| Agroecosystems play a vital role in the global balance of atmospheric greenhouse gases (GHGs) and act as a main souce of GHGs emissions. Shift in agricultural production regime is considered to be a key factor influencing GHGs emissions from croplands, thus, to give an insight into GHGs emissions from cropping systems as affected by the shift in agricultural production regime and in turn to comprehensively evaluate the global warming potential (GWP), greenhouse gas intensity (GHGI) and its relevant ecological benefits derived from agricultural production would be highly needed.The paddy rice-winter wheat rotation, as one of the major cropping systems in Southeast China, was selected in the present study to simultaneously measure CO2, CH4and N2O fluxes with static opaque chamber-gas chromatograph method. The primary objective of this study was to gain an insight into the effect of water regime-specific conventional agricultural management on GHGs emissions, and in turn to explore the related processes and mechanisms involved. This study consists of five field experiments. In2010, we presented field measurements of CH4and N2O fluxes from rice seedling nurseries under the water regimes of continuous flooding and moist irrigation without waterlogging in Southeast China. Different N fertilizer sources were simultaneously integrated in this field experiment, which include inorganic N, organic N or combined organic/inorganic N fertilizers. The main objectives are to gain an insight into a complete accounting of CH4and N2O emissions from typical rice seedling nurseries and thereby to examine which water management and fertilizer regimes would be an effective option for mitigating climatic impacts of rice seedling nurseries on the perspective of GWP of CH4and N2O. In2011, field experiments were carried out both in conventional transplanted and direct seeded rice paddies, to examine the effects of rice cultivation pattern together with the application of chemical N fertilizer, slow-release N fertilizers, nitrification inhibitors and Fe (III) fertilizers on CH4and N2O emissions during the rice-growing season. The fertilizer sources consisted of urea (U), sulfur-coated urea (U-S), urea formaldehyde (UF), urea with ferrihydrite (U+Fe) or urea with dicyandiamide and hydroquinone (U+DCD+HQ). Five fertilizer treatments and a control treatment without fertilizer application under either rice cultivation pattern were simultaneously devised in this field experiment. In2007, a field experiment was conducted to investigate the effect of sewage irrigation on CH4and N2O emissions during the rice-growing season, where sewage and unpolluted rive water was simultaneously taken into account in this field experiment. In2007-08cropping rotation, a split-plot experiment was performed to study the effects of water regime during the rice-growing season and fertilizer application on N2O emissions over the annual rotation cycle. Water regime during the rice season consisted of continuous flooding (F), flooding-drainage-flooding (F-D-F) and flooding-drainage-flooding-moisture irrigation (F-D-F-M). Chemical N were applied at the rates of0,100,200or300kg N. hm’2in rice season and0,75,150or250kg N·hm-2in wheat season, respectively. In2009-10cropping rotation, ferric hydroxide and ferrihydrite in the form of amorphous granular selected as the iron fertilizer materials were amended in the rice-and wheat-cropping seasons, respectively. The objective was to gain an insight into a complete GHGs (including CO2, CH4and N2O) accounting of the net greenhouse gas balance (NGHGB) and greenhouse gas intensity (GHGI) as affected by Fe (III) fertilization in paddy rice-winter wheat rotation systems. The Fe (III) fertilizer was applied identically at the rate of4and8t-ha"1for both rice and wheat growing seasons, respectively.The results of this study are displayed as follows:1. The total CH4emissions from the rice seedling nurseries were significantly affected by water regime and fertilizer. Relative to the continuous flooding (F), moist irrigation (M) decreased total CH4by50%,25%and14%for the inorganic N (IF), combined application of inorganic/organic manure N (CF) and organic manure N (OF) plots, respectively. Compared with IF applied plots, OF applied plots increased CH4by44%and148%in the rice seedling nurseries under the F and M irrigation regimes, respectively. Total N2O emissions from the rice seedling nurseries were significantly affected by water regime, but the effects of fertilizer and their interaction were not significant. Relative to the continuous flooding, total N2O