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The rates of biochar addition also depend on the degree of the effect on ammonia volatilization induced by biochar. Feng et al. (2017) examined ammonia volatilization loss from paddy field with two additional rates of biochar and found that 3% biochar additions increased ammonia volatilization, and biochar added with 0.5% rate did not significantly increase the ammonia volatilization. A similar result was reported by Sun et al. (2017) who found no significant differences in ammonia volatilization loss when biochar was added at the rate of 0.5% and 1%
to paddy soil, while a remarkable increase was detected when biochar was added at the rate of 2% and 4%. The increased ammonia volatilization is attributed to increased pH of the soil and reduced nitrification processes induced by biochar application.
However, the effects of biochar application on the ammonia volatilization in vegetable production systems have been neglected. Further studies on ammonia volatilization are still needed as data to assess the proper rate of biochar addition combined with consideration of the benefit of promoting yields and reducing nitrogen leaching (Zhang et al., 2012; Zhao et al., 2014; Sun et al., 2017).
1.6 Biochar in China: Status quo of research and trend of development
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Fig. 1.5 Common agricultural residue can serve as raw material for biochar production in China
For example, the straw production in China accounts for 20%-30% of the worldwide straw production (Wang et al., 2010). Chinese use straw as fodder for feeding livestock or as a building material from the ancient era. However, with the advance of agricultural modernization, most fodder and building material have been replaced by industrial production and therefore plenty of straw is left in the field. Data shows the annual straw produced from conventional crops (like maize, paddy and wheat) has reached up to 650 million tons in China, while less than 20% of straw is returned to field and the rest exceeding 50% is largely wasted (Li et al., 1998; Shi, 2011). The abandonment of straw brings serious non-point organic matter emissions and nutrient losses. If part of the wasted straw is used for producing biochar, carbon and nutrients can be returned into the agricultural system with biochar addition into soil. The pyrolysis process of biochar creates a link between agricultural residue and renewable resources, through the efficient utilization of agricultural residue.
(a) Paddy straw (b) Wheat straw (c) Maize straw
(d) Rice hull (e) Corn cob (f) Peanut shell
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Furthermore, Chen et al. (2013) holds the view that not all of the agricultural residue should be used for biochar production in China. Some of agricultural residue has been effectively used for returning fields or compost. Only the agricultural residue that used to be treated as waste should be transferred to biochar for reuse. However, Meng et al. (2011) estimate that more than 30 million tons of biochar can be produced by using 20% of the straw in China. This amount of biochar (30 million tons) can be used as a carbon basis for processing fertilizer or other soil amendments, and then can be applied to the cultivated land with an area of up to 70 million ha.
1.6.2 How to use 1.6.2.1 Suitable fields
Beneficial effects of biochar in terms of increased crop yield and improved soil quality have been widely reported (Glaser et al., 2002; Chan et al., 2008; Novak et al., 2009a; Yao et al., 2012; Zhang et al., 2012) and thus agricultural biochar production is rapidly developing in China (Meng and Chen, 2013). However, it is impossible that biochar can be applied to all cultivated fields. It means that farmers should be encouraged to add biochar to some special fields, where they can get more benefits from biochar addition. Fields with intensive production and fields with low fertility should be prioritized for biochar addition (Chen et al., 2013).
Fields with intensive production always correspond with the soil having relatively high fertility. Vast crop biomass is harvested from intensive production systems every year. Massive amounts of nutrients are applied to ensure the nutrients supplying to crops growth. Plenty of nutrients, not utilized timely, are left in soil and then get lost. Soil nutrient retention reduces gradually and even soil fertility degrades with such large fluxes. Crop production from intensive fields plays a primary role in supplying food to meet the demand from the large Chinese population. It is critical to maintain sustainable development by recovering soil fertility of intensive fields. Biochar addition is helpful to improve nutrient retention in intensive planting and thereby creates benefits to stabilize yield and reduce loss.
On the other hand, China uses 9% of the worlds cultivated fields to feed 22% of the world
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population (Chen et al., 2013). The requirement of crop yields is high due to the low per capita cultivated area. However, the area of fields with low fertility accounts for 70% of total cultivated land in China (NBS, 2015), which has brought tremendous pressure on food supply.
