The review, which compiled 67 observation points for methane and 13 for nitrous oxide from peer-reviewed studies across Sumatra, Java, Bali, Kalimantan and Sulawesi, found average emissions of 288.9 kilograms of methane per hectare per season and 0.71 kilograms of nitrous oxide per hectare per season. Those averages are notably higher than the emission factors previously used to estimate Indonesia’s paddy footprint, which stood at 160.9 kilograms of methane and 0.297 kilograms of nitrous oxide per hectare per season. The gap matters: national inventories built on the older factors may be undercounting the climate cost of the country’s most important staple crop.

When the authors grouped the methane data by soil type and compared the distributions with Box and Whisker plots, backed by F-tests of the means, a striking hierarchy emerged. Continuously flooded paddies on the alluvial soils of Kalimantan and Sulawesi topped the list with a mean of 1,062 kilograms of methane per hectare per season, followed by the Andisols of Java at 505, the Ultisols of Sumatra at 446, the Alfisols of Java and Bali at 135, and the Inceptisols of Java and Bali at just 64 kilograms per hectare per season. The median lines of most soil-type datasets fell outside the boxes of the others, a statistical signal that the differences between soil groups are real rather than noise.

The extreme methane flux from alluvial paddies in South Kalimantan carries a warning that goes beyond soil classification. The Barito River, which irrigates many of those fields, is heavily polluted: in 2013 alone it carried an estimated 94.8 tonnes of biological oxygen demand, 121.5 tonnes of chemical oxygen demand and 51.34 tonnes of total suspended solids per day, much of it from coal mining, industry and microplastics. Polluted water carries more degradable organic carbon, and the more polluted the river, the higher the methane emissions from the water it delivers to the fields. In effect, industrial contamination upstream is being converted into greenhouse gas downstream, one flooded paddy at a time.

Soil chemistry explains much of the rest of the pattern. Andisols have remarkably low bulk density, measured between 0.32 and 0.76 grams per cubic centimeter in Java’s topsoil, compared with 1.23 for Alfisols. Lighter, more porous soils let methane escape more easily, which is why a Java Andisol trial with only 3 tonnes per hectare of farmyard manure emitted 513 kilograms of methane per hectare per season, while an Alfisol trial with 7.5 tonnes of the same amendment emitted just 342. Ultisols, rich in iron and manganese oxides, follow a different route: as flooding drives redox potential below 150 millivolts, microbes strip electrons from Fe3+ and Mn4+ before finally reducing CO2 to methane, making those electron acceptors stepping stones on the path to methanogenesis.

Management practice, however, is the principal driver. The dataset showed that the combination of rice straw and chemical fertiliser produced the highest and most dispersed methane emissions, significantly different from straw alone, granular compound fertiliser alone or untreated controls. Incorporating 6 to 9 tonnes of rice straw per hectare can raise methane emissions 1.8 to 3.5 times, and the interaction of organic amendments with synthetic fertiliser appears to supercharge the anaerobic decomposition that feeds methanogenic bacteria. Nitrous oxide tells a different story: the calculated fertiliser-induced emission factor for West Java paddies averaged just 0.37%, or 0.69 kilograms per hectare per season, in line with figures from China and India at 0.39% and 0.13% respectively. Roughly 70% of nitrous oxide from paddies is the product of nitrifier and denitrifier microbes, and rice roots modulate the flux by leaking oxygen that shifts redox status around the rhizosphere.

The proposed fix is biochar. Pyrolyzing rice straw and rice husk, the two great by-products of the harvest, in low-oxygen conditions yields rice straw biochar and rice husk biochar at 30 to 75% of dry weight, depending on temperature. Field trials compiled in the review show the material works: 22.5 tonnes of rice straw biochar per hectare cut methane emissions by 47.3 to 86.4% compared with fields receiving 6 tonnes of raw straw, while 2.5% biochar by soil weight reduced emissions by as much as 91.2% in one study. Rice husk biochar at 2% and 4% by soil weight reduced methane by 45.2 and 54.9% respectively. The mechanism is twofold: biochar suppresses methanogenic activity while boosting methanotrophic proteobacteria that consume methane, and its recalcitrant carbon stores roughly 50% of the biomass carbon in the soil for years, sequestering far more than compost, which retains only 10 to 20% of biomass carbon after 5 to 10 years. Nitrous oxide falls too, because biochar inhibits nitrification and denitrification and immobilises available nitrogen, with reductions of up to 73.1% reported at high application rates.

Scaling that promise to a national policy, however, collides with hard arithmetic. Indonesia’s paddies produce an estimated 68.31 million tonnes of rice straw and 15.30 million tonnes of rice husk per year. Pyrolyzed at 400 degrees Celsius, the median temperature for good biochar characteristics, that would yield 28.09 million tonnes of rice straw biochar and 6.44 million tonnes of rice husk biochar annually if every gram were converted. But farmers already sell or use 31 to 39% of straw and husk as animal feed, mushroom substrate and ornamental plant media, and another 7 to 16% goes to industry and bioenergy. Only about 60% is realistically available as biochar feedstock, which works out to just 2.78 tonnes of biochar per hectare per year across the national paddy area, far below the 20 to 40 tonnes per hectare that trials suggest is needed for effective emission control.

There is a further irony buried in the pyrolysis reactor itself. Heating biomass to 350 to 700 degrees Celsius releases synthesis gas containing carbon monoxide, hydrogen, carbon dioxide and methane, the last formed by degradation of methoxyl groups in lignin. Models of the pyrolysis system indicate that every kilogram of rice straw biochar produced at 400 degrees Celsius releases roughly 50 grams of methane. Producing the 16.85 million tonnes of straw biochar realistically available would therefore emit about 0.84 million tonnes of methane, with husk biochar adding another 0.06 million tonnes, for a total of roughly 0.9 million tonnes. Set against the estimated 3.08 million tonnes of methane that Indonesian paddies emit per year, biochar manufacturing would add nearly 30% of the annual paddy total before a single field benefit is secured, and the size of that benefit varies widely across the trial record.

The review’s authors argue the case is not closed. Biochar’s extraordinary stability against physical aging, chemical oxidation and microbial degradation means applications persist in the soil for years, and successive annual applications have proven better at reducing emissions than high one-off doses. That persistence could eventually close the gap between the 2.78 tonnes per hectare available from rice waste alone and the 20 tonnes or more that effective control demands. Farmers and policymakers could also widen the feedstock base to wood, manure, plant litter, urban waste and other lignocellulose-rich materials, while pushing pyrolysis technology toward low or zero-emission designs. Until then, the review stands as a sobering reality check: the climate solution growing in Indonesia’s rice fields is real, but it is not yet big enough to dig the country’s paddies out of their methane debt.

Subject of Research: Greenhouse gas emissions from Indonesian flooded paddy fields and the mitigation potential of rice biomass biochar

Article Title: CH 4 and N 2 O emissions and their potential control by rice biomass biochar: The case of continuously flooded paddy fields in Indonesia – A review

Article References: Hafif, B., Kartika, K., Randriani, E., Pitono, J., Gusmaini, G., Ridwan, R., & Nurida, N. (2024). CH4 and N2O emissions and their potential control by rice biomass biochar: The case of continuously flooded paddy fields in Indonesia – A review. One Ecosystem, 9, Article e109657. https://doi.org/10.3897/oneeco.9.e109657

Image Credits: AI Generated

DOI: 10.3897/oneeco.9.e109657

Keywords: emissions, potential, control, rice, biomass, biochar, case, continuously, flooded, paddy, fields, Indonesia