Saturday, 15 July 2006

Assessing the Quality of Plant Residues and Managing their Breakdown Rate to Enhance the Sustainability of Lowland Rice Cropping Systems.

Gina Villegas Pangga, Farming Systems and Soil Resources Institute, Univ of the Phillippines, Los Baños, Laguna 4031, Philippines and Graeme Blair, Dept of Agronomy and Soil Science, Univ of New England, Armidale, New South Wales, Australia.

The initial benefit of plant residues appears to be related to their quality and decomposition rate and associated nutrient release. Plant residues are vital resource in the replenishment of nutrients in soil and in affecting the sustainability of rice production. A series of perfusion experiments were conducted to determine the breakdown rate of straw from different rice varieties (Oryza sativa L.). The correlation matrix of the initial composition of straw samples and the cumulative C release established the relationships between residue quality parameters and decomposition rates. A Straw Quality Index (SQI) was developed to describe the decomposition rate of the rice straw as follows: SQI = -56.85 + (11.68 X %N) + (1.25 X %DOM) + (2.59 X % lignin) r2 = 0.81 The equation uses important descriptors of plant residue quality such as Nitrogen (N), Digestible Organic Matter (DOM), and lignin concentration. These findings indicate that SQI has a potential in assessing the quality of straw residue materials in predicting their usefulness in crop-residue management systems. A glasshouse pot experiment was conducted to study nutrient availability after rice straws with different qualities were incorporated to an Alfisol sandy loam soil. The effects of soil water condition (flooded and non-flooded) and straw residue treatments on the availability of nutrients in the soil were examined at different sampling times (active tillering, maximum tillering and maturity). There was higher concentration of total C (CT ) and total N (NT) remaining in the soil in straw residues with high SQI than those residues with low SQI. Flooding significantly decreased CT concentration in all straw-residue treatments. Decomposition rate of straw residue was faster under aerobic than anaerobic conditions. The results demonstrated that nutrient availability is dependent on the quality of residues after the straw has been incorporated into the soil. To validate the efficiency of SQI as a tool in assessing the quality of rice straw, a field experiment was conducted on a low-fertility clayey soil (Aquandic Epiaqualf) at Los Baños, Philippines. The SQI was also tested using multi-purpose plant species (Gliricidia sepium and Macaranga tanarius). There was a direct relationship between SQI and rice dry grain yield (r2 = 0.87). The study recognized the value of plant residue quality as an important aspect of low-input sustainable rice production systems. The management systems in this study improved rice grain production, however, there were no significant changes in soil organic matter after 3 rice crops. Management of plant residue quality and quantity demonstrated great potential in enhancing the sustainability of lowland rice cropping systems.

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