Growth of maize in acid soil amended with biochar, derived from gasifier reactor and gasifier stove, with or without organic fertilizer (biodigester effluent)

Huy Sokchea and T R Preston*

Center for Livestock and Agriculture Development (CelAgrid)
PO box 2423, Phnom Penh , Cambodia
huysokchea@yahoo.com
* Finca Ecológica, TOSOLY, UTA (Colombia)
AA #48, Socorro, Santander, Colombia

Abstract

A biotest with maize as indicator plant was used to measure the value as an amender of acid soil (pH 4.6) of biochar derived from gasification of rice husks. The experiment was designed as a 5*2*2 factorial in a completely randomized design (CRD) with 3 replicates. The factors were: source of biochar (from a downdraft gasifier reacgtor or an updraft gasifier stove). level of biochar (0, 2, 4, 6 or 8% added to the soil); and application of biodigester effluent (0 or 10 g N/m²).

The biochar from the stove contained more ash (less organic matter) and the pH was higher compared with biochar from the gasifier. The yield of the aerial fraction and of total biomass of maize  was 30% higher when the soil (pH 4.6) was amended (at 6 to 8% of the soil) with  biochar from an updraft gasiifier stove than from a downdraft gasifier reactor. There was no effect of the level of biochar on maize growth in the absence of biodigester effluent but growth was increased 90% when biochar was incorporated at 6% of the soil and biodigester effluent was applied at 10g N/m² over 30 days. Soil pH was raised from 4.6 to 4.9 and water holding capacity by 50% when 6-8%  biochar was added to the soil. 

Key words: Biotest, CEC, downdraft, pH, updraft, WHC

Introduction

The present world human population of some 6 billion is estimated to at least double in the next 50 years (PRB 2008). The implications for food production are serious especially considered in the light of the probable impacts of climate change in reducing yields of essential cereal grain crops such as rice.  At the same time, soil deterioration from depletion of organic matter is an increasingly serious global problem that contributes to hunger and malnutrition. When the soil is intensively cultivated with high levels of chemical fertilization, the organic matter in the soil is quickly decomposed into carbon dioxide by soil microbes and this gas released into the atmosphere, leaving the soil compacted and nutrient-poor as well as adding to global warming (Mingxin Guo 2008).

The pH of the soil solution is also very important because soil solution plays a key role in carrying the nutrients such as nitrogen (N), potassium (K), and phosphorus (P) to support plant growth. Acid soils are common in the tropics. When soil pH is below 4 to 5, growth rates of crops such as maize are reduced. Desirable soil pH values for optimum maize growth are in the range of 6.5 to 7.0 (Nielsen 2005).

Biochar is the by-product from processes such as gasification and pyrolysis where biomass is heated to high temperatures in situations where the supply of oxygen is limited. Biochar is composed of the residual mineral matter from the original biomass and carbon resulting from the incomplete combustion of the biomass. Because of the high temperatures (from 600 to 1000 °C) reached in the gasification and pyrolysis processes, the physical and chemical properties of the carbon-rich residue in biochar are changed.

According to Glaser (2006) the carbon in biochar is intimately associated with “poly-condensed aromatic moieties which are assumed to be responsible for its chemical and biological recalcitrance in the environment”. This author also emphasized the importance of the highly porous structure of biochar as responsible for its high capacity to adsorb organic molecules.

As most of the mineral matter in biomass is composed of salts of K, Na and Ca, it has a strong alkaline reaction giving rise to a pH of between 8 and 10 (Rodriguez et al 2009). Thus application of biochar as a soil amender would be specially appropriate in acid soils with a low content of organic matter.  Biochar is unlikely to have a major role as a fertilizer but, because of its structure,  it can be expected to increase water and air holding capacity, and be a good habitat for some microbes and plant nutrients.

Biochar can be produced in different processes according to the temperature (Table 1).  

Table 1.  Product yield from pyrolysis (or gasification) of wood (expressed as yield in terms of % dry weight conversion to products) (from IEA 2010)

Much of the discussion on the use of biochar has centered on producing it by pyrolysis. There are few reports on the properties of biochar produced as a byproduct of the processing of fibrous biomass for purposes of production of electricity (Phalla and Preston 2005) or for use in gasifier stoves (Olivier 2010). These two processes differ in the configuration of the reaction and specifically the flow of air. The gasifier described by Phalla and Preston (2005) is a downdraft gasifier whereas the gasifier stove uses the updraft principle. It is possible that the properties of the biochar produced in these two systems will be different.

The object of the present study was therefore to compare the soil amendment properties of biochar produced from rice husks used as the fuel in the two types of gasifier; the updraft (TLUD) gasifier stove designed for cooking (Olivier 2010) compared with the downdraft gasifier to produce a combustible gas as fuel for an internal combustion engine (Phalla and Preston 2005). 

