Effect of biochar and biodigester effluent on growth of maize in acid soils

Lylian Rodríguez, Patricia Salazar and T R Preston

UTA-TOSOLY, AA 48 Socorro, Colombia
lylianr@utafoundation.org

Abstract

The hypothesis that was tested in the present study was that there would be a synergistic response in growth of maize when biodigester effluent, rich in NH₄-N, was combined with biochar, derived by gasification of sugar cane bagasse. Two experiments were carried out to measure changes in soil fertility as a function of the growth of maize plants over a 30-40 day period following seeding. In each experiment a completely randomized design was used with 4 replications of the treatments applied to samples of soil held in one litre capacity plastic bags. In experiment 1, 8 treatments were compared in a 2*2*2 factorial arrangement. The factors were: with or without biochar at 50g/kg soil; fertile soil or sub-soil; and with or without biodigester effluent (100 kg N/ha). In experiment 2, ash from a wood-burning stove replaced the biochar used in experiment 1.

Biochar increased green biomass growth of the maize on the fertile soil in absence or presence of biodigester effluent and in the sub-soil when effluent was applied, but had no effect on heavily leached soil without effluent.  Application of effluent had no effect on green biomass growth in the fertile soil irrespective of the presence or not of biochar. By contrast, the effluent dramatically increased green biomass growth when biochar was applied to the sub-soil but had no effect in the absence of biochar. Effects on growth of the roots mirrored those on the green biomass except in the case of the sub-soil without effluent when the biochar markedly increased root growth. Soil pH was increased from 4-4.5 to 6.0-6.5 due to addition of biochar.  Wood ash brought about increases in the weight of both the aerial part and roots of the maize but the relative increases were only half of those observed when biochar was used.  Soil pH was increased to values between 9 and 10.

It is concluded that there are synergistic effects on plant growth in heavily leached, acid soils when biodigester effluent is combined with biochar produced by gasification of sugar cane bagasse.

Key words: Ash, bagasse, biomass, biotest, gasifier, roots, sugar cane, wood stove

Introduction

Recent interest in the use of biochar as a soil amender (Lehmann et al 2006; Lehmann 2007) has its origin in the discovery, by Dutch soil scientist Wim Sombroek in the 1950's, of pockets of rich, fertile soil in the Amazon rainforest (otherwise known for its poor, thin soils). He gave it the name of Terra Preta ("black earth").  Carbon dating has shown that the carbon in these soils dates back to between 1,800 and 2,300 years (Glaser 2007).

Terra preta . is rich in minerals including potassium, phosphorus, calcium, zinc, and manganese - however it’s most important ingredient is charcoal, the source of terra preta's dark colour. The exact origin of the charcoal in Terra preta is not fully understood but it appears to have arisen from controlled burning of trees and related biomass sources. The fact that it has remained in the soil  for thousands of years implies that it can be an effective medium for long term sequestration of carbon derived originally from the atmosphere through photosynthesis. This also indicates that the form of the charcoal in Terra preta soils is different to the charcoal prepared in the traditional manner as a source of fuel for cooking. This has given rise to the term “biochar” to differentiate this “stable” form of charcoal, that is not oxidized by soil micro-organisms, as compared with  charcoal which eventually is degraded by soil microbes to carbon dioxide. According to Glaser (2007) the chemical structure of biochar is characterized by the presence of poly-condensed aromatic moieties  and that these are responsible for the stability against microbial degradation.

The apparent  high fertility of Terra preta soils,, has led to research to measure the immediate effects of “biochar” addition to soils  on plant growth. Major increases (up to 324%)  in yield of a range of crops through addition of biochar at rates varying from 0.5 to 135 tonnes/ha were recorded in the review by Sohi et al (2009). However, these authors state that addition of nutrients from inorganic or organic fertilizers is usually essential for high productivity and to increase the positive response from the bio-char amendment. Chan et al (2008) recorded a linear increase in yield of radish (Raphanus sativus) by addition of up to 50 tonnes/ha of biochar provided additional N fertilizer was also supplied. Glaser (2007) also indicated that there would be benefits in plant growth from combining the biochar with chicken manure.

The explanations for the effects of addition of biochar to soils in increasing crop yields include greater water holding capacity, increased Cation Exchange Capacity (CEC), and providing a medium for adsorption of plant nutrients and  improved  conditions for soil micro-organisms (Sohi et al 2009). Biochar efficiently adsorbs ammonia (NH₃) according to Oya and Iu (2002) and Iyobe et al (2004) and acts as a binder  for ammonia in soil, therefore having the potential to decrease ammonia volatilization from soil surfaces .

