Growth of rice in acid soils amended with biochar from gasifier or TLUD stove, derived from rice husks, with or without biodigester effluent

Sisomphone Southavong and T R Preston*

Champasack University
Champasack province, Lao PDR
spdeuk@yahoo.com
* Finca Ecológica, TOSOLY, UTA (Colombia)
AA #48, Socorro, Santander, Colombia

Abstract

The trial was carried out at the experimental farm of An Giang University to measure changes in soil fertility as a function of the growth of rice plants (bio-test) cover a period of 30 days. The experiment was arranged in a completely randomized design with 3 replications of the treatments applied to samples of soil held in one and half litre capacity plastic bags and compared in a 5*2*2 factorial arrangement. The factors were: five levels of biochar (0, 2, 4, 6 and 8%); two types of biochar (Downdraft Gasifier or Updraft Gasifier Stove); and with or without biodigester effluent at 100 kg N/ha.

The biomass growth of rice (over 30 day period from planting) showed a curvilinear increase as the level of biochar was raised from 0 to 2-4%, followed by a slight decline with higher levels. There were no differences due to source of biochar (gasifier or TLUD stove). Application of biodigester effluent at 100 kg N/ha increased biomass growth five-fold with no interaction due to type or level of biochar. Biochar raised soil pH from  4.5 to 5.13 and 5.40 with the the higher value for stove biochar. There were no effects of treatment on cation exchange capacity of the soil but water holding capacity was increased from 38 to 59% with no differences due to source or level of biochar.

Key words: CEC, nitrogen, pyrolysis, soil pH, Terra Preta, water holding capacity

Introduction

Viet Nam has approximately two million hectares (ha) of acid sulphate soils, a large proportion of which are in the Red River Delta in the north and the Mekong Delta in the south. These soils need to be reclaimed for agricultural production, since toxic elements such as aluminium and iron accumulate in crop roots, harming growth and ultimately yield (http://ssc.undp.org/uploads/media/Acid.pdf).

Rice occupies a position of overwhelming importance in the global food system. Over a third of the world’s population, predominantly in Asia, depends on rice as a primary dietary staple. Many of these people live in densely populated countries on an average annual income of less than $US 100, of which a third or more is typically spent on rice (Barker et al 1985). Lack of food security is especially common in sub Saharan Africa and South Asia, with malnutrition in 32 and 22 per cent of the total population, respectively (FAO 2006).

Soil improvement is not a luxury but a necessity in many regions of the world. Conventional ways of improving soil fertility are by addition of chemical fertilizer (NPK) and/or organic matter. A recent development, based on observations of methods used by indigenous peoples in Amazonia (Lehmann 2007), is the application of biochar, which is a form of charcoal derived by pyrolysis.  Pyrolysis is the process of heating fibrous biomass in a restricted supply of oxygen, which prevents complete combustion of the biomass (which happens in open fires). According to Lehmann and Joseph (2009), biochar is the carbon-rich product obtained when biomass, such as wood, manure or leaves, is heated in a closed container with little or no available air. In more technical terms, biochar is produced by thermal decomposition of organic material under a limited supply of oxygen and at temperatures of around 700°C.  

Gasification is a process for deriving a combustible gas by burning fibrous biomass in a restricted current of air; most of the gasifiers developed for this process are of the "down-draft" type (Figure 1). The process is a combination of partial oxidation of the biomass with the production of carbon which at a high temperature (600-800°C) acts as a reducing agent to break down water and carbon dioxide (from the air) to hydrogen and carbon monoxide, both of which are combustible gases (Figure 2). Biochar is the solid residue from the process.

Figure 1: Principles of biomass gasification	Figure 2: Chemical reactions in the gasifier

Biochar is also produced in gasifier stoves designed for cooking. The design is different from the downdraft gasifier in that the flow of air is upwards so as to produce a flame for cooking, as seen in this recent version of a "TLUD" gasifier stove being constructed in Vietnam (Photos 1-3). 

Photos 1-3: The TLUD gasifier stove developed in Vietnam (Olivier 2010)

Application of  biochar to soils may be a partial solution to reducing the negative impact of farming on global warming. It has been shown that biochar has multiple uses. When added to the soil it can significantly improve soil fertility (Rodriguez et al 2009) and also act as a sink for carbon (Lehmann 2007). In this way, the carbon is removed from the atmosphere in a process called sequestration (Zwietenoe 2006). Besides that, biochar can act as a soil conditioner, enhancing plant growth by supplying and, more importantly, retaining nutrients by providing other services such as improving the physical and biological properties of soils (Glaser et al 2002; Lehmann and Glaser 2003; Lehmann and Rondon 2005).