Hypotheses

• Biochar produced from rice husks in an updraft gasifier stove will have similar effects on soil fertility as biochar produced from rice husks in a downdraft gasifier

• There will be synergistic effects on yield of maize from both sources of biochar with effluent from a biodigester charged with pig manure.

Materials and methods

The experiment was done in An Giang University of Agriculture, An Giang province, Long Xuyen city, Vietnam. It lasted for 30 days, starting from September 04 to October 04, 2010. The local ambient temperature was about 33-37°C.

Experimental design

The experiment was designed as a 5*2*2 factorial in a completely randomized design (CRD) with 3 replicates. The factors were:

Source of biochar:

• Downdraft gasifier (DDG) versus gasifier stove (GS)

• Level of biochar: 0, 2, 4, 6, and 8%

• Level of biodigester effluent: None or 10 g N/m²

Source of biochar:

Downdraft gasifier

The gasifier used to produce the biochar is divided into three parts (hopper, reactor and ash collector) with four steps in the process of gasification: Drying, Pyrolysis, Combustion, Reduction (Photos 1 and 2). Through these processes, the main end products are producer gas (CO 20%, H₂ 20%, CH₄ 3%). When rice husk  is the feedstock, the residual biochar is on average about 17% of the dry weight of the feedstock with content of 72% ash and 28% carbon (Sokchea, unpublished data).

Updraft gasifier stove

This type of gasifier stove has 4 features:  top-lit, forced-air, updraft, and batch.

1.      The lighting of the biomass takes place at the top of the reactor (top-lit).

2.      Air is forced through the biomass and char within the reactor by means of a fan or blower (forced-air).

3.      The air or gases rise within the reactor (updraft).

4.      When all of the biomass is gasified, the reactor is emptied of char, and the process is repeated (batch).

The gasifier stove (Photos 3 and 4) is divided into three parts (cooker, reactor and ash storage) with 4 steps of gasification: drying, pyrolysis, combustion, reduction. The stove with diameter of 25cm can burn for around  1h with 5kg of rice husk as feedstock. Biochar yield is 25% from rice husk with 35.6% ash and 64.4% carbon. Biomass gasification proceeds from top to bottom at a rate of about 17 mm per minute (Olivier 2010).  

Biodigester effluent

The effluent was taken from two “plug-flow” tubular polyethylene (0.5 m³ liquid volume) biodigesters (Photo 5) charged daily with pig manure collected from a nearby pig farm. The daily charge was 5 kg of fresh manure and 20 litres of water with 20 days of retention time.The biodigester effluent was applied every 5 days at the amount of 10g N/m² with the duration of 30 days, according to the treatment.

Photo 5: The plug-flow tubular polyethylene biodigesters

Procedure

Maize (Zea mays) was chosen as the most suitable indicator plant (Chamnanwit 2001).  The soil (pH 4.3) was taken from the campus of An Giang University, located in An Giang province, Long Xuyen city. It was broken down into small pieces and quantities of 1 kg mixed with one of the two kinds of biochar at different levels, according to the treatments. The mixed soil and biochar was put into plastic bags (n=60) of 1.5 liter capacity. Seeds of maize were soaked over-night for better germination before planting three seeds in each bag. Water was sprayed daily on the bags. The biodigester effluent was applied every 5 days in quantities according to the N content to provide the equivalent of 10 g N/m² over the 30 day period.  After germination, 1 or 2 plants were removed to leave a single plant for the rest of the experiment (Photo 6).

Photo 6. The maize biotest system

At the end of 30 days, the plants were removed from the bags by soaking the contents in water to release the soil. Each component of the plant (Leaves, stems and roots) was weighed separately and samples taken  for determination of DM.

Chemical analysis

The soil was analysed for DM, pH and ash before, and at the end of the experiment. Biochar was analysed foir ash and pH. The DM content was determined using the micro-wave radiation method of Undersander et al (1993). Ash and N were determined following AOAC (1990) procedures. The pH of soil samples was determined using a digital pH meter with glass electrode;  the samples were collected and ground to become powder and then, 5g of sample weighed h by putting into the sterilized tubes and pour 25 ml into each tube, after that shook for 2 hours before centrifugation of 10 minutes and then measured with electronic pH meter machine cations exchange capacity (CEC), The biodigester effluent was analyzed for nitrogen content (Table 2).

Statistical analysis

The data were analyzed by the GLM option in the ANOVA program of the Minitab software (Minitab 2000).  Sources of variation were:  biochar source, effluent,  biochar level, interactions between biochar*level, biochar*effluent and error.

