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The DNDC (DeNitrification-DeComposition)-Rice model, one of the most advanced process-based models for the estimation of greenhouse gas emissions from paddy fields, has been discussed mostly in terms of the reproducibility of observed methane (CH 4 ) emissions from Japanese rice paddies, but the model has not yet been validated for tropical rice paddies under alternate wetting and drying (AWD) irrigation management, a water-saving technique. We validated the model by using CH 4 and nitrous oxide (N 2 O) flux data from rice in pots cultivated under AWD irrigation management in a screen-house at the International Rice Research Institute (Los Bañ os, the Philippines). After minor modification and adjustment of the model to the experimental irrigation conditions, we calculated grain yield and straw production. The observed mean daily CH 4 fluxes from the continuous flooding (CF) and AWD pots were 4.49 and 1.22 kg C ha À1 day À1 , respectively, and the observed mean daily N 2 O fluxes from the pots were 0.105 and 34.1 g N ha À1 day À1, respectively. The root-mean-square errors, indicators of simulation error, of daily CH 4 fluxes from CF and AWD pots were calculated as 1.76 and 1.86 kg C ha À1 day À1, respectively, and those of daily N 2 O fluxes were 2.23 and 124 g N ha À1 day À1, respectively. The simulated gross CH 4 emissions for CF and AWD from the puddling stage (2 days before transplanting) to harvest (97 days after transplanting) were 417 and 126 kg C ha À1 , respectively; these values were 9.8% lower and 0.76% higher, respectively, than the observed values. The simulated gross N 2 O emissions during the same period were 0.0279 and 1.45 kg N ha À1 for CF and AWD, respectively; these values were respectively 87% and 29% lower than the observed values. The observed total global warming potential (GWP) of AWD resulting from the CH 4 and N 2 O emissions was approximately one-third of that in the CF treatment. The simulated GWPs of both CF and AWD were close to the observed values despite the discrepancy in N 2 O emissions, because N 2 O emissions contributed much less than CH 4 emissions to the total GWP. These results suggest that the DNDC-Rice model can be used to estimate CH 4 emission and total GWP from tropical paddy fields under both CF and AWD conditions.

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The Mekong Delta produces 21 Mt of rough rice (Oryza sativa L.) and an estimated 24 Mt of straw (dry weight) annually. Approximately one fourth of the straw is burned on the field, which is a common practice in intensive rice cultivation systems in this region because there is limited time to prepare the field for the next crop. The spread of intensive rice production in the Delta may increase the total biomass of burning crop residues, significantly impacting greenhouse gas (GHG) emissions in Vietnam. In this study, GHG emissions from the major uses of straw (burning and mushroom beds) were monitored in a triple rice cropping system located in the central Mekong Delta. Between September 2011 and November 2012, both wind tunnel and closed chamber methods were used to measure the emissions of major GHGs from straw-burning and straw-mushroom cultivation systems, respectively. The global warming potential (GWP) was then determined. Methane (CH 4 ) and non-methane volatile organic carbon emissions (NMVOC) increased with lower modified combustion efficiency [MCE: emissions ratio of Carbon composing carbon dioxide (CO 2 -C) and carbon monooxide (CO-C) (CO 2 -C/(CO-C + CO 2 -C))]. Furthermore, higher moisture straw stacks generated lower nitrous oxide (N 2 O) emissions. Small straw stacks (5 or 10 kg dry straw) with higher moisture content emitted more carbon monoxide (CO), CH 4 and NMVOC. These results suggest that factors that increase the straw moisture content, such as rainfall, can cause smoldering combustion in small straw stacks or when straw is scattered on the ground, thereby inhibiting N 2 O emissions but enhancing CO, CH 4 and NMVOC. The measured N 2 O emissions contributed negligible amounts to the GWP compared with measured CO and CH 4 , which are relatively intense GHG emissions; this was likely a result of the slow and inefficient burning that was observed from the smaller straw stacks with higher moisture content. In this study, rice straw burning threatened to generate more GHGs than straw-mushroom (Volvariella volvacea (Bul. ex Fr.) Singer) cultivation under the studied agroecosystems.

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Temporal changes in nitrogen concentrations and stream discharge, as well as sediment and nitrogen losses from erosion plots with different land uses, were studied in an agricultural watershed in the Taihu Lake area in eastern China. The highest overland runoff loads and nitrogen losses were measured under the upland at a convergent footslope. Much higher runoff, sediment and nitrogen losses were observed under upland cropping and vegetable fields than that under chestnut orchard and bamboo forest. Sediment associated nitrogen losses accounted for 8-43.5% of total nitrogen export via overland runoff. N lost in dissolved inorganic nitrogen forms (NO(3-)-N + NH4+-N) accounted for less than 50% of total water associated nitrogen export. Agricultural practices and weather-driven fluctuation in discharge were main reasons for the temporal variations in nutrient losses via stream discharge. Significant correlation between the total nitrogen concentration and stream discharge load was observed. Simple regression models could give satisfactory results for prediction of the total nitrogen concentrations in stream water and can be used for better quantifying nitrogen losses from arable land. Nitrogen losses from the studied watershed via stream discharge during rice season in the year 2002 were estimated to be 10.5 kg N/ha using these simple models.

Field evaluations of slow-release N fertilizers are needed to improve N use effi ciency while reducing NH 3 volatilization. Ammonia volatilization losses, fl oodwater NH 4 + , and aboveground rice (Oryza sativa L.) tissue N accumulation were compared between non-coated urea (NCU) and polyolefi n-coated urea (POCU) applications in a double-rice production system of subtropical China. A factorial treatment design included two urea sources such as POCU and NCU applied at 75 and 150 kg N ha -1 , and a no-N control. For early and late rice seasons, fl oodwater NH 4 + and pH increased signifi cantly within 7 to 10 d aft er NCU applications and then decreased rapidly. Nitrogen losses through NH 3 volatilization accounted for 16 to 30% and 4 to 8% of the total N applied for NCU and POCU, respectively. Applications of POCU signifi cantly increased N accumulation in aboveground rice biomass due to a slower N release and better synchrony between in-season rice N demand and N supply from this fertilizer. Applications of POCU at a rate of 75 kg N ha -1 provided the optimal amounts of N for early and late rice crops and reduced NH 3 volatilization losses. Th e use of slow release urea fertilizers should increase N use effi ciency and maintain the high grain yield in the double rice production systems of subtropical China.