Effect of biochar from rice husks (combusted in a downdraft gasifier or a paddy rice dryer) on production of rice fertilized with biodigester effluent or urea

Huy Sokchea, Khieu Borin and T R Preston*

Center for Livestock and Agriculture Development (CelAgrid)
PO box 2423, Phnom Penh , Cambodia
huysokchea@yahoo.com   ;   huysokchea@gmail.com
* TOSOLY, AA48 Socorro, Colombia

Abstract

The objective of this study was to measure the effect of biochar produced from rice husk by different types of combustion (drowndraft gasifier and paddy rice drying machine) and the interaction between two kinds of fertilizer (biodigester effluent and urea) on soil fertility and paddy rice grain yield.  The experiment was done at the ecological farm of the Center for Livestock and Agriculture Development (CelAgrid), located in Phnom Penh city, Cambodia. The experiment was designed as a 2*2*2 factorial in a completely randomized block design (CRBD) with 4 replicates and in 32 plots each of 20 m².  The first factor was type of biochar (from a downdraft gasifier or a rice dryer); the second factor was the level of biochar (0 and 3 kg/m²); the third factor was source of fertilizer N (Biodigester effluent or urea at 100 kg N/ha/crop).

The rice husk biochar increased yields of rice grain and straw by 30 and 40%, respectively; but there were no differences between biochar produced in a downdraft gasifier compared with that from a rice dryer, nor between urea and biodigester effluent as N fertilizer. Biodigester effluent increased rice grain yield more than urea in the absence of biochar but there were no differences between the two fertilizers when biochar was applied. Biochar increased soil pH, water holding capacity and cation exchange capacity. These criteria were not affected by the source of N fertilizer, nor by the source of the biochar.

Key words: CEC, exchangeable cations, grain, pH, straw, water holding capacity

Introduction

The population of Cambodia was almost 15.1 million in 2010, and will increase to 23.8 million in 2050 but with 40 percent still being under the poverty line (PRB 2010). Poverty, population growth and environmental degradation (air, soil and water pollution) are increasingly being considered as major subjects for research and development. Agriculture is very important in Cambodia with around 37.1% of GDP generated from agricultural productivities (FAO 2003). Soil is one of the most important factors in determining crop yields. For agriculture to be sustainable there is an immediate need to combat the problem of soil erosion and to increase food production. According to MAFF (1996), soil fertility depends on the agro-ecosystem. There are four important rice agro-ecosystems in Cambodia: rain fed lowland rice, rain fed upland rice, deep-water or floating rice, and dry-season (flood recession) rice. There are 2.3 million ha in lowland rice in Cambodia but most of the soils are sandy and poor in nutrients. Erosion occurs not only in the upland areas but also in the lowland areas. In practice, water run-off occurs on all land, and the top soil is lost when no protective and conservation measures are in place. The most common rain-fed lowland soils are sandy, acidic, extremely infertile and low in organic carbon and cation exchange capacity.  

Global climate change raises major questions about management of fibrous residues from rice growing – straw and rice husks.  Decomposition of organic matter in flooded rice gives rise to emissions of methane, which is about 22 times more climate forcing  than CO₂. Rice-based systems are estimated to contribute from 9 to 19% of global methane emissions. An opportunity to address these issues in a completely new way arises from research on anthropogenic soils in Brazil, called terra preta. These soils are characterized by high content of black carbon (carbonized organic matter or biochar) most probably due to the application of charcoal, according to Sombroek (1966).

Agricultural fires were found to account for 8-11% of the annual global fire activity. Burning crop residue before and/or after harvest is a common farming practice. About 30% more GHG emissions can be reduced when the biochar is applied to soil. The biochar option can address issues emerging from soil organic carbon depletion and carbon sequestered in soil actually removes CO₂ from the atmosphere. Biochar formation decelerates the carbon cycle with important implications for carbon management. Terra Preta may be the best proof that soil organic carbon (SOC) enrichment is possible if done with a form of carbon such as biochar. Terra Preta soils show not only a doubling in the organic carbon content but also a higher cation exchange capacity (CEC) (Jonah et al. 2010). 

Materials and methods

Location and duration

The experiment was conducted for 110 days at the Centre for Livestock and Agriculture Development (CelAgrid).  

Experimental design

The experiment was arranged as a 2*2*2 factorial in a completely randomized block design (CRBD) with plot size 20 m² (4*5m) and 4 replicates. The first factor was type of biochar (from gasifier or paddy rice dryer machine);  the second factor was fertilizer (biodigester effluent or urea); the third factor was level of biochar (0 and 3 kg/m²). There were 32 plots in total with the overall area of 640 m² (Table 1). 

Experimental materials

The rice seeds were bought from DomnukTeuk group, Phnom Penh, Cambodia. The urea was bought from the local market while the effluent was produced by a concrete dome biodigester, charged with pig manure. The biochar (dryer) was collected from the rice grain dryer in CelAgrid farm;  the biochar (gasifier) was bought from the local rice milling station.

