Black Carbon (Biochar) in Rice-Based Systems: Characteristics and Opportunities

The total amount of crop residues produced each year in rice-based systems of Asia can be roughly estimated at about 560 million tons of rice straw and about 112 million tons of rice husks (based on 2005 production, a harvest index of 0.5, and a husk/paddy ratio of 0.2). These residues constitute a valuable resource, but actual residue management practices do not use their potential adequately and often cause negative environmental consequences. In the past decades, increasing opportunity costs of organic fertilizer use and shortened fallow periods due to cropping intensification caused a continuous decline in the recycling of crop residues (Pandey 1998). Residue burning is widely practiced and causes air pollution, human health problems, and considerable nutrient losses. The declining return of organic materials to soils does not seem to affect soil quality in mostly anaerobic systems (rice-rice) with good soils but residue recycling is important to maintain soil fertility on poor lowland soils, in mixed cropping systems (rice-upland crop), and in upland systems (Dawe et al. 2003; Ladha et al. 2003; Tirol-Padre and Ladha 2006). Global climate change raises further questions about rice residue management. Decomposition of organic matter in flooded rice is always related to emissions of methane, which is about 22 times more radiatively active than CO2, and rice-based systems are estimated to contribute 9% to 19% of global methane emissions (Denman et al. 2007). In addition, the rapidly increasing interest in renewable energy sources adds new options and consequences for rice residue management and rice-based systems.

An opportunity to address these issues in a completely new way arises from research on anthropogenic soils in the Amazonian region called terra preta de índio (Sombroek 1966). These soils are characterized by high contents of black carbon (carbonized organic matter, biochar) most probably due to the application of charcoal by Amerindian populations 500 to 2,500 years ago. They are also distinguished by a surprisingly high and stable soil fertility contrasting distinctively with the low fertility of the adjacent acid and highly weathered soils, which was at least partially attributed to their high content of black carbon (Lehmann et al. 2003). The high stability of black carbon in soils and its beneficial effect on soil fertility led to the idea that this technology could be used to actively improve poor soils in the humid tropics (Glaser et al. 2001; Lehmann and Rondon 2006). However, most studies in this context concentrated on extensive production systems, on crops other than rice, and on wood as the source of black carbon. But black carbon can be produced by incomplete combustion from any biomass and it is a by-product of modern technologies for bioenergy production (pyrolysis).