The pH of biochar produced by gasification of sugar cane bagasse and rice husks is about 9 (Rodriguez et al 2009; Kong Saroeun and Preston 2008). Application of biochar has been shown to increase the pH of acid soils (Rodriguez et al 2009), thus it could be used to increase the yield of acid-sensitive crops (FFTC 2008; Lickacz 2002).

Animal manure is a potential replacement for chemical fertilizer and is traditionally used by poor farmers. However, in most cases it is not properly managed so that the efficiency of utilization of the manure is very low. The introduction of low-cost biodigesters in Southeast Asia (Bui Xuan An et al 1997) has made it possible for small-scale farmers to convert manure into biogas and a nutrient rich effluent. When applied to vegetables and plants, it can lead to increases in biomass yield and a higher content of crude protein.  Examples of these effects were observed in Chinese cabbage (San Thy and Pheng Buntha 2005), water spinach (Kean Sophea and Preston 2001; Ho Bunyeth and Preston 2004) and cassava (Le Ha Chau 1998). 

Hypotheses

• On the acid soils in Vietnam it is expected there will be positive effects on plant growth from application of biochar in combination with biodigester effluent.
• The biochar from an updraft (TLUD) gasifier stove will have similar properties in stimulating plant growth as biochar from a downdraft gasifier.

Objectives

• To determine effect of biochar from different sources and effluent from the biodigester (charged with pig manure) on growth of rice in acid soils.

Materials and methods

Location and duration

The experiment was conducted at the experimental farm of An Giang University, Long Xuyen City, An Giang, southern Vietnam. The trial was over a period of 40 days from 1 September to 10 October 2010.  

Experimental design

The experiment was arranged in a completely randomised design (CRD) as a 5*2*2 factorial with 3 replications. 

The factors were:

• Biochar from rice husks used as fuel in a downdraft gasifier (Photo 3) or in an updraft (TLUD) gasifier stove (Photo 4).
• Level of biochar added to soil: 0, 2, 4,  6 and 8%
• Fertilizer: Biodigester effluent (10 g N/m²) or none

Procedure

One kg of acid soil (DM basis) with or without biochar was put in plastic bags of 1.5 litre capacity (Photo 4). Five seeds of rice (a local variety purchased from the market) were planted in each bag. Water was applied uniformly to all bags every morning and evening. Biochar (gasifier) derived from rice husks was brought from Celagrid, Cambodia (Photos 6 and 9). Biochar (stove) was made locally by burning rice husks in a “gasifier stove” (Photos 7 and 10).

The effluent was taken from a “plug-flow” tubular polyethylene (0.5 m³ liquid volume) biodigester (Photo 5) charged daily with pig manure collected from the farmer’s farm (daily charge was 5 kg of fresh manure and 20 litres of water) with 20 days of retention time. The N content of the effluent was 600 mg/litre with 535 mg/litre as NH₄-N. It was applied 5 days after seed germination and then every 5 days for 30 days (total of 5 times). The quantities were calculated according to the N content of the effluent to give the equivalent of 100 kg N/ha (10 g N/m²).

Photo 4: General view of the experimental layout	Photo 5: The plug-flow tubular polyethylene biodigester

Photo 6: Biochar produced from gasifier	Photo 7: Biochar produced from stove	Photo 8: Experimental soil

Photo 9: The 9 KW downdraft gasifier (Ankur Technologies) gasifier installed in CelAgrid, Cambodia	Photo 10: The updraft gasifier stove

Data collection

Observations were made of germination and growth of the rice plants. When the seeds were germinated, 2 to 4 plants were removed to leave only one seedling in each bag. The height of the plants was measured at day 5, 10, 15, 20 25 and 30 (total period of 30 days). In addition, the colour of the plant, germination and growth of plants were observed every day. At the end of the trial, the plants and roots were removed from the bags, washed free of soil, and weighed for fresh biomass. The root length was measured. The green parts (leaves and stems) and the roots were separated and analyzed immediately for DM content. Samples of soil and biochar were analysed at the beginning and end of the trial for pH, ash and CEC (Cation Exchange Capacity). Water holding capacity was also recorded.

Chemical analysis

The DM content of the rice plant (leaf, root and stem) and the soil was determined using the micro-wave relation method of Undersander et al (1993). Soil samples were analyzed for organic matter (OM) by AOAC (1990) method. Biodigester effluent was analyzed for nitrogen (N) content according to AOAC (1990) method. The pH of soil samples was determined using microprocesser pH meter (5 g soil samples were mixed with 25 ml of water and agitated in a mechanical shaker for two hours then centrifuged for 10 minutes before measuring). Cation Exchange Capacity of the soil was analysed according to Houba et al (1988). Water holding capacity was determined by saturating the soil with water and then leaving it in a funnel lined with filter paper during 24 hours.

