Black carbon from rice residues as soil amendment and for carbon sequestration

1Haefele SM, 2Konboon Y, 3Knoblauch C, 4Koyama S, 1Gummert M, 1Ladha JK

1 International Rice Research Institute, 2 Ubon Ratchathani Rice Research Center, 3 University of Hamburg, 4 Japan International Cooperation Agency Tsukuba, IRRIIRRIINTERNATIONAL RICE RESEARCH INSTITUTEINTERNATIONAL RICE RESEARCH INSTITUTE

1 Background and objectives

On highly weathered soils in tropical and subtropical climates, maintenance of soil organic matter is essential to sustain system productivity and avoid rapid soil degradation. But climatic conditions as well as soil characteristics favor the rapid decomposition of organic matter. However, several recent studies indicated that carbonized plant residues (black carbon), the product of incomplete combustion of organic material, could combine characteristics highly beneficial for soil nutrient dynamics with high stability against chemical and microbial breakdown. Our project investigates i) past and present uses of carbonized rice residues, ii) the agronomic and environmental effects of carbonized rice residues in rice-based systems, iii) the bio-degradability of carbonized residues in rice soils, and iv) the possibilities to integrate residue use into the rice production process.

Fig 2: Production of CH4 and CO2 in two temperate Fluvisols. Upper panels show CH4 (black) and CO2 (grey) production under anoxic incubation conditions, lower panels CO2-production under oxic conditions. Error bars indicate standard deviation of quadruplicate incubations, the diamond the content of Corg in the different treatments.

Carbonized rice husk can also be the end product of rice hull furnaces used by small commercial rice mills to dry the paddy. A new semi-automatic down-draft rice husk furnace, developed by IRRI, Hohenheim University (Germany) and Nong Lam University (Vietnam) has an adjustable feed mechanism for setting the retention time of the rice husk inside the burning chamber.

2. Past and current use of carbonized rice residues

Carbonized rice husk are traditionally used in Japan. In and outside of Japan they are mainly used in seed beds, in hydroponics and for the cultivation of ornamental plants.

Poster identification card

3. The effect of carbonized rice husks in greenhouse and field experiments

To investigate the effect of carbonized residue application, we conducted greenhouse studies and established field experiments in different rice production systems (irrigated and rainfed). Preliminary results indicate beneficial agronomic effects on poor soils only. Analysis of the effect on soil characteristics has not yet been completed.

4. Bio-degradability of carbonized rice husks in incubation experiments

The decomposition of carbonized organic matter in soils was evaluated under oxic and anoxic conditions by measuring the carbon dioxide and methane production. Under both conditions, carbonized rice husks were inert and not decomposed.

5. Integrated use of rice residues (husks)

Husks constitute about 20% of the paddy weight. Using a newly developed rice hull furnace, they could substitute fossil fuels in the paddy drying process, supply a material for soil improvement and contribute to carbon sequestration in the soil.

No rice husk added

Carbonized rice husk added

Untreated rice husk added

No rice husk added

Carbonized rice husk added

Untreated rice husk added

loamy Fluvisol sandy Fluvisol

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Oxi

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Fig 1: Experimental setup. Anoxic incubations (left panel) were done with field fresh soil in sealed glass flasks. CH4 and CO2-concentrations were measured repeatedly and gas production rates calculated. Oxic carbon turnover was studied in soil samples incubated in gas tight glass flasks under ambient air (right panel). A NaOH trap was used to precipitate CO2 produced. After the incubation period (24-48 h) CO2 production was quantified by titration with HCl.

Preliminary results: If dried rice husks were added, the initial CO2 and CH4 production rates increased by a factor of 3 to 7. In case of the addition of carbonized material, no effect could be measured. These results indicate the strong resistance of carbonized organic matter towards microbial breakdown.

At high feed rates the husks are incompletely burned resulting in a higher ash recovery, i.e. the furnace produces a high percentage of carbonized rice husk without loosing much efficiency. Hence, rice husks could simultaneously substitute fossil fuels (CO2 emission neutral) during the paddy drying process, supply a material for soil improvement and contribute to carbon sequestration in the soil.

Conclusions:

Carbonized crop residues could contribute to increase soil carbon storage (carbon sequestration), reduce climate-relevant gas emissions and improve the natural soil resource in rice-based production systems.

In Japan, carbonized rice husk (Kuntan) has been produced and used in agriculture since a long time. Several books on the utilization of Kuntan in agriculture were already published in the 1910’s in Japan. In and outside of Japan, Kuntan was and is used to cover and protect rice nurseries, as an additive to the culture medium for ornamental plants, and as a component of growth media for seedlings and hydroponics. It was also reported to function as absorptive for moisture and gas and as water purifier.

Because of the known use and production of carbonized rice husk in Japan, the same material and procedure for our experiments was used. Weight loss due to carbonization was 68%, but the bulk density did not change (128 g per liter). Data of the chemical analysis is given below. Note that these results depend highly on the temperature and duration of the carbonization process.

C N P K Ca Mg Si Fe Zn

-----------------------------------g kg-1--------------------------------- ppm

Plain rice husk 362 6.9 1.4 4.5 0.6 0.8 95 0.9 17

Carbonized husk 1 398 19.7 7.8 13.4 1.4 3.7 187 1.2 63

Carbonized husk 2 398 4.9 1.1 7.2 1.6 1.2 220 5.6 36

Charcoal (wood) 805 9.4 1.2 11.2 11.8 1.3 1 0.4 21

Greenhouse experiments were conducted at IRRI and URRRC (NE Thailand), comparing treatments without, with plain and with carbonized husk application. Fertilizer treatments (none and medium NPK rate) were superimposed. Four field trials with identical treatments were established in irrigated systems in the Philippines and India (Los Baños, Modipuram), in a rainfed upland system in the Philippines (Siniloan), and in a rainfed lowland system in NE Thailand (Ubon).

Total organic carbon in the percolating soil solution

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01-Aug-05 01-Sep-05 01-Oct-05 01-Nov-05

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carbonized husk and fertilizer carbonized husk no fertilizer plain husk and fertilizer plain husk no fertilizer

Siniloan, Philippines

Los Banos, Philippines

Greenhouse experiment

Preliminary results: greenhouse experiments did indicate small biomass and yield increases due to the application of carbonized rice husks. Even high application rates did not result in increased leaching of organic carbon. Results of the field trials did not show any significant agronomic effect (biomass or grain yield) in irrigated systems with medium soil fertility (Los Baños, Modipuram), but considerable yield increases were observed in rainfed systems with poor soil fertility. Particularly interesting at these sites and in the greenhouse experiments was that the addition of carbonized husks did increase the fertilizer use efficiency. Analysis of soil related results is still ongoing.

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Kuntan production in Japan (photo provided by A. Kubota)

Kuntan production at IRRI

Presented by SM Haefele, IRRI (shaefele@cgiar.org)