Abstract
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.
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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.
Keywords
• Biochar;
• Carbon sequestration;
• Carbon leaching;
• Crop residues;
• Mean residence time;
• Rice
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1. Introduction
In 2008, total rice residue production in Asia could be roughly estimated at 623 million t of rice straw and about 125 million t of rice husks, based on the 2008 paddy production (FAOSTAT online database), 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 even have negative environmental consequences. In the past five decades, increasing opportunity costs of organic fertilizer use, relatively cheap inorganic fertilizer, and shortened fallow periods resulting from cropping intensification caused a continuous decline in the recycling of crop residues (Pandey, 1999). In intensive systems, where 2–3 crops are grown each year, the time for residue incorporation and decomposition is very short. Un-decomposed remains and decomposition products often disrupt soil preparation, crop establishment, and early crop growth. Therefore, residue burning is still widely practiced (although officially banned in most countries), contributing to air pollution, human health problems, and substantial nutrient losses (Tipayarom and Kim Oanh, 2007 and Gustafsson et al., 2009). In addition, field burning of residues generates considerable amounts of methane (Miura and Kanno, 1997), which is a potent greenhouse gas contributing to global climate change. And, only recently, several studies indicated a large contribution to global warming from soot particles in the atmosphere, which are due to biomass burning (Gustafsson et al., 2009 and Jacobson, 2010). Residue incorporation into the soil as a recommended alternative practice reduces atmospheric pollution but would further increase methane emissions from rice soils (Yan et al., 2003, Bossio et al., 1999 and Knoblauch et al., 2010), which are estimated to contribute 9–19% of global methane emissions (Denman et al., 2007).
An opportunity to examine these problems in a completely new way originates from the research on Terra Preta soils, which are characterized by high contents of biochar (carbonized organic matter, black carbon) due to the application of charcoal by Amerindian populations 500–2500 years ago (Sombroek, 1966). These soils 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 biochar (Lehmann et al., 2003a). The high stability of biochar 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). Positive yield as well as biomass responses resulting from charcoal applications were reported repeatedly and attributed to direct nutrient additions, higher nutrient retention and availability, increased cation exchange capacity, improved soil physical characteristics, and positive effects on soil microorganisms ( Lehmann and Rondon, 2006 and Glaser et al., 2002).
But, biochar from wood is not a feasible and sustainable option in most farming areas of Asia. However, biochar can be produced by incomplete combustion from any biomass and is a by-product of modern technologies for bioenergy production such as gasification and pyrolysis. Therefore, rice residues could be used to produce energy, and the biochar by-product could serve to recycle nutrients and maintain or even improve soil fertility. The supposedly high stability of carbonized residues could help to reduce greenhouse gas emissions from rice-based systems and sequester carbon in rice soils. This coupling of bioenergy production and the use of biochar in rice production systems would offer several important advantages:
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Rice residues are a by-product of food production. Therefore, bioenergy based on rice residues does not impair food security; higher food production increases bio-energy output simultaneously.
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The complete removal of residues from the field leads to a soil organic matter and soil quality decrease in most agricultural systems. However, studies in flooded rice-based systems have shown that soil quality is maintained over decades even if all residues are removed (Dawe et al., 2003 and Pampolino et al., 2008).
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Residue removal from rice fields for energy production directly reduces emissions of greenhouse gases and air pollution caused by residue incorporation or field burning.
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The high cropping intensity especially in irrigated rice systems ensures a more constant residue supply for bioenergy production and reduces the necessary transport distances to preferably medium-sized bioenergy plants.
However, most studies on biochar as a soil amendment concentrated on extensive production systems, on crops other than rice, and on biochar made from wood. Therefore, our study intended to (i) examine the effect of biochar from rice husks on soil characteristics in a range of different rice soils, (ii) investigate the stability of carbonized rice residues in the soil under the special conditions of different rice-based systems, and (iii) test the agronomic effect of applications of carbonized rice residues.