 Biodigestor effluent from live stock excreta contains a high proportion of the nitrogenous constituents as  ammonium salts. Pedraza et al (2002) observed that the proportion of ammonia-N in the effluent from plug-flow, tubular plastic biodigesters was in the range of 0.65 to 0.75. Similar findings were reported by San Thy et al (2003). In their study, the proportion of ammonia-N to total-N increased from 0.077 to 0.12  in fresh pig manure to  0.46 to 0.65 in the effluent. The combination of biodigester effluent and biochar therefore should be synergistic in improving soil fertility and plant growth 

The hypothesis that was tested in the present study was that there would be a synergistic response in growth of maize when biodigester effluent is combined with biochar.

Materials and methods

Location

The study was carried out in the "Finca Ecológica", TOSOLY, Morario, Guapota, Department of South Santander, Colombia (6° 18" N, 73° 32" W, 1500 msl) between September and December 2008. Air temperature ranges between 19 and 28°C in the day, falling to around 12°C during the night. Rainfall is between 2700 and 3000 mm/year and is relatively evenly distributed.

Treatments and design

Two experiments were carried out using the maize “biotest” for measuring changes in soil fertility as a function of the growth of maize plants over a 30-40 day period following seeding (Boonchan Chantaprasarn and Preston (2004). In each experiment a completely randomized design was used with 4 replications of the treatments applied to samples of soil held in one litre capacity plastic bags (Photo 1).

Experiment 1: Effect of biochar added to soil (pH 4.0) at 50 tonnes/ha with and without biodigester effluent (100 kg N/ha)  on growth of maize

Eight treatments were compared in a 2*2*2 factorial arrangement with 4 replications.

The factors were:

Biochar: With or without biochar

Soil type: Fertile soil or sub-soil

Biodigester effluent: With or without effluent

Experiment 2: Effect of wood ash added to soil at 50 tonnes/ha with or without biodigester effluent

The eight treatments and the design were the same as in Experiment 1 but with wood ash replacing the biochar. The factors were:

Wood ash: With or without

Soil type: Fertile soil or sub-soil

Biodigester effluent: With or without

Materials

Biochar

The biochar (Photo 2) was the solid residue from a down-draft gasifier (Photo 3; Ankur PTY, India), charged with sugar cane bagasse derived from sugar cane stalks passed two times through a 3-roll crusher driven by a diesel engine (Photo 5). It contained 35% ash and 65% carbon and had a pH of 9.0. The bagasse was sun-dried to about 12% DM and hand-separated into large and small pieces, the latter being the feedstock for the gasifier. The particle size of this fraction was between 1 and 30mm (Photo 4). After gasification of this fraction the residual biochar represented 10% by weight of the air-dry bagasse (88% DM) fed into the gasifier.

Wood ash

This was the residue after burning firewood in a closed stove (Photo 6). The pH was 9.5.

Soil samples

Two types of soil were used in each experiment. The fertile soil was taken from areas (top 10cm) in a coffee plantation shaded with Guamo trees (Inga hayesii Benth) (Photo 7). The pH of this soil was 4.5. The su-soil; second sample (sub-soil) was from soil that had been excavated during construction work (Photo 8). The pH was 4.0.

Samples (about 1 kg) of the respective soils were placed in polyethylene bags with or without addition of the biochar (or ash) which was mixed thoroughly with the soil according to the imposed treatment. Water was sprayed uniformly on the bags at 2-day intervals throughout the growth period of 40 days.

Biodigester effluent

The effluent was taken from the exit stream of a “plug-flow” tubular polyethylene (3.0 m³ liquid volume) biodigester (Photo 9) charged daily with the washings (500 litres)  from 4 pens each holding on average 8 pigs of 50 kg mean live weight. The diet of the pigs (DM basis) on average contained 20% soybean meal, 30% rice polishings and 50% fresh sugar cane juice. The N content of the effluent was 700 mg/litre with 420 mg/litre as NH₄-N. It was poured on the surface of the soil in the bags at weekly intervals (5 applications) at the overall rate of 100 kg N/ha. Similar amounts of water were added to the bags not receiving effluent.

Maize seeds

These were of a local variety. Three seeds were placed in each bag. After germination, one or two seedlings were removed to leave only one plant for the experimental growth period of 40 days.

Measurements

At 40 days after seeding the height of the maize was measured at the tip of the highest leaf. The complete plant was then removed from the bag and the aerial part separated from the roots which were washed free of soil. Both fractions were weighed.  The pH of the soil was measured with a digital pH meter at the time of harvesting the maize.

Statistical analysis

The data were analysed using the General Linear Model in the ANOVA option of the Minitab (2003) software. Sources of variation were: Blocks, Biochar, Effluent, Soil type, and the interactions of Biochar*Effluent, Biochar*Soil, Effluent*Soil, Biochar*Effluent*Soil and error.

