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Two-year field experiments were conducted at Tamil Nadu Rice Research Institute, Aduthurai, Tamil Nadu, India, to evaluate the effect of continuous flooding (CF) and alternate wetting and drying (AWD) irrigation strategies on rice grain yield and greenhouse gas emissions from double-cropping paddy rice. Field observation results showed that AWD irrigation was found to reduce the total seasonal methane (CH4) emission by 22.3% to 56.2% compared with CF while maintaining rice yield. By using the observed two-year field data, validation of the DNDC-Rice model was conducted for CF and AWD practices. The model overestimated rice grain yield by 24% and 29% in CF and AWD, respectively, averaged over the rice-growing seasons compared to observed values. The simulated seasonal CH4 emissions for CF were 6.4% lower and 4.2% higher than observed values and for AWD were 9.3% and 12.7% lower in the summer and monsoon season, respectively. The relative deviation of simulated seasonal nitrous oxide (N2O) emissions from observed emissions in CF were 27% and −35% and in AWD were 267% and 234% in the summer and monsoon season, respectively. Although the DNDC-Rice model reasonably estimated the total CH4 emission in CF and reproduced the mitigation effect of AWD treatment on CH4 emissions well, the model did not adequately predict the total N2O emission under water-saving irrigation. In terms of global warming potential (GWP), nevertheless there was a good agreement between the simulated and observed values for both CF and AWD irrigations due to smaller contributions of N2O to the GWP compared with that of CH4. This study showed that the DNDC-Rice model could be used for the estimation of CH4 emissions, the primary source of GWP from double-cropping paddy rice under different water management conditions in the tropical regions.

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A field experiment was conducted at the Main Research Station, University of Agricultural Sciences, Dharwad, India, on medium black soils during the kharif (wet) season of 1999. The experiment was laid out in a randomized complete block design with varying N/P ratios (0.67 to 2.00) along with a control with a constant level of potassium (60 kg ha -1). The results revealed that the number of green leaves plant -1 , the dry matter accumulation in the leaves, leaf area (dm 2 plant -1 ) and leaf area index (LAI) increased up to the flowering stage (65 DAS) and thereafter declined. In the early stages (seedling and button stages) there was no significant variation with respect to the number of green leaves plant -1 among the treatments except in the control. Similarly, leaf area and LAI did not vary at the seedling stage. Treatments receiving N/P ratios of >1.0 or 1.0 with higher doses of nitrogen (120 kg N ha -1 ) gave a significantly higher number of green leaves plant -1 , leaf area and LAI as compared to N/P ratios of <1.0 and the control in later stages. The dry matter accumulation in the leaves (g plant -1 ) differed in all the stages, but higher values were recorded in these same treatments. Thus, due to the higher number of green leaves, higher LAI and greater dry matter accumulation in the leaves, the treatments with an N/P ratio of >1.0 or 1.0 with 120 kg N ha -1 produced higher seed yields (3188 to 3554 kg ha -1 ) than other N/P ratios (2761 to 3009 kg ha -1 ). The highest yield (3554 kg ha -1 ) was recorded with an N/P ratio of 1.0 in the treatment receiving 120 kg N and 120 kg P 2 O 5 ha -1 . The correlation coefficients between these photosynthetic attributes and seed yield were also positive and significant.