Photo 1:  Biochar from rice dryer	Photo 2: Biochar from downdraft gasifier

Biochar

The biochar (dryer) was obtained from a machine used to dry paddy rice (Photo 3). The feedstock used in the furnace of the dryer was rice husk. The temperature in the furnace was around 500 ⁰C. The other source of biochar was a commercial down-draft gasifier (Photo 4) producing a combustible gas (approximately 20% hydrogen and 20% carbon monoxide) which was used as fuel in an internal combustion gas engine driving an electrical generator to produce electrical power for the rice mill. The temperature inside the gasifier was around 600⁰C. 

Photo 3: Dryer machine utilizing rice husk as feedstock	Photo 4: Downdraft gasifier, designed for rice husk feedstock

The biochar was sprayed on the flooded soil surface (Photo 5), and immediately afterwards the plots were ploughed to break down the large particles of soil and to ensure the texture was suitable to transplant the germinated rice.  

Photo 5: The biochar was broadcast on the flooded soil surface

Bio-digester effluent

The brick and concrete fixed-dome biodigester (Photo 6) had a capacity of 15 m³. It was charged with manure from pigs fed brewery residues and a commercial concentrate.

Photo 6:  Concrete biodigester

Fertilization and irrigation

The biodigester effluent and the urea were applied in three steps: the first time was 25 days after transplanting the rice, and then after two successive intervals of 20 days. The total quantity was the equivalent of 100 kg N/ha.  The effluent was pumped from the biodigester to PVC drums situated in each block (Photo 7). From the drums the effluent was applied by hand. Urea was broadcast by hand. The plots were irrigated with water from a well. The amounts applied were sufficient to maintain the water levels in the plots (Photo 8).

Photo 7:  Effluent flow system	Photo 8: Water supply from the well

Planting and harvesting of the rice

The rice seed variety name was Phka Romdoul and it was sown in a nursery for germination. After 20 days, it was transplanted in the experimental plots. Two plants were planted in each hole which was at 30 cm distances. Midway through the growing season (50 days) the tillers were counted on 16 randomly selected plants in each plot (Photo 9).

At the time of harvest (Photo 10), the rice plants from each plot were gathered (Photo 11) and separated into grain and stems + leaves which were weighed separately.

Photo 9: Counting the tillers	Photo10: Harvesting the rice plants	Photo11: Collecting the rice biomass

Analytical procedures

The rice grain and straw (stems + leaves) were analysed for DM by the micro-wave radiation method of Undersander et al (1993). Nitrogen and ash were determined following AOAC (1990) procedures. Organic carbon was calculated as OM/1.724 (Walkley et al 1934). Soil samples were analysed for texture, separating the fractions into clay, fine silt, coarse silt, fine sand and coarse sand using the Pipette Method (Day 1965). The cation exchange capacity (CEC) was determined by titrating with 1M Calcium Chloride at pH 7 (Rhoades 1982). The water holding capacity was determined by weighing 15 g of soil into a glass funnel fitted with filter paper and then saturating the soil with water (Photo 12). After 24 h the soil was weighed to determine the quantity of water that had been retained.

Photo 12: Adding water to saturate the soil then allowing the water to drain for 24 hours to determine water holding capacity

 For measurement of the pH, the soil samples were dried in the microwave oven, then ground to a powder. Five grams of the ground sample was put in a beaker and 25 ml of distilled water were added. The suspension was stirred 3 times at 15 minute intervals, and then filtered. pH was measured on the filtrate using a digital pH meter. 

Statistical analysis

The data were analyzed by the General Linear Model of the ANOVA program in the Minitab software (Minitab 2000). Sources of variation were: Biochar source, fertilizer source, biochar level and interaction biochar source*fertilizer source*biochar level and error.

Results and discussion

Soil texture, pH, WHC and CEC

According to Turenne (2011), soil texture is determined by the size of the particles: very coarse sand: 2.0-1.0 mm, coarse sand: 1.0-0.5 mm, medium sand: 0.5-0.25 mm, fine sand: 0.25-0.10 mm, very fine sand: 0.10-0.05 mm, silt: 0.05-0.002 mm and clay: < 0.002 mm. There are three elements that define soil type: texture, structure, and porosity. Soil texture is determined by the percentages of sand, clay and silt while soil structure is the way the clay, sand and silt particles join together with organic matter to form aggregates or clusters of particles. The data in Table 2 indicate that the soil in the experimental area would be classified as “loam” soil (Berry et al 2007).