Results

Effects of biochar

The height and the fresh weights of the aerial part and the roots of the maize were increased by addition to the soil of biochar and biodigester effluent and were higher for the maize grown in the fertile compared to the sub-soil (Table 1).  Soil pH was increased by addition of biochar and was higher in the fertile soil.

There were interactions due to the treatments on weights of aerial part and roots  of the maize (Table 1 and Figures 1 and 2) for biochar*effluent and biochar*effluent*soil type with tendencies for interaction (P=0.075) for effluent*soil.   

Biochar increased green biomass growth of the maize on the fertile soil in absence or presence of biodigester effluent and in the sub-soil when effluent was applied, but had no effect on sub-soil without effluent (Figure 1).  Application of effluent had no effect on green biomass growth in the fertile soil irrespective of the presence or not of biochar. By contrast, the effluent dramatically increased green biomass growth when biochar was applied to the sub-soil but had no effect in the absence of biochar. Effects on growth of the roots mirrored those on the green biomass except in the case of the sub-soil without effluent when the biochar markedly increased root growth (Figure 2).

Soil pH was increased by nearly 2 units due to addition of biochar (Table 1 and Figure 3).  There was no effect on soil pH due to application of effluent but values were 0.5 pH units higher on average for the fertile soil compared with the sub-soil. 

Figure 3.  Effect of biochar  and effluent on soil pH in fertile soil and sub-soil

Effects of wood ash

In view of the major increase in soil pH (from 4-4.5 to 6.0-6.5) caused by the addition of biochar, it was hypothesized that one reason for the stimulatory effect of biochar on growth of maize might have been caused by the increase in soil pH.

Experiment 2 was designed to study the effect of ash per se in the absence of the carbon which is the other major component of the biochar used in this study. The addition of the wood ash, admittedly at ash levels some 30% higher than when biochar was used, brought about increases in the weight of both the aerial part and roots, of the maize but the relative increases were much less than when biochar was used (Table 2; Figures 4 and 5).

Soil pH was raised by 5-6 pH units (Figure 6) to values between 9 and 10. This high degree of alkalinity may have been a deterrent to plant growth.   

Discussion

The 10% yield of biochar from gasification of sugar cane bagasse is similar to values reported by Miech Phalla and Preston (2005) for Mulberry stems (10.9%), Cassava stems (12.8), Coconut shells (13.7%) and branches from the leguminous tree Cassia stamea (10.9%), processed in a similar model of gasifier (Ankur PTY, India).

The increase in growth of the maize brought about by the biochar is in agreement with the majority of reports in the literature (see Sohi et al 2009). Two factors appear to distinguish the biochar used in these studies and that used in most reported experiments. First, the biochar was the product of gasification and therefore would have been submitted to higher temperatures than biochar derived by pyrolysis; and secondly it had a very high content of ash. Such a high ash content (35%) in the biochar used in these studies has not apparently been observed in other experiments. 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 4.6 to 6.3 in our experiment. The results from using wood ash as soil amendment in Experiment 2 were confounded by the higher level (of ash) that was used and the resulting major increase in soil pH (from 4.4 to 9.5). The much lower growth response of the maize to the wood ash compared with the biochar could be interpreted as the consequence of the absence of the carbon and related organic compounds in the biochar, and/or the negative effect of the excessive alkalinity (soil pH of 9.5) which would have reduced phosphorus availability with formation of insoluble calcium phosphate.

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). The significant interaction between application of biodigester effluent and biochar in the sub-soil, but not in the fertile soil, confirms the importance of the relationship between nutrient supply and response to biochar. These findings emphasize the major benefits that biochar combined with biodigester effluent can confer on poor soils with little or no organic matter and low nutrient status (Photos 10 and 11). Similar synergistic effects on plant growth by combining charcoal with chicken manure were observed by Steiner et al (2007). 

Conclusions

• Biochar produced as a byproduct of the gasification of sun-dried, sugar cane bagasse (the cane stalks were passed two times through a 3-roll mill traditionally employed for making “panela”), contained 35% ash.
  
  
• Application of the biochar (50 g/kg of soil) to a fertile soil (from a shaded coffee plantation) increased above ground biomass growth five-fold with no additional benefit from simultaneous application of biodigester effluent. When applied to a sub-soil, there was a synergistic effect of the biochar and the biodigester effluent; the biochar alone increased yield eight-fold but combined with biodigester effluent the increase was twenty-fold. Effects on the root biomass were similar.
  
  
• The initial pH of both soils was in the range of 4.0-4.5 and was increased to 6.0-6.5 by addition of the biochar. Effluent application did not affect soil pH.
  
  
• Application of ash from a wood-burning stove at 50g/kg soil also increased maize yield but to a level of only one third of that achieved with biochar. The increase in soil pH was double that observed with biochar reaching levels of between 9 and 10.

References

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Received 11 March 2009; Accepted 1 June 2009; Published 1 July 2009

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