emissions from moist irrigated plots were increased by186%,132%and72%for the IF, CF and OF plots, respectively. Total N2O emissions were equivalent to0.20-0.24%and0.41-0.57%of the N applied in the rice seedling nurseries under continuous flooding and moist irrigation regimes, respectively. Relative to the continuous flooding, moist irrigation decreased the net GWPs by13-46%over the20-year horizon or11-39%over the100-year horizon. Compared with inorganic fertilizer, organic manure application significant increased CH4emissions, but no significant effects on N2O emissions, which led to the net GWPs increased by43%and107-132%in the rice seedling nurseries under the continuous flooding and moist irrigation regimes, respectively. The results of this study suggest that moist irrigation instead of continuous waterlogging and decreased organic N fertilizer inputs that have been increasingly experienced in Chinese rice seedling nurseries would benefit for mitigating the combined global warming potentials of CH4and N2O in rice seedling nurseries in China.2. Seasonal CH4emissions differed significantly (P=0.01) between conventional transplanted (TPR) and direct seeded rice (DSR) cultivation patterns, irrespective of various fertilizer sources applied. As compared to TPR production mode, DSR decreased CH4by59%. No pronounced relationship (P=0.14) was found across fertilizer treatments under either rice cultivation mode, and also the interaction between rice cultivation pattern and fertilizer application (P=0.40). The N2O emissions differed significantly (P=0.02) between the two rice cultivation methods. In comparision with TPR, seasonal amounts of N2O emissions during the rice growing season for DSR were enhanced by38%across different fertilizer treatments. Seasonal N2O emissions under either rice cultivation mode were significantly distinguished by fertilizer source (P<0.001). Under identical N input rates, in contrast with urea applied plots (U) treatment, N2O emissions were increased by12%and11%for the plots with urea and sulfur-coated urea (U-S), while decreased by38%and33%for the urea formaldehyde applied treatment (UF), and also decreased by54%and60%for plots with nitrification inhibitor dicyandiamide and urease inhibitor hydrochinone (U+DCD+HQ) under TPR and DSR production modes, respectively. However, under the TPR mode, the treatment with urea and ferrihydrite (U+Fe) vs. the U treatment enhanced N2O by18%. Seasonal CH4and N2O emissions significantly correlated with the harvested rice above-ground biomass both under TPR and DSR production modes in the present study.3. Sewage irrigation significantly increased seasonal amounts of CH4emissions. In contrast with river water irrigation, sewage irrigation increased seasonal CH4emissions by27%and33%for the plots with and without chemical N addition, respectively. Compared with the control without fertilizer application, chemical N fertilizer application decreased CH4emissions by8%and12%for the plots under sewage and river water irrigation, respectively. Seasonal amounts of N2O emissions were significantly affected by the water type of irrigation, fertilizer application (P<0.001) and tended to be affected by their interaction (P=0.06). Relative to river water irrigation, sewage irrigation increased N2O by68%and170%for the plots with and without N application, respectively. The direct emission factor of fertilizer N for N2O was estimated to be0.71%and0.52%for the plots under sewage and river water irrigation, accompanied by a seasonal N2O-N background emission of0.81kg·hm-2and0.30kg·hm-2, respectively. The sewage N induced indirect emission factor of N2O would be0.73-0.92%. In terms of the combined GWP of CH4and N2O as affected by irrigation type and N fertilization, compared with river water irrigation, sewage irrigation increased significantly the net GWPs whether over a20-year horizon or a100-year horizon. N application significantly increased N2O emissions, while it tended to decrease CH4emissions, the synthetic fertilizer application slightly increased the net GWPs over a100-year horizon, but this impact was not pronounced whether under river water irrigation or sewage irrigation. In contrast, the net GWPs in terms of rice yield were significantly decreased by nitrogen application. Overall, higher net GWPs in rice paddies irrigated by sewage suggests that sewage instead of unpolluted river water irrigation would intensify the radiative forcing derived from CH4and N2O emissions in rice production.4. In contrast with the water regimes of flooding-drainage-flooding (F-D-F) and flooding-drainage-