Effect of soil amender (biochar or charcoal) and biodigester effluent on growth of water spinach (Ipomoea aquatica)

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

A biotest was carried out at the research centre of Champasack University, Lao PDR to determine the effect of biochar, charcoal and biodigester effluent on growth of water spinach. The fifteen treatments in a completely randomized 3*5 factorial arrangement with 3 replications were: soil amender (biochar or charcoal or none) at 40 tonnes/ha and level of effluent (0, 25, 50, 75 or 100 kg N/ha) applied to samples of soil held in fifteen litre capacity plastic baskets. Sixty seeds of water spinach were planted in each basket. After germination, some seedlings were removed to balance the number in each basket (40 seedlings) for the rest of the experiment. The plants were irrigated every morning and evening. Measurements were made of height, number of leaves, and weight of above-ground biomass after 28 days and again (re-growth) after a further 28 days.

Both soil amenders (biochar and charcoal) gave similar improvements in water holding capacity, from 27.4% to 39.0 and 37.6, respectively. Soil pH was increased from 4.7 to 6.6 due to addition of biochar and to 6.3 with charcoal. Biochar increased foliage yield of the water spinach in both the first and second harvests, but there was no apparent effect on foliage growth from application of charcoal. In the first harvest, there were curvilinear responses to biodigester effluent for biochar and charcoal amenders, with the peak occurring at between 50 and 75 kg N/ha. For the un-amended soil the response was linear with the highest yield at 100 kg N/ha. In the second harvest, the response to effluent for the biochar amender was again curvilinear with the peak at 50-75 kg N/ha; by contrast the response to effluent with the charcoal amender was linear with maximum yield requiring 100 kg N/ha. On the un-amended soil there was no relationship between effluent level and biomass yield.

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

Introduction

The world is faced with major perturbations, a financial crisis precipitated by simultaneous and interrelated/interactive events including Peak Oil (the end of inexpensive energy), other global resource depletion and climate change all of which are undermining world food economy. There is an urgent need to respond to these challenges in order to produce and deliver food to maintain the present world population, let alone the increased population predicted by 2030 of 8-10 billion people (Leng 2009).

The fertility of soils is important in agriculture particularly in making decisions on planting of crops. Terra Preta ("black earth") was discovered by Dutch soil scientist Wim Sombroek in the 1950's, when he discovered pockets of rich, fertile soil in the Amazon rainforest (otherwise known for its poor, thin soils). Carbon dating has shown them to date back between 1,800 and 2,300 years (Glaser et al 2002). Biochar is a form of charcoal produced from biomass, by a process known as pyrolysis. Pyrolysis means heating in the absence of oxygen, which prevents complete burning of the organic biomass (which happens in open fires). It is rich in a stable form of carbon which is not oxidised by soil micro-organisms.

The application of biochar (charcoal or biomass-derived black carbon [BC]) to soil is proposed as a novel approach to establish a significant, long-term, sink for atmospheric carbon dioxide in terrestrial ecosystems. Apart from positive effects in both reducing emissions and increasing the sequestration of greenhouse gases, the production of biochar and its application to soil will deliver immediate benefits through improved soil fertility and increased crop production (Lehman et al 2006). Moreover, some researchers claim that biochar may be an immediate solution to reducing the global impact of farming (and in reducing the impact from burning of agricultural waste). It has been shown that biochar has multiple uses, when added to soil it can significantly improve soil fertility 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; Davies 2007).

The increase in crop yield with biochar application has been reported elsewhere for crops such as cowpea (Yamato et al 2006), soybean (Tagoe et al 2008), maize (Yamato et al 2006; Rodríguez et al 2009), and upland rice (Asai et al 2009). Haefele (2007) and Haefele et al (2008) discussed the possibility of biochar applications for rice-based cropping systems. Reichenauer et al (2009) applied biochar in tsunami-affected paddy fields in Sri Lanka, and the experimental results showed that the 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 per ha for the biochar treatment. Boun Suy Tan (unpublished data) has also indicated that applying biochar (from a downdraft gasifier) to the soil at 40 tonnes/ha in combination with compost could triple the yield of rice from 1.25 to 3.76 tonnes/ha. It is believed that biochar acts as a soil conditioner enhancing plant growth by retaining nutrients and by providing other services such as improving soil physical and biological properties (Glaser et al 2002; Lehmann and Glaser 2003; Lehmann and Rondon 2005).