The main reasons for low fertility of Chinese agricultural soil are:
Poor soil physical condition. It is very difficult to preserve soil water when there is a high sand content and nutrients are inclined to be lost via leaching. Biochar addition can improve the hydraulic condition of sandy desert soil and increase crop yield by 22 Mg/ha (Laghari et al., 2015). However, Jeffery et al. (2015) reports a significant finding that the hydrophobicity of biochar can prevent water entering its internal pore structure resulting in no effect on sandy soil water retention. Jeffery et al. (2015) also points out that the hydrophobicity of biochar can be altered by improving the biochar production process.
Soil salinization is the accumulation of water-soluble salt in soil, usually along with intensive greenhouse agriculture, causing a deterioration or loss of soil function. Studies clearly show that biochar addition can mitigate negative effects of salt on plant growth by its strong adsorption (Thomas et al., 2013).
Soil pH value. Soil acidification is a common trend in Chinese intensive agricultural fields, while biochar is mostly neutral to basic. Thus, biochar is widely accepted as improving soil pH value or decreasing acidity. Meta-analysis results reveal that biochar addition can on average increase the soil pH value from 5.3 to 6.2 (Verheijen et al., 2010).
Soil heavy metal and organic pollution. Biochar has the potential to control contamination in situ. (Cao and Harris, 2010). It can be used as an additive for reducing the bioavailability and mobility of toxic trace metals (Beesley and Marmiroli, 2011; Uchimiya et al., 2011b), or used as a contaminant mitigation agent to control different soil organic pollutants (Beesley et al., 2010) in water and soil. The short-term effects of biochar on soil heavy metal mobility organic pollution are controlled by intraparticle diffusion and soil pH increase (Zhang et al., 2013). A recent study also provides evidence that the surface ligand and other function groups of biochar may make an effort to stabilize heavy metal (Cu, Ni, Cd and Pb) (Uchimiya et al., 2011a) or organic pollution in the long term (Zhang et al.,
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2013).
In these low fertility soils, biochar is regard as a reliable soil amendment. It will be rewarding if there is an improvement in crop production with the addition of biochar and this can mitigate the negative constraints on soil fertility.
1.6.2.2 Usage
Biochar, with its characteristics of small volume and light weight, is difficult to transport and add directly into soil. It is easy to get lost and produces serious dust (Meng and Chen, 2013) during transport. Additionally, much labor is needed to add biochar to soil because there is no special farm machinery for biochar addition in China. Most of the biochar remaining on the surface of the soil will be blown away by the wind and cause near-surface air pollution. Based on the existing agricultural facilities and measures, some studies recommend that biochar should be added into soil in the form of biochar-base products (Novak and Busscher, 2013).
This usage mode avoids the need of special farm machinery and extra labor for biochar addition.
For example, when the rate of biochar in biochar-based products accounts for approximately 60% of the total weight, 0.5×103 kg/ha biochar is added into the soil with the application rate of biochar-based fertilizer or amendment at 0.8×103 kg/ha (Meng et al., 2011). With the increase in biochar addition times, the amount of biochar in soil is accumulating and its effects on soil properties and fertility gradually appears.
The specialized uses of biochar can further improve the economic prospects for biochar utilization (Novak and Busscher, 2013). Biochar can be impregnated with chemical fertilizers to serve as a slow-release fertilizer (Khan et al., 2008) and can also be blended with compost (Steiner et al., 2010), which increases biochar’s nutrient content (Cao and Harris, 2010). The alkalinity of biochar makes it possible to be a remediation tool for acid mine soils and biochar can also be used as a nutrient recovery agent due to its adsorption (Streubel et al., 2011).
1.6.3 Biochar production patterns in China
At present, in the Chinese biochar production industry there are two main patterns:
centralized and distributed. Each of them develops according to the local agricultural
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production pattern (individual farmers or large agribusiness companies) and transportation condition (convenient or not). Centralized biochar production industries are inclined to be built in some agricultural areas having large agribusiness companies which provide sufficient agricultural residues for biochar production. In addition, these areas are always accompanied by relatively complete agricultural production chains, decreasing the cost of labor and raw material transportation and promoting the development of centralized biochar production industries.
However, in the other agricultural areas with individual farmers, it is hard to collect agricultural residue used as raw material for biochar production from small-sized fields. The agricultural residue such as straw decomposes easily due to environmental moisture. A novel production pattern is enabled. The suggestion is that small distributed biochar factories should be built nearby these agricultural fields (Chen et al., 2013). These small factories can offer initial biochar production by primary biochar production technology, which effectively cuts down the transportation cost of biochar production and utilization. Simultaneously, distributed biochar production could provide more work opportunities for local villagers that would further reduce labor costs.