contributed to 76% of rice exports (30.24 million tons) around the globe (Bandumula, 2018).
1.1. Rice residue
As Asians depend on rice for their staple food, it is vital that sustainable agricultural practices are implemented in rice paddies, especially in agricultural waste management and soil fertility.
Just like most plantations, it is common for rice plantation managers to overuse chemical fertilizers, namely nitrogen fertilizers. This has led to increased operational cost, while also causes environmental degradation. Besides, rice production is believed to account for about 20% of methane emissions from man-made sources. Most rice farmers see this as an unavoidable problem, as rice plantations require high amounts of nitrogen and constant flooding, which encourages decomposition of organic material by anaerobic microbes.
Harvested rice will undergo milling, where most of the rice production residue is produced. About 600–800 million tons of rice straw is produced every year in Asia, while globally about 800–1000 million tons of rice straw is produced (IRRI, 2020). Furthermore, the world annual production of rice husk and rice bran are 120 tonnes and 76 million tons respectively (Kahlon, 2009, Bodie et al., 2019). Rice husk is partially used as fuel in rice mills to generate heat to dry paddy and burning of rice residue may be practiced in the field. (Shafie, 2015, Pode, 2016).
It is the belief of rice farmers that direct application of rice husk and rice straw into paddy fields would ensure optimal nutrient cycling, although this is not the case. Some farmers burn rice residue because it is a cheaper and easier option to managing rice waste (Ahmed et al., 2015). There is also a believe among rice farmers that it will contribute to soil nutrient cycling (Ahmed et al., 2015). However, prolong burning of rice waste contributes to air pollution and increases greenhouse gas emissions. Thus, not only a sustainable approach to rice production is needed, but also in rice residue management. Converting rice husk into biochar has the potential to address both of these issues.
Furthermore, rice husk ash is an example of plant biomass that highly used in the making of materials (Soltani et al., 2015; Bahrami et al., 2016). One of the main reason is the high contain of silica (Si) and it is important in the making of ceramic nanocomposite (Bahrami et al., 2015a, Bahrami et al., 2017a, Soltani et al., 2017a). Furthermore, rice husk has electrical and termomechanical properties and as promising electric resistivity (Bahrami et al., 2017b, Soltani et al., 2017b, Bahrami et al., 2020). Besides that, rice husk ash is an important material in the silica nitride formation due to rice husk high Si content (Soltani et al., 2017c)
1.2. Objective
There have been numerous review papers on biochar research, however as of to this date, there has been no review paper focusing exclusively on rice husk biochar properties, usage and application in Asia agriculture. The objectives of this review article is to compile studies related to preparation of rice husk biochar factors affecting it properties and application of rice husk biochar in agriculture especially in Asian countries.
2. Rice husk biochar (RHB)
Biochar can be defined as any biological residue from any organic based materials produced through gasification or pyrolysis at 300-600˚C under exclusion of oxygen (Lehmann, 2007, Brewer et al., 2011, Schulz and Glaser, 2012, Tan et al., 2017, Agegnehu et al., 2017, Xiao et al., 2017, Godlewska et al., 2017, Ghorbani et al., 2019, Semida et al., 2019, Wang et al., 2019c). Some common feedstocks of biochar include forestry by-products, organic industrial waste, manures and agriculture residue. Biochar has great prospects to offer many solutions, as it not only has the potential to improve soil properties in a non-destructive manner, but also allows for an environmentally friendly approach to agriculture residue management (Asai et al., 2009, Chen et al., 2011, Carter et al., 2013, Barrow, 2012, Barus, 2016, Badar and Qureshi, 2014, Chen et al., 2018, Bu et al., 2019).
A major by-product of rice production is rice husk, rice straw and rice bran. Rice husk is generated in the first phase of rice milling where the paddy rice is husked. Rice husk can be converted into biochar just like any other plant residues. Rice husk biochar (RHB) account for 20% of rice weight and it contains 50% cellulose, 25–30% lignin, 15–20% silica and 10–15% moisture (Singh, 2018). Converting rice husk into rice straw into RHB and recycling it back to the paddy field as a soil amendment can be an effective solution in rice waste management. The biochar yield form rice husk is approximately 35% of its feedstock material (Shackley et al., 2011, Shackley et al., 2012).
2.1. Morphology and mineralogy of RHB
RHB contains SiO2, Al2O3 and Fe2O3 (Kulkarni et al., 2014). Fig. 1 shows the morphology of RHB at different pyrolysis temperature (Claoston et al., 2014). Cloaston et al. (2014) found that the morphology of RHB changes to honeycomb like structure when subjected to 500 °C pyrolysis temperature and many pores formed on the RHB surface. RHB pores pyrolyzed at 350 °C was not fully developed while the regular pattern of the RHB was destroyed at pyrolysis temperature of 650 °C.