Statistical analysis

The data were analyzed according to the General Linear Model option in the ANOVA programme of the Minitab (2000) software. Sources of variation were level of biochar, effluent, biochar type, interactions biochar level*effluent, biochar level*biochar type, effluent*biochar type and error.

Results and discussion

Chemical composition of experimental materials

The OM content was higher for biochar derived from the gasifier stove than from the updraft gasifier (Table 1). Both values were considerably lower than was reported for biochar obtained from an updraft gasifier in Colombia charged with sugar cane bagasse (65% OM; Rodríguez et al 2009). The difference can probably be explained by the much higher content of ash in rice husks (about 20%) compared with sugar cane bagasse (2 to 5%).

Water holding capacity

The biochar from both sources increased the water holding capacity of the soil with a curvilinear trend according to the level of biochar in the soil (Table 2 and Figures 3 and 4).

There was no difference between the two sources of biochar. The results are similar to those reported by Glaser et al (2002) where water retention capacity was 18% higher in adjacent soils one of which had been amended by charcoal.

Effect of biochar and effluent on rice biomass yield

The source of biochar had no effect on yield of rice biomass, both aerial part and root; however,  soil pH was higher with biochar from the stove (Table 3). Rice biomass yield was increased from 3 to 5 times by application of biodigester effluent. The response to level of biochar was curvilinear (Figures 5 and 6) with increases in yield as the biochar was increased from 0 to 2-4%, followed by a decline with higher levels.

The increase in growth of the rice brought about by moderate levels of biochar (2-4%) is in agreement with the preliminary report of Boun Suy Tan (2010) in which application of 40 tonnes/ha (about 4% of the soil assuming a cultivation depth of 10cm) of biochar (from rice husk gasifier) doubled the yield of rice grain (from 1.5 to 3.7 tonnes/ha). The slight depression in yield with higher levels of biochar is similar to results of Duong Nguyen Khang et al (2010) with maize as the indicator plant.

Many researchers have emphasized the importance of nutrient supply, especially nitrogen, as a determinant of plant growth response to soil amendment with biochar (see review by Sohi et al 2009). Similar synergistic effects on plant growth by combining charcoal with chicken manure were observed by Steiner et al (2007).

Effect of biochar and effluent on soil pH

The pH of the soil increased linearly with level of biochar addition and was higher for stove than for gasifier biochar (Figure 7) in absence of effluent and the converse when effluent was applied (Figure 8). A positive effect of biochar in improving soil pH was observed by Rodríguez et al (2009), where the pH of an acid soil increased from 4.6 to 6.3 with addition of 5% biochar to the soil. In a very acid soil, Agusalim Masulili et al (2010) reported that application of biochar from rice husk at 10 tonnes/ha increased soil pH from 3.75 to 4.40.

Cation exchange capacity (CEC)

Surprisingly, the biochar produced from rice husk derived from both gasifier and TLUD stove had no effect on cation exchange capacity (Figure 9). This is in contrast to reports by Bot and Benites (2005) and Agusalim Masulili (2010).

Figure 9: Mean values for cation exchange capacity (CEC) in soil amended with different levels of biochar and application of biodigester effluent

Conclusions and recommendations

• The biomass growth of rice (over 30 day period from planting) showed a curvilinear increase as the level of biochar was raised from 0 to 2-4%, followed by a slight decline with higher levels. There were no differences due to source of biochar (gasifier or TLUD stove).

  
  

• Application of biodigester effluent at 100 kg N/ha increased biomass growth five-fold with no interaction due to type or level of biochar.

  
  

• Biochar raised soil pH from 4.5 to 5.13 and 5.40 with the the higher value for stove biochar.

  
  

• There were no effects of treatment on cation exchange capacity of the soil but water holding capacity was increased from 38 to 59% with no differences due to source or level of biochar.

Acknowledgement

The authors would like to express their appreciation to the MEKARN program funded by SIDA-SAREC project, Can Tho University and An Giang University for providing the opportunity and budget to carry out the study. We gratefully thank Ms. Dao THi My Tien, Ms. Bui Phan Thu Hang, Mr. Nguyen Ba Trung and Ms. Nguyen Huu Yen Nhi for their help in facilitating the execution of the experiment.

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Received 11 January 2011; Accepted 14 January 2011; Published 1 February 2011

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