| Five different cultivation patterns with different grain yield and nitrogen use efficiency targets were established in the double-rice ecosystem in Hunan province, including NN (no N as control), FP (farmers’ practice), YE (10%～15%higher in yield and15%～20%higher in N use efficiency), HY (30%～40%higher in yield), HE (20%～30%higher in yield and30%～50%higher in N use efficiency). Greenhouse gases (CH4, N2O, CO2) from rice field under different cultivation patterns were studied in situ by using static-chamber/gas chromatography. The net ecosystem carbon budget (NECB),δSOC and Eo, Ei were simultaneously estimated to give an overall evaluation of net global warmming potential (Net GWP) as well as greenhouse gas intensity (GHGI) for each cultivation pattern. Result showed that:1. Annual CH4cumulative emissions for each cultivation pattern ranged from577.0±31.4kg hm-2yr-1(FP) to896.9±77.9kg hm-2yr-1(HE). The dynamics of CH4fluxes turned out to be similar, which mainly occoured in the rice grown season. The CH4emissons of late rice season were relatively higher than that of early rice season. Though, the CH4fluxes during the fallow period were extremely lower than during the rice grown season, its cumulative amounts can not be ignored, which accounted for9.7%～19.1%in a total year for each pattern. Different cultivation patterns significant affected CH4emissions (p<0.05). CH4emissions were efficiently suppressed by field drying, while distinctly stimulated by straw return. The cumulative CH4emissions of HE reached490.5±65.7kg hm-2,62%～87%significantly higer than other patterns during the late rice season (p<0.05). Besides, inorganic fertilizer displayed a certain negative role in CH4emissions in this study. Compared to NN, FP and YE decreased CH4emissions by19%and5%, respectively. Rapeseed cake (HY) did not show a significant impact on CH4emissions.2. Annual N2O cumulative emissions for each cultivation pattern ranged from0.29±0.11(NN) to0.88±0.29(HY) kg N hm-2yr-1. N2O emissions were neglectful under waterflooding rice field, except for some separately sharp emission peaks. The fallow period in double-rice cropping ecosystem was an important source of N2O emissions, during which contributed about42%～62%throughout an entire year in this study for each pattern. The accumulation of organic carbon and nitrogen due to rice growing created proper conditions for emiting N2O, which led to the accordance between N2O emissions with rice yield. Overall, the factors of N2O were very low, just0.1%and0.14%for FP, YE,0.16%and0.15%for HY and HE, respectively. The N2O bursted in the early stage of early rice, yet peaked during the flooding period after field drying in late rice. The different cultivation patterns significantly affected N2O emssions (p<0.05). Compared to NN, patterns incorporated with N fertilizer increased N2O emissions by114.6%～193.5%(p<0.05). Besides, the correlations between N2O emissions and total N fertilizer were significant during rice grown season (p<0.05).3. The rice yield of different cultivation patterns ranged from3.37±0.15(NN) to7.52±0.46(HY) t hm-2in the eary rice, and5.5±0.12(NN) to9.13±0.98(HY) t hm-2in the late rice. The different rice crop seasons and cultivation patterns both had significant impact on rice yield (p<0.05). Also, the rice yields in late rice season were relatively higher than in the early rice for each cultivation pattern. The application of N fertilizer significantly improved the rice yield (p<0.05). Secondly, reasonable fertilization and appropriate field management also had a great influence on rice yield. In this study, YE, HY and HE increased rice yield obviously as well as N fertilizer agronomic efficiency (p<0.05). Compared to FP, which had the lowest N fertilizer agronomic efficiency16.7±1.5kg grain kg-1N in the early rice and6.3±2.3kg grain kg-1N in the late rice, YE, HY, HE increased by88.6%,37.1%,58.1%, and150%,200%,200%, respectively.4. The dynamics of Re during rice grown season followed the trends of crop growing. While, in non-rice grown season, the Re was very low. The estimated NECB and δSOC of different cultivation patterns all demonstrated positive values. The correlations between NECB and rice yield were significant (p<0.05). Straw return could apparently enhance the effect of carbon sequestration by cropland soil. The NECB of HE with straw return in the late rice season was as large as2699.4±350kg C hm-2,176.2%,139.5%,73.9%and45.7%(p<0.05) higher than FP, YE, HY and HE, respectively.The annual δSOC of different cultivation patterns were0.003±0.068t C hm-2yr-1(NN),0.19±0.044t C hm-2yr-1(FP),0.28±0.056t C hm-2yr-1(YE),0.41±0.131t C hm-2yr-1(HY),0.53±0.1t C hm-2yr-1(HE), and the differences between them were significant (p<0.05). Relative to NN, other patterns all significantly increased δSOC (p<0.05). Reasonable fertilization and appropriate field management also improved δSOC, relative to FP, the YE, HY and HE still increased δSOC. The carbon emission aroused by field management (Eo, Ei) could not be overlooked, which dominated by Ei. Relative to Eo, the Ei were140%～320%higher for different cultivation patterns except NN. The N fertilizer was the main souce of Ei, which could contribute49%-75%.5. The GWP caused by CH4emissions under different cultivation patterns dominated the Net GWP of the double-rice cropping ecosystem, accouting for84.3%-94.2%, while the GWP of N2O could be neglected. On the contrary, the GWPs of Eo and Ei were vital to the ecosystem’s Net GWP. Except for NN, the GWP of field management (Eo, Ei) for other patterns accounted for13.2%～25.6%of the GWP caused by CH4. In addition, the effect of carbon sequestration by soil would offset the caused GWP in some extent. Despite of strengthening the ability of carbon sequestration for soil through straw return, the GWP caused by the stimulated CH4emissions in the meantime would over excess it. So, how and when to return the straw need further researches. NN produced the highest GHGI2.02±0.22kg CO2-eq kg-1yield, while HY was the lowest, being only1.12±0.19kg CO2-eq kg-1yield. Relative to FP, the YE and HY could be advocated due to their increased rice yield and N fertilizer agronomic efficiency but reduced GHGI. However, HE produced the higher Net GWP and GHGI, which led to be further studied. |
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