Yasukazu Hosen

Validation of the DNDC-Rice model by using CH4 and N2O flux data from rice cultivated in pots under alternate wetting and drying irrigation management

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 (CH4) 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 CH4 and nitrous oxide (N2O) flux data from rice in pots cultivated under AWD irrigation management in a screen-house at the International Rice Research Institute (Los Baños, 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 CH4 fluxes from the continuous flooding (CF) and AWD pots were 4.49 and 1.22 kg C ha−1 day−1, respectively, and the observed mean daily N2O fluxes from the pots were 0.105 and 34.1 g N ha−1 day−1, respectively. The root-mean-square errors, indicators of simulation error, of daily CH4 fluxes from CF and AWD pots were calculated as 1.76 and 1.86 kg C ha−1 day−1, respectively, and those of daily N2O fluxes were 2.23 and 124 g N ha−1 day−1, respectively. The simulated gross CH4 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−1, respectively; these values were 9.8% lower and 0.76% higher, respectively, than the observed values. The simulated gross N2O emissions during the same period were 0.0279 and 1.45 kg N ha−1 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 CH4 and N2O 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 N2O emissions, because N2O emissions contributed much less than CH4 emissions to the total GWP. These results suggest that the DNDC-Rice model can be used to estimate CH4 emission and total GWP from tropical paddy fields under both CF and AWD conditions.

Nitrogen oxide emissions and soil microbial activities in a Japanese andisol as affected by N-fertilizer management

Abstract This experiment was conducted in maize field plots to study the effects of N-fertilizer management on nitrogen oxide emissions, soil microbial activities and crop yield, and the relationships among them in a Japanese Andisol. N fertilization increased both nitrification and dehydrogenase activities in soil and the emissions of NO and N20. Nitrification and dehydrogenase activities showed a close relationship with the NO emission, while the relationship with the N20 emission was not significant. Deep application (8 cm) of N fertilizer generally decreased both nitrification activity and NO emission. Controlled release of N fertilizer decreased the dehydrogenase activity and up to 86% of the NO emission. Our findings indicate the possibility that the low biological NO production in soil by deep application and controlled release of N fertilizer contributed to the low NO emission. Application of controlled release N fertilizer significantly increased the grain yield and N uptake by grain. NO emission showed a negative relationship with maize yield as affected by N-fertilizer management, suggesting the possibility of mitigating NO emission from Andisols without reducing crop production.

Greenhouse gas emissions from rice straw burning and straw-mushroom cultivation in a triple rice cropping system in the Mekong Delta

Abstract 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 (CH4) and non-methane volatile organic carbon emissions (NMVOC) increased with lower modified combustion efficiency [MCE: emissions ratio of Carbon composing carbon dioxide (CO2-C) and carbon monooxide (CO-C) (CO2-C/(CO-C + CO2-C))]. Furthermore, higher moisture straw stacks generated lower nitrous oxide (N2O) emissions. Small straw stacks (5 or 10 kg dry straw) with higher moisture content emitted more carbon monoxide (CO), CH4 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 N2O emissions but enhancing CO, CH4 and NMVOC. The measured N2O emissions contributed negligible amounts to the GWP compared with measured CO and CH4, 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.

Do abiotic factors cause a gradual yield decline under continuous aerobic rice cultivation? A pot experiment with affected field soils

Abstract Aerobic rice is a water-saving technology in which rice grows in non-puddled and non-saturated (aerobic) soil without ponded water. A gradual decline in rice yield was found in field plots at the farm of the International Rice Research Institute, Los Baños, Philippines, where rice has been cultivated continuously for 10 cropping seasons under aerobic rice conditions. We investigated whether abiotic soil factors lead to the observed yield decline. An aerobic rice pot experiment was conducted using field soils from flooded rice plots and from the 10-season-long aerobic rice cultivated plots (referred to as 1st-season and 11th-season aerobic rice, respectively). Subtreatments consisted of soil sterilization by oven heating (at 95°C or higher for 24 h) and a control treatment. The above-ground biomass of 1st-season aerobic rice was significantly greater than that of 11th-season aerobic rice in both the oven-heating and control treatments. Oven heating increased soil N availability and above-ground biomass accumulation over the control in both 1st-season and 11th-season aerobic rice, but the above-ground biomass in the oven-heated 11th-season aerobic rice was still significantly lower than that of the oven-heated and even the untreated (control) 1st-season aerobic rice. These results suggest that abiotic factors contribute to the gradual yield decline observed in the field plots.

Effect of urea placement on the time-depth profiles of NO, N2O and mineral nitrogen concentrations in an Andisol during a Chinese cabbage growing season

Abstract To evaluate the effects of urea placement on the concentration profiles of soil gases (NO and N2O) and mineral nitrogen, we conducted an experiment in a Chinese cabbage field in Tsukuba, Japan, over one cultivation season. Soil gas and mineral nitrogen concentrations at different depths (0.05, 0.1, 0.15, 0.2, 0.3, 0.45 and 0.6 m) were measured 1–2 times per week in experimental plots fertilized by either urea incorporation (U-I; uniformly spread over the soil surface and incorporated down to approximately 0.15–0.2 m) or by urea deep band (U-DB; placed in 0.12-m-deep trenches cut at intervals of 0.6 m). Considerable NO was observed in the top 0.2 m of soil in the U-I treatment during the first 2 weeks after fertilization (WAF), whereas in soil in the U-DB treatment NO was observed only at depths of 0.1 and 0.15 m for approximately 6 WAF. In U-I, the maximum soil N2O concentration was observed 5 days after fertilization (DAF). Significantly high N2O concentration in soil of U-DB was observed 2 WAF, and lasted longer than that in UI (4 vs 2 weeks). The NH4 + concentrations in U-I exceeded background levels during the first 2 WAF, with a maximum 5 DAF. The high NH4 + concentrations in soil in the U-DB treatment, mainly located within the 0.05–0.15-m soil zone, lasted approximately 6 weeks, with a maximum at 9 DAF. The NO3 − concentrations in both U-I and U-DB increased shortly after the application of urea; the increases were higher in U-I than in U-DB. These time-depth series of concentration profiles clearly demonstrate the effect of urea placement on NO and N2O production and transportation in soil and provide a better understanding of the emission dynamics of these gases than previous reports. In addition, comparison of NO and N2O emissions among field, laboratory and numerical experiments firmly suggests that deep urea placement is highly effective in reducing NO emissions, with less effect on N2O emissions from Andisols.