Water spinach (Ipomoea aquatica) is a vegetable that is consumed by people and animals; it has a short growth period, is resistant to common insect pests and can be cultivated either in dry or flooded soils. Moreover, it has been found that water spinach has a high potential to convert nitrogen from biodigester effluent into edible biomass with high protein content (Kean Sophea and Preston 2001). Hongthong Phinmasan et al (2004) reported that water spinach as the only source of feed for growing rabbits appears to support acceptable growth rates of close to 20 g/day with a DM feed conversion of 2.7. This simple feeding system may be attractive for small-holder farmers in the tropics, due to the possibility to raise rabbits with a local resource (water spinach) that is easy to grow and needs no processing.

The pH of biochar produced by gasification of bagasse and rice husks is 9.5 (Kong Saroeun and Preston 2008) and biochar produced from rice husk by gasifier stove is 9.8 (Southavong and Preston 2011). As these soil conditioners have high pH value, they should be used in the low pH soil (acid soil) because they can increase the pH of the soil and thus increase the yield of acid sensitive crops (Lickacz 2002; FFTC 2008). Positive results from application of biochar to acid (pH 4.5) soils in Colombia were reported by Rodríguez et al (2009). Of special importance in this study was the apparent interaction between biodigester effluent and biochar especially in very poor soil (Figure 1).

Effluent is the liquid waste from anaerobic biodigesters (Bui Xuan An et al 1997). 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; Nguyen Van Hiep and Preston 2006), mulberry (Phiny et al 2009), cassava (Le Ha Chau 1998), maize (Rodríguez et al 2009; Sokchea and Preston 2011) and rice biomass (Southavong and Preston 2011).

Charcoal is a black substance that resembles coal and generally is made from wood that has been burned, or charred, in a reduced flow of oxygen so that what is left is an impure carbon residue. Charcoal is reported to have beneficial effects in soil by helping to clean the soil of pollutants; it also acts as a soil conditioner http://www.wisegeek.com/what-is-charcoal.htm. It is used as a top dressing for gardens, bowling greens and lawns, and as a substitute for lime in soil additives because of the potash content (http://www.buyactivatedcharcoal.com/natural_fertilizer). Ogawa (1987) reported that charcoal applied to the soil could stimulate the activity of soil microorganisms and promote the formation of root nodules and vesicular-arbuscular mycorrhizae in soybean roots.

The objectives of the present study were:

• To evaluate the effect on growth of water spinach of biochar and charcoal as soil amenders in combination with effluent from a biodigester charged with pig manure.
• To test the long-term effect of biochar application to soil in improving soil and crop production.

Materials and Methods

Location, duration and climate of the study area

The experiment was conducted at the research centre of Champasack University, about 13 km from Pakse City, Champasack province, southern Laos. The trial covered the period of March to May 2011. The climate in this area is tropical monsoon with a rainy season between May and October and a dry season from November to April. The mean air temperature is 28.2°C. Average annual rainfall is 2,000mm/year.

Experimental design

The experiment was arranged in a completely randomized design (CRD) as a 5*3 factorial with 3 replications (Tables 1 and 2 and Photo 1).

The factors were:

• Level of biodigester effluent equivalent to: 0, 25, 50, 75 or 100 kg N/ha
• Soil amender: biochar, charcoal (both at 4kg/m2) or none

Materials

The biochar (Photo 2) was produced locally by burning rice husks in an updraft (TLUD) gasifier stove (Olivier 2010) (Photo 4). Charcoal was bought locally from a farmer nearby the University campus. The effluent were taken from a “plug-flow” biodigester (5 m³ liquid volume) made from tubular polyethylene with UV filter (Photo 5) and charged daily with washing (1 m³) from pig pens holding on average 21 pigs of 50 kg mean live weight. Water spinach seeds were bought locally from the market.

