Effect of biochar and biodigester effluent on growth of water spinach (Ipomoea aquatic) and soil fertility

Sisomphone Southavong, T R Preston* and Ngo Van Man**

Champasack University
Champasack province, Lao PDR
spdeuk@yahoo.com
* Finca Ecológica, TOSOLY, UTA (Colombia)
AA #48, Socorro, Santander, Colombia
** Nong Lam University, Ho Chi Minh city, Vietnam

Abstract

The experiment was conducted at the Integrated Farming Demonstration Centre, Champasack University, Lao PDR to investigate the effect of biochar and biodigester effluent on biomass yield of water spinach and on soil fertility. The treatments were arranged in a randomized complete block design (RCBD) as a 3*2 factorial with 4 replications. The factors were application of biochar to soil at 40 tonnes/ha or none and three levels of biodigester effluent at 0, 50 or 100 kg, N/ha. Twenty four plots were prepared with a total area of 96 m². Each plot had an area of 4 m² (1*4m). Spacing between plots was 80cm and between replications was 120cm. Biochar was applied to the soil at 16kg/4m² or 40 tonnes/ha. Water spinach was established from seed with spacing between rows of 20 cm and between seeds 2-3 cm.

The water holding capacity of the soil was increased by application of biochar but there were no differences due to the level of biodigester effluent. Soil pH was increased by application of biochar from 4.68 to 6.22. There was no apparent effect of level of effluent on soil pH. The biomass yield of water spinach in both first and second harvests was increased due to the application of biochar.

Key words: rice husk, soil pH, soil texture, TLUP gasifier stove, water holding capacity

Introduction

The soil is a very crucial factor in food production. The most important problem of tropical agriculture is the inability of the land to sustain annual food crop for more than a few years at a time. Since animals, in turn, depend on plants, it becomes obvious that all agricultural activities directly or indirectly depend on the soil (Akinrinde 2006).

An increasing number of global threats such as climate change, poverty, declining agricultural production, scarcity of water, fertilizer shortage and the resulting social and political unrest seem overwhelming (Lehmann and Joseph 2009). The urgency to address these threats creates an ever increasing demand for solutions that can be implemented now or at least in the near future. These solutions need to be widely implemented both locally by individuals and through large programmes in order to produce effects on a global scale. This is a daunting and urgent task that cannot be achieved by any single technology, but requires many different approaches (Lehmann and Joseph 2009).

At best, common renewable energy strategies can only offset fossil fuel emissions of CO₂ – they cannot reverse climate change. One promising approach to lowering CO₂ in the atmosphere, while producing energy and biochar, is by pyrolysis and gasification of biomass (Lehmann 2007). This technology relies on capturing the off-gases from thermal decomposition of wood or grasses to produce heat, electricity, or biofuels. Biochar is a major by-product of this pyrolysis, and has remarkable environmental properties (Lehmann 2007). Biochar is produced by so-called thermal decomposition of organic material under limited supply of oxygen (O2), and at relatively low temperatures (<700°C). This process often mirrors the production of charcoal, which is one of the most ancient industrial technologies developed by mankind – if not the oldest (Harris, 1999). In soil, biochar was shown to persist longer and to retain cations better than other forms of soil organic matter. Furthermore, the cation retention of fresh biochar is relatively low compared to aged biochar in soil, and it is not clear under what conditions, and over what period of time, biochar develops its adsorbing properties (Lehmann 2007).

Previous biotest experiments conducted in our laboratory (Sisomphone et al 2012a; Sisomphone et al 2012b) showed that there were effects on growth of water spinach from combining biochar (the residue from the gasification of rice husks) with biodigester effluent, as additives to an acid soil (pH 4.6). It was therefore hypothesized that applying similar method in a field trial would also increase biomass yield of water spinach.

Materials and methods

Location

The experiment was carried out in the integrated farming demonstration center of Champasack University located in the Huay Leusy village, about 13 km from Pakse district, Champasack province, Lao PDR between May and Sep 2010, the mean air temperature of 28.2°C and average annual rainfall of 2000mm/year.

Experimental design

Six treatments were arranged in a randomized complete block design (RCBD) as a 3*2 factorial with 4 replications.

The factors were:

• Addition of biochar: with or without at 40 tonnes/ha
• Application of biodigester effluent: 0, 50 or 100 kg N/ha

Materials

The biochar (Photo 2) was derived from rice husk (Photo 3), produced locally in an updraft (TLUD) gasifier stove (Olivier 2010; Photo 4). The effluent used in the experiment was taken from a “plug-flow” biodigester made of tubular polyethylene with UV filter of 5 m³ liquid volume (Photo 5) charged daily with washings (1 m³) from pig pens holding on average 21 pigs of 50 kg mean live weight fed rice bran and taro silage. Water spinach seeds (dry land species) were bought locally from the market.

Land preparation, plot size and planting

Land was ploughed by using a two-wheel tractor. Then twenty four plots were prepared with a total area of 96 m². Each plot had an area of 4 m² (1*4m); spacing between plots was 80cm and between replications was 120cm. Biochar was applied to the soil at 16kg/4m² or 40 tonnes/ha (Photo 6). Water spinach seeds were soaked overnight in warm water before planting in the next day for better germination. The spacing between rows was 20 cm and between seeds 2-3 cm (Photo 7).