Soil pH was increased by application of biochar (Table 4) as was tillering capacity. Agusalim (2010) also showed that the application of rice husk biochar as a soil amendment could increase the number of rice tillers, compared to untreated soil. The water holding capacity of the soil was increased by application of biochar but there were no differences between the sources of biochar nor between urea and biodigester effluent fertilizers (Table 4). These results are similar to those reported by Agusalim (2010) where water holding capacity was increased from 11.3% for untreated control soil to 15.5% for soil treated with rice husk biochar. Sokchea et al (2011) and Sisomphone et al (2011) reported increases in WHC of soil from 43 to 53% and 40 to 50%, respectively, as a result of biochar application.  The higher values in these latter reports probably reflected differences in soil characteristics between the different experiments.   Lehmann et al (2009) suggested that biochar application may enhance the water holding capacity of the soil, and Chan et al (2007) also showed that biochar application in the soil improved some physical properties of soils, such as increased soil aggregation and water holding capacity.

At the beginning of the experiment and after application of biochar, the cation exchange capacity (CEC) was increased by both kinds of biochar (Table 5a; Figure 1a). Content of calcium, sodium and magnesium were not affected by biochar addition but content of potassium was increased two-fold. However, in the samples taken after harvest (Table 5b; Figure 1b) there appeared to be no effect of the biochar on the CEC, while the content of the calcium, sodium and magnesium were increased, while that of potassium had decreased. As in the samples taken at the beginning of the experiment, availability of potassium was increased by bochar with no effect on the other elements. We have no explanation for the changes in cation status which appeared to have occurred in the soils after harvest.

According to Lehmann (2003) the availability of potassium, phosphorus and zinc are upgraded when biochar is applied but calcium and copper less so Increase in CEC of up to 40% over initial CEC by addition of biochar was reported by Topoliantz (2002).  James et al (2010) also showed that biochar increased the CEC of the soil, and that this was associated with soil fertility improvement and decreased fertilizer runoff. Many authors (Liang et al 2006; Yamato et al 2006; Priyadarshini et al 2010 and Agusalim (2010) have reported increases of CEC in soils through application of biochar.  

Figure 1a: Effect of  biochar on cation exchange capacity of soil samples taken after application of biochar at the beginning of the experiment	Figure 1b: Effect of  biochar on cation exchange capacity of soil samples taken after harvesting the rice

Grain and straw yield

Incorporating biochar in the soil increased yields of grain and straw by 30 and 40%, respectively (Table 6; Figures 2 and 3); but there were no differences between the two sources of biochar, nor between urea and biodigester effluent as fertilizer.  

Figure 2. Effect of source and level of biochar on rice grain yield	Figure 3. Effect of source and level of biochar on rice straw (stem + leaf) yield

Increases in rice yield from application of biochar were reported by Bounsuy (2010) in Cambodia. He recorded yields of 3.76 tonnes/ha with application of 40 tonnes/ha of biochar compared with 1.82 tonnes/ha with 20 tonnes/ha of biochar.  Priyadarshini et al (2010) described linear increases in rice yield from application of biochar.  According to Afeng et al (2010), biochar amendment at 10 and 40 tonnes/ha increased the rice yield by 12% and 14% in unfertilized soils and by 8.8% and 12.1% in the soil with N fertilization. However, Singhal et al (2011) showed that application of 2 tonnes rice-husk-biochar per ha increased the grain yield from less than 4 tonnes per ha for the control treatment to more than 5 tonnes/ha for the biochar treatment.  

There were no interactions between the effects of level of biochar and source of fertilizer on tillering rates of the rice plants and soil pH (Table 7; Figures 4 and 5). Tillering was increased by effluent compared with urea when no biochar was applied but there were no differences between the two fertilizers in the presence of biochar. In the absence of biochar, grain yield was higher with effluent but the contrary was the case when biochar was applied. Soil pH showed the same trends as grain yield. It was to be expected that grain yield would be higher with effluent as, besides nitrogen, this fertilizer also contained a range of other plant nutrients.  

Figure 4: Effect of  biochar and nitrogen sources on rice tillering	Figure 5: Effect of two kinds of biochar on soil pH amendment

Figure 6: The interaction between nitrogen source and biochar level on rice grain yield

Conclusions

• Incorporating 3 kg/m2 of rice husk biochar in a loam soil (p increased pH 5.5) increased yields of rice grain and straw by 30 and 40%, respectively. However, there were no differences between biochar produced in a downdraft gasifier compared with that from a rice dryer, nor between urea and biodigester effluent applied at 100 kg N/ha.

• Biodigester effluent increased rice grain yield more than urea in the absence of biochar but there were no differences between the two fertilizers when biochar was applied.

• Biochar increased soil pH, water holding capacity and cation exchange capacity in the soils at the beginning of the experiment, but had no effect in the samples taken after harvest. These criteria were not affected by the source of N fertilizer.

Acknowledgement

The authors express their appreciation to the MEKARN program, financed by Sida (Sweden), for the grant which made possible this research, and to staff members and students in CelAgrid for their assistance in the experimental work. The senior author is especially grateful to the ØRSKOV Foundation for funds which facilitating his participation in the Cantho University MSc course, for which this research paper in one of the requirements.  

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Received 8 February 2012; Accepted 14 December 2012; Published 4 January 2013

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