Plant root exudates play important roles in the rhizosphere. We tested three media (nutrient solution, deionized water and CaSO4 solution) for three periods of time (2, 4 and 6 h) for collecting root exudates of soil‐grown rice plants. Nutrient culture solution created complications in the analyses of exudates for total organic C (TOC) by the wet digestion method and of organic acids by HPLC due to the interference by its components. Deionized water excluded such interference in analytical analyses but affected the turgor of root cells; roots of four widely different rice cultivars excreted 20 to 60 % more TOC in deionized water than in 0.01 M CaSO4. Furthermore, the proportion of carbohydrates in TOC was also enhanced. Calcium sulfate solution maintained the osmotic environment for root cells and did not interfere in analytical procedures. Collection for 2 h avoided under‐estimation of TOC and its components exuded by rice roots, which occurred during prolonged exposure. By placing plants in 0.01 M CaSO4 for 2 h, root exudates of soil‐grown traditional, tall rice cultivars (Dular, B40 and Intan), high‐yielding dwarf cultivars (IR72, IR52, IR64 and PSBRc 20), new plant type cultivars (IR65598 and IR65600) and a hybrid (Magat) were collected at seedling, panicle initiation, flowering and maturity and characterized for TOC and organic acids. The exudation rates were, in general, lowest at seedling stage, increased until flowering but decreased at maturity. Among organic acids, malic acid showed the highest concentration followed by tartaric, succinic, citric and lactic acids. With advancing plant growth, exudation of organic acids substituted exudation of sugars. Root and shoot biomass were positively correlated with carbon exudation suggesting that it is driven by plant biomass. As root exudates provide substrates for methanogenesis in rice fields, large variations in root exudation by cultivars and at different growth stages could greatly influence CH4 emissions. Therefore, the use of high‐yielding cultivars with lowest root excretions, for example IR65598 and IR65600, would mediate low exudate‐induced CH4 production. The screening of exciting rice cultivars and breeding of new cultivars with low exudation rates could offer an important option for mitigation of CH4 emission from rice agriculture to the atmosphere.

Nutrient release from legume and cereal crop residue is important to N cycling and the success of conservation and sustainable farming systems. Residue type, placement, and degree of incorporation, and soil water regimes largely control availability and loss of soil N and were evaluated in the laboratory. Four residues, i.e., vetch (Vida villosa Roth.), soybean (Glycine max [L.] Meir.), corn (Zea mays L.), and wheat (Triticum aestimm L.) having C/N ratios ranging from 8 to 82 were applied on the soil surface or incorporated in repacked cores of a NicoIIet loam (fine-loamy, mixed, mesic Aquic Hapludoll) and incubated for 17 or 35 d at 60 and 90% water-filled pore space (WFPS) with enriched "N-NO, (76.7%). Denitrification losses from all treatments were negligible at 60% WFPS. At 90% WFPS, total denitrification losses from residue-incorporated soils represented 87 to 127% of the initial soil NO 3 level (80.5 mg N kg-'); losses increased with decreasing residue C/N ratio. Denitrification was greatest during the first 8 d, as was CO 2 evolution. Initial denitrification with surface-placed residues was less than with incorporated residues, but cumulative losses over 35 d did not differ significantly. Substantial N immobilization occurred with incorporated or surface-placed wheat, corn, and soybean residue with wide C/N ratios at 60% WFPS, whereas, with low-C/N-ratio vetch, significant mineralization occurred. After 35 d, 51 and 36% of N in incorporated and surface-placed vetch residue, respectively, was mineralized. Residue C/N ratio was inversely related to initial rates of residue decomposition, and effects of residue type and placement and soil water on denitrification and mineralization were most important during early (8-10 d) decomposition. I NCREASED COSTS and environmental concerns promote research on agricultural production systems that reduce soil erosion and balance synthetic chemical use with renewable biological resources. With conventional tillage, crop residues are incorporated into the soil; in conservation farming (reduced-or ridgetillage management systems), residues are left on the soil surface or are partially incorporated just before

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