Fertilizing and irrigation

Samples of the effluent were analyzed for N before applying to the water spinach plots. Effluent from the biodigester was applied to the treatments at the beginning of planting and then at 7 day interval (total 4 times). The quantities were calculated according to the N content of the effluent to give the equivalent of 50 or 100 kg N/ha. Water was applied uniformly to all plots every morning and evening. On rainy days no additional water was applied.

Measurements

The heights of the plants and number of leaves were measured every 7 days over a total period of 28 days by selecting 10 representative plants in each plot. At the end of the trial, the green biomass (leaf + stem) was harvested by using the frame (0.8*3m) and weighed and allowed to re-grow for a further 28 days. Samples of the foliages were analysed for dry matter (DM) content. Samples of soil were analysed at the beginning and end of the trial for pH, OM, water holding capacity and N. Biochar was analysed for DM, pH and ash content.

Chemical analysis

The DM content of the water spinach, biochar and soil samples was determined using the micro-wave radiation method of Undersander et al (1993). Organic matter (OM) of biochar and soil and N content of effluent were determined by AOAC (1990) methods. The pH of soil was determined using digital pH meter by adding 5g of ground sample (DM basis) into a beaker with 25 ml of distilled water. The suspension was stirred and kept over-night. In the next morning before measuring the pH the sample was stirred again for 5- 10 minutes, then kept for another 5 - 10 minutes to let the solid part sink down and then the measurement was taken in the liquid part. Soil samples were analysed for texture, separating the fractions into clay, fine silt, coarse silt, fine sand and coarse sand using the Pipette Method http://www.geology.iupui.edu/research/SoilsLab/procedures/psd/index.htm

The cation exchange capacity (CEC) was determined by titrating with 1M Calcium Chloride at pH 7. Water holding capacity was measured by weighing 5 g of soil (DM basis) into a glass funnel fitted with filter paper and then saturating the soil with water. After 24 h the soil was weighed to determine the quantity of water that had been retained.

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 effluent, biochar, interaction effluent*biochar, block and error. Tukey test in the Minitab software was used to separate mean values that differed when the F-test was significant at P<0.05.

Results and discussion

Soil characteristics

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 (Turenne 2011). 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 3 indicate that the soil in the experimental area would be classified as “clay” soil (Foth 1990; Berry et al 2007). 

Chemical composition of experimental materials

Water-holding capacity and pH of the soil

Biochar has high total porosity, and it can both retain water in small pores and thus increase WHC and let the water flow through the larger pores after heavy rain from topsoil to deeper soil layers (Asai et al 2009). The water holding capacity of the soil was increased by application of biochar but there were no differences with level of biodigester effluent (Table 6, Figure 2). The level of improvement with biochar was similar to the value reported by Sisomphone et al (2012a) and Sisomphone et al (2012b) when 4% (by weight) biochar was added to the soil. 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 lower values in this present report probably reflected differences in soil characteristics between the different experiments. Lehmann (2009) suggested that biochar application may enhance the soil moisture retention, while Chan et al (2007) showed that biochar application improved some physical properties of soils, such as increased soil aggregation and water holding capacity. A positive improvement of WHC was also reported by (Karhu et al 2011). Soil pH was increased by application of biochar from 4.68 to 6.22 (Figure 2); there was no apparent effect of level of effluent on soil pH. 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 and Southavong and Preston (2011) where the soil pH increased from 4.5 to 5.13 and 5.40 when biochar was added to soil at 2 to 8% with the higher value for biochar from the stove than from the down draft gasifier. Agusalim Masulili et al (2010) also reported that application of biochar from rice husk at 10 tonnes/ha in a very acid soil increased pH from 3.75 to 4.40 and in the study by Zhang et al (2012) there was also a positive effect due to biochar.

Effect of biochar and effluent on water spinach biomass yield

Biochar increased foliage yield of the water spinach in both the first and second harvests. The long-term effect of biochar to enhance the fertility of the soil was observed by Sisomphone et al (2012a) and it is in agreement with the majority of reports in the literature (rice biomass [Sisomphone and Preston 2011], rice grain yield [Zhang et al 2010; Zhang et al 2012]; maize [Rodriguez et al 2009; Sokchea and Preston 2011]; water spinach [Sisomphone et al 2012b]). This response (equivalent to 18.1 tonnes/ha) is similar to the 18.3 tonnes/ha yield of water spinach reported by Sisomphone et al (2012b). Lehmann (2007) stressed that nutrients of the soil are retained and remain available to plant due to application of biochar hence it increased crop yield. It has been well documented that biochar amendment to crop lands enhances crop productivity through improving soil quality (Asai et al 2009; Major et al 2010; Sohi et al 2010; Zwieten et al 2010; Gaskin et al 2010; Haefele et al 2011).

Conclusions and recommendations

• Biochar increased foliage yield of water spinach in both first and second harvests but there was no apparent effect on foliage growth from application of level of effluent.
  
  
• Soil pH was increased from 4.68 to 6.22 due to addition of biochar.
  
  
• Water holding capacity of the soil was increased by application of biochar.

Acknowledgement

The authors are very grateful to the Sida MEKARN program funded by sida SAREC project, Sweden, for the support for this research as part of the requirements for the MSc degree at Cantho University in "Animal Production; Specialized in Response to Climate Change and Depletion of Non-renewable resources". Dr. Phetsamay Vyraphet, Dean Faculty of Agriculture and Forestry for his patient and valuable guidance and advice during my experiment. Champasack University is also acknowledged for provision of research facilities.

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Received 29 December 2011; Accepted 29 January 2012; Published 7 February 2012

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