Procedure and data collection

Fifteen kg of acid soil (pH 4.68) with or without soil amender (biochar or charcoal) were put into plastic baskets (35*48cm) according to the experimental layout in Table 2. Water spinach seeds (dry-land species) were soaked in water over-night (for better germination) before planting. Sixty seeds were planted in each basket. After germination, some seedlings were removed to balance the number in each basket (40 seedlings) for the rest of the experiment. The distance between rows was 8cm with 2-3cm between seeds. The baskets were lined with a plastic net so that excess water could drain away easily (Photo 6). Water was applied uniformly to all baskets every morning and evening. On rainy days no additional water was applied. The colour, germination and growth of the plants were observed every day.

The heights of the plants and number of leaves were measured every 7 days over a total period of 28 days. At the end of the trial, the green biomass (leaf + stem) was harvested 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 and charcoal were analysed for DM, pH and ash content.

Fertilizing

The fertilizer (biodigester effluent) was applied at the beginning and at 7-day intervals interval (total of 4 times) during the growing period. The quantities were calculated according to the N content of the effluent based on the treatments (25% at each application). For the re-growth period, there was no further addition of effluent.

Chemical analysis

The DM content of the water spinach and soil samples was determined using the micro-wave radiation method of Undersander et al (1993). Organic matter (OM) and N of soil and effluent were determined by AOAC (1990) methods. The pH of soil was determined using a digital pH meter.

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, soil amender, interaction effluent*soil amender and error. The 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

Chemical composition of experimental materials

The pH content of the biochar was much higher than of charcoal (Table 3), a result similar to that reported by Southavong and Preston (2011). The OM content was much higher for charcoal than for biochar (Table 3). The N content of the effluent was much lower compared to reports by Rodríguez et al (2009); Southavong and Preston (2011) and Sokchea and Preston (2011). The reason for this may have been the more dilute influent to the biodigester as a result of washing the pens frequently.

Water holding capacity

Both soil amenders (biochar and charcoal) gave similar improvements (about 50%) in water holding capacity (Table 4). The value was considerably lower than was reported for biochar obtained from an updraft gasifier in Colombia charged with sugar cane bagasse and biochar derived from a TLUD gasifier stove (Southavong and Preston 2009). These authors compared two types of biochar and 5 different levels ranging from 0 to 8%. The increase in water holding capacity was from 37.9 to 59.6% (an increase of over 50%). The difference can probably be explained by the soil properties in the two studies. Sokchea and Preston (2011) experimented with similar soil to that used by Southavong and Preston (2009), and reported an increase from 43 to 62% in water holding capacity when biochar was added.

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, but there was no apparent effect on foliage growth from application of charcoal. In the first harvest (Figure 3; Table 5, there were curvilinear responses to biodigester effluent for biochar and charcoal amenders, with the peak occurring at between 50 and 75 kg N/ha. For the un-amended soil the response was linear with the highest yield at 100 kg N/ha. In the second harvest (Figure 4; Table 5), the response to effluent for the biochar amender was again curvilinear with the peak at 50-75 kg N/ha. The biochar showed the long term effect in improving the biomass yield of WS in agreement with Sombroek et al (2003). Glaser et al (2002), Lehmann and Glaser (2003) and Lehmann and Rondon (2005) reported that when biochar is applied to soil it helps to retain the nutrients which remain available to plants thus increasing the plant growth and yield; by contrast the response to effluent with the charcoal amender was linear with maximum yield requiring 100 kg N/ha. On the un-amended soil there was no relationship between effluent level and biomass yield.

Effect of soil amender on soil pH

The pH of the soil was significantly increased when biochar was applied. There were no effects on soil pH due to level of effluent (Table 6; Figure 6). In the research reported by Rondon et al (2007) the biochar was made by pyrolysis of eucalyptus logs and contained only 0.3% of ash. Their data showed an increase in soil pH from 5.0 to 5.4 after applying 40g biochar per 1 kg of soil, much less than the increase from 4.7 to 6.6 in our experiment.

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 charcoal.
  
  
• Soil pH was increased from 4.7 to 6.6 due to addition of biochar.
  
  
• Both soil amenders (biochar and charcoal) gave similar improvements in water holding capacity.

Acknowledgement

The authors would like to express their sincere thanks to Sida MEKARN program funded by sida SAREC project for financial support 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", special thanks to Dr. Phetsamay Vyraphet for useful and valuable advice, students of Faculty of Agriculture and Forestry and Champasack University for providing the study site to carry out this trial.

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Received 24 July 2011; Accepted 27 January 2012; Published 7 February 2012

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