Decomposition of rice straw and microbial carbon use efficiency under different soil temperatures and moistures
Introduction
Residue management impacts on soil organic matter (SOM) and long-term fertility is becoming more relevant in the context of soil quality. Open-field burning has been used traditionally to dispose of crop residues and sanitize agricultural fields against pests and diseases (Ponnamperuma, 1983). Because of air pollution concerns, thermal straw management is now being reconsidered in many regions of the world (Ocio et al., 1991, Miura and Kanno, 1997). In rice cropping systems, the restriction on open-field burning can result in 7–12 tons of residues per hectare being left on soils (Kludze and Delaune, 1995). The rice residues can impede seedbed preparation and contribute to disease and weed problems. There are currently few options for rice straw because of its poor quality for forage, bioconversion, and engineering applications (Jenkins et al., 1997). Rice growers are therefore seeking alternative disposal options, such as incorporation of the straw into the soil. Besides introducing an extra cost, rice straw incorporation in association with flooding likely impacts soil fertility through nutrient and pest interactions (Cassman et al., 1995, Cassman et al., 1997, Olk et al., 1996) and environmental quality through greenhouse gas emissions (Delwiche and Cicerone, 1993, Bossio et al., 1999).
The incorporation of rice residues and continuous flooding has become common in tropical areas through intensification of rice cropping practices (Cassman and Pingali, 1995). In CA, a large rice producing region in the temperate zone, straw incorporation combined with winter flooding has been promoted to enhance straw decomposition and waterfowl habitat (Elphick and Oring, 1998). Under these alternative rice residues management practices, rice growers must rely on decomposition of residues under extended flooding to dispose of the straw. Previous studies have shown that the incorporation of rice straw can negatively affect rice yield through N immobilization (Rao and Mikkelsen, 1976) or N availability (Cassman et al., 1997). Other studies have shown positive residual effects on rice yield after straw incorporation (Cassman et al., 1996). However, a dearth of information exists regarding straw decomposition and C use efficiency by soil microorganisms under prolonged flooded conditions and its effect on long-term N availability (Mikkelsen, 1986, Cassman et al., 1996).
Among other controversial effects of soil flooding, the addition of plant residues to highly reduced soils is known to enhance methane emission (CH4) (Cicerone et al., 1992). Dickinson and Cicerone (1986) listed rice paddies as the most important anthropogenic source of CH4. Up to 20% of CH4 emitted annually is attributed to flooded rice fields (IPCC, 1992). Bossio et al. (1999) found a five-fold increase in CH4 emission in straw incorporated plots compared to burned plots in CA rice paddy soil. Other studies have found similar increases in rice soils where straw incorporation is practiced (Yagi and Minami, 1990, Sass et al., 1991, Denier van der Gon and Neue, 1995, Wassmann et al., 1996). Population pressures will cause further increases in rice production leading to an increase in atmospheric CH4 if current rice farming practices are continued (Neue et al., 1996). Incorporation of rice residues would be expected to aggravate this trend.
Microbial processes primarily drive changes in N availability and CH4 emissions. The magnitude of microbial processes is generally constrained by C availability in arable soils (Paul and Clark, 1996). For these reasons, the objectives of the present study were to determine the effect of flooding and temperature on microbial C use efficiency and rice straw decomposition. We examined: (a) C mineralization and CH4 emission; (b) effect of C:N ratio, flooding and temperature on degradation of cellulose and lignin; and (c) straw-C use efficiency by soil microorganisms.
Section snippets
Soil and rice straw
The soil was collected from an ongoing rice straw residue management trial located at Maxwell, CA, on Willows clay (fine, montmorillonitic, thermic, Typic Pelloxerert). The soil is 51% clay, 5% sand, and 44% silt with a pH (water) of 6.6, a total C content of 20.65 g kg−1, a total N content of 1.95 g kg−1, and a CEC of 42 meq 100 g−1.
The soil (0–15 cm) was sampled in the fall after open-field burning of rice residues. The gravimetric soil water content was determined after drying at 105°C for
Results and discussion
The addition of straw to rice paddy soil and its decomposition under different temperatures and flooding regimes showed wide differences in C mineralization, CH4 emission, microbial biomass-C, cellulose and lignin degradation, and C utilization efficiency. These changes show the extensive effect that soil management and climate factors of temperature and moisture have on the processes that affect rice straw decomposition.
Conclusion
It has been widely accepted that under aerobic conditions the level of waste products from microbial activity is usually not high, while under anaerobic conditions products from fermentative metabolism tend to accumulate. For this reason, there is a perception that decomposition processes under reduced conditions are slower and more incomplete compared to aerobic conditions. However, this observation comes mainly from short-term studies rather than long-term studies as shown here in this study
Acknowledgements
The authors are grateful to Dr. D. Harris (UC Davis Stable Isotope Facility) for advice, and to Dr. C. van Kessel, Professor in Agronomy and Range Science at UC Davis, for reviewing the manuscript. We would also like to thank Susan Lo for her valuable technical assistance, and Timothy Doane for his helpful comments. The California Energy Commission (Grant No.), College of Agricultural and Environmental Science of the University of California at Davis and the California Rice Research Board
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