| The field experiment was carried out at the Tao Yuan Agro-ecological Experimental Station (28°55'N,110°30'E), in Hunan Province, China. The paddy soil is classified as stagnic anthrosols developed from Quaternary red earth. These long-term experiments have been ongoing since 1990, with six different fertilizing systems including balanced mineral fertilizers or combined organic/mineral fertilizers as following:nutrient K deficit (NP), nutrient P deficit (NK), balanced mineral nutrients (NPK), and combined organic/mineral fertilizers at full rate (FOM) or at reduced rate (ROM) and no fertilizer as a control (CK). CH4 and N2O emission were investigated through the static-chamber and a modified gas chromatograph method, to study the pattern of CH4 and N2O emission and its influence factors in double rice cropping systems, This paper discussed the effect of environmental and crop factors on CH4 and N2O emission and estimate contribution of methane emission on GWP under long-term different fertilizing systems. The results showed that:1. Paddy rice field is the important source of CH4 and N2O. Under the long-term different fertilizing systems, CH4 emission from double-crop rice fields exhibited similar seasonal variation. At the early rice season, methane emission increased after transplanting. The highest CH4 emission peak mainly appears in the 35 Day after transplanting, during the period of tillering. At the late rice eason, methane emission had only one peak and reached the highest fluxes one week after transplanting. N2O fluxes during the early rice season were lower because of continued flooding, with a flux peak occasionally. While during the late rice season, N2O flux were promoted by drainge and field drying, especially at later growth stage. The CH4 and N2O flux from rice field is crossed under water management. During the flooding CH4 flux is in peak, but N2O flux is very small. When the rice field is in drainage N2O flux is in peak, but at the same time CH4 flux is very small.2. The average CH4 fluxes ranged between 8.5 and 19.4 mg m-2 h-1 for the early rice season and between 17.2 and 43.8 mg m-2 h-1 for the late rice season. Out of the total emissions,67-69% from the double-cropping system occurred during the late rice season. Cumulative emissions of CH4 from two rice crops ranged from 140.6 to 861.7 Kg/ha, ranking as following: FOM> ROM> NPK> NK> NP> CK. N2O weighted average and accumulated fluxed in organic-inorganic fertilizing system(FOM) were constantly the highest during the double rice growth period. In NP and NK treatments, N2O fluxes were higher than in NPK treatment. Under the condition of our studies, rice straw incorporation definitely enhanced methane and N2O emissions and balance fertilization significantly controlled N2O emission.3. The environmental factors of long-term different fertilizer systems affecting CH4 and N2O emission from double-crop rice fields were discussed, including water depth, soil Eh, soil pH and 5cm soil temperature. The CH4 flux from early rice season don't correlation with water depth, soil Eh, soil pH. There was significantly positive correlation between CH4 emission flux and 5 cm soil temperature. The results showed that when water depth was in a level suitable for CH4 production, water depth has no significant effect on seasonal of CH4 emission. Soil temperature was a mainly control factor. But for N2O emission during the early rice season, there was no relationship between N2O emission and soil factors. methane fluxes from late rice season were significantly positive and negative related with water depth, the soil pH,5cm soil temperature and soil Eh, N2O emission was significantly positive and negative related with water depth, the soil pH,5cm soil temperature and soil Eh in FOM,ROM,NPK treatments.4. Seasonal CH4 accumulation emission rates during the early rice or late rice season were all significantly correlated with soil organic matter content, soil total N, plant height and rice biomass product, the N2O flux from rice field don't correlation with rice height and rice biomass, only at the later rice paddy field, the N2O accumulation emission were significantly positive related with the soil organic matter content and total N.5. The biomass and grain yields of long-term different fertilizer systems affecting methane and nitro oxide emission from double-cropp rice fields were discussed, The results showed that, rice straw was incorporated into the soil in situ at full and half rates produced obviously higher grains during the early rice season without obvious effect on grain yields for the late rice season while methane emissions were much higher. Therefore, it is not recommendable to return rice straw before rice transplanting6. Based on total emission of CH4 and N2O over the double-crop rice fields, the estimation of combined GWPs for CH4 and N2O shows that over a 20 years horizon or a 500 years horizon, the value of the GWPs per unit crop grain yield in different fertilization treatments over a 20 years horizon was ranked in the order of FOM>ROM>CK>NK>NP > NPK, over a 500 years horizon it was CK> NK= FOM>ROM>NP> NPK. Compared to the chemical fertilizer treatment, Organic fertilizer (rice straw or green fertilizer) application significantly increased climatic impacts from CH4 and N2O emissions in the double-crop rice fields. Synthetical analyse the GWPs per unit crop grain yield of different fertilization, the treat in NPK is more scientific choice at this study condition. |
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