Results and discussion

Composition of biochar

The biochar from the stove contained more ash (less organic matter) and the pH was higher (Table 2) compared with biochar from the gasifier. The organic matter content was much higher in the biochar derived from rice husks in this study than was reported for biochar derived from gasification of sugar cane bagasse for which the organic matter was 65% (Rodriguez et al 2009). This presumably reflects the much higher content of ash in rice hulls  compared with sugar cane bagasse.

Effects of pH

The soil pH increased linearly with increasing level of biochar and was higher for biochar from the stove than that from the gasifier (Figure 1).. However, the order of increase was less than that reported by Rodriguez et al (2009). In  the study by these authors the soil pH was raised from 4.6 to 6.8 when 5% biochar was added to the soil, an increase of 50% compared with  the much smaller  increment (from 4.6 to 4.9) observed in the present experiment when the level of gasified rice husk was increased from 0 to 5%.

Figure1: Effect of level of different sources of biochar on soil pH

Water holding capacity and cation exchange capacity (CEC)

Water holding capacity was increased by level of biochar with no difference between sources of biochar (Figure 2). By contrast there was no effect of biochar on cation exchange capacity (Figure 3). These results were similar to those reported by Sothavong and Preston (2011) who applied similar treatments to similar samples of the same soil but using rice as the indicator plant. Lehman (2007) emphasized that while cation exchange capacity of soil can be increased by addition of biochar, this depended on the temperature at which the biochar was produced  and on the length of time it had been in the soil . 

Figure 2: Effect of level and source of biochar on water holding capacity of the soil	Figure 3: Effect of level and source of biochar on cation exchange capacity (CEC) of the soil

Biomass yield

The yield of the aerial fraction and of total biomass of the maize was higher when the biochar was from the stove than from the gasifier (Table 3), and when effluent was applied. Root yield tended to be increased with the stove biochar (P=0.077) and was increased two-fold when effluent was applied. Level of biochar tended to increase root (P=0.11) and total biomass (P=0.10) There were no interactions among the treatments; however, the pattern of the responses to the level of addition of biochar was quite different (Figures 4-6).  With the addition of biodigester effluent, biomass yields of the three components of the plant were increased as the biochar level was increased reaching a maximum with 8% of biochar added to the soil (equivalent to 80 tonnes biochar/ha, assuming the biochar would be incorporated in the upper 10 cm of the soil profile). By contrast, in the absence of effluent there appeared to be no effect of the biochar. These effects were similar to those reported by Sothavong and Preston (2011) although the optimum level of biochar in presence of effluent tended to be higher in the present study (with 6% biochar) compared with that of Sothavong and Preston (2011) where the maximum response with rice plants was with 4% biochar. Doung Nguyen Khang et al (2010) also found that the optimum maize growth was achieved with 4% biochar (from gasifier stove) in the presence of effluent, with reduced yields at higher levels, while there was no response to biochar when effluent was not applied.

Figure 4: Effect of effluent and level of biochar on root biomass yield of maize	Figure 5: Effect of effluent and level of biochar on green biomass yield of maize	Figure 6: Effect of effluent and level of biochar on total biomass yield of maize

In general the responses to biochar addition to the acid soil (pH 4.6) in the present study were much less pronounced than were reported by Rodriguez et al (2009) where maize yields on a fertile, but acid (pH 4.6),  soil were increased three- to five-fold with addition of 5% biochar in presence or absence of biodigester effluent. The implication is that the major difference in responses in the two studies reflected differences in the origin of the parent feedstock (sugar cane bagasse in the study of Rodríguez et al [2009] compared with rice husks in the present study). The fact that the soil pH was increased only slightly with biochar from rice husks and that there was no effect on cation exchange capacity lends support to this idea that the nature of the parent feedstock may be a major factor in determining plant growth responses to soil amelioration with biochar. This hypothesis merits future studies to compare widely different sources of feedstock and of sources of biochar with different ratios of ash to organic matter.

Conclusions

• The yield of the aerial fraction and of total biomass of maize in a growth period of 30 days was 30% higher when acid soils (pH 4.6) were amended (at 6 to 8% of the soil) with  biochar from an updraft gasiifier stove than from a downdraft gasifier reactor.
• There was no effect of the level of biochar on maize growth in the absence of biodigester effluent but growth was increased 90% when biochar was incorporated at 6% of the soil and biodigester effluent was applied at 10g N/m² over 50 days.
• Soil pH was raised from 4.6 to 4.9 and water holding capacity by 50% when 6-8%  biochar was added to the soil.

Acknowledgement

The senior author would like to express his appreciation to the Ørskov Foundation for their financial support during the conduct of this experiment. The MEKARN Project, financed by Sida, Sweden, provided the finance for purchase of materials for the research and for the chemical analyses. The administration, students and staff of the An Giang University are thanked for their active participation in, and support for, the research activities.

References

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Received 1 November 2010; Accepted 14 February 2011; Published 1 April 2011

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