Effects and fate of biochar from rice residues in rice-based systems

Although crop residues constitute an enormous resource, actual residue management practices in rice-based systems have various negative side effects and contribute to global warming. The concept of a combined bioenergy/biochar system could tackle these problems in a new way. Rice residues would be used for energy production, thereby reducing field burning and the use of fossil fuels, and the biochar by-product could help to improve soils, avoid methane emissions, and sequester carbon in soils. To examine some of these promises, we conducted field experiments from 2005 to 2008 in three different rice production systems. Objectives were to study the effect of biochar from rice husks on soil characteristics, assess the stability of carbonized rice residues in these different systems, and evaluate the agronomic effect of biochar applications. The results showed that application of untreated and carbonized rice husks (RH and CRH) increased total organic carbon, total soil N, the C/N ratio, and available P and K. Not significant or small effects were observed for soil reaction, exchangeable Ca, Mg, Na, and the CEC. On a fertile soil, the high C/N ratio of CRH seemed to have limited N availability, thereby slightly reducing grain yields in the first three seasons after application. On a poor soil, where the crop also suffered from water stress, soil chemical and physical improvements increased yields by 16–35%. Together with a parallel study including methane and CO2 emission measurements at one site, the results strongly suggest that CRH is very stable in various rice soils and systems, possibly for thousands of years. However, the study also showed that CRH was very mobile in some soils. Especially in poor sandy soil, about half of the applied carbon seemed to have moved below 0.30 m in the soil profile within 4 years after application. We concluded that biochar from rice residues can be beneficial in rice-based systems but that actual effects on soil fertility, grain yield, and soil organic carbon will depend on site-specific conditions. Long-term studies on biochar in field trials seem essential to better understand biochar effects and to investigate its behavior in soils.

Research highlights
► Objectives were to study the effect of biochar from rice husks on soil characteristics, assess the stability of carbonized rice residues in different systems, and evaluate the agronomic effect of biochar applications.
► Our results indicate that carbonized rice husks are stable in various rice soils and environments, probably for thousands of years.
► On a poor soil, where the crop also suffered from water stress, application of carbonized rice husks increased yields by 16% to 35% over the control.
► Carbonized rice husk was very mobile in some soils, and about half of the applied carbon moved below 0.30 m within 4 years after application in a poor, sandy soil.
► Biochar technology is promising for rice-based systems but actual effects will depend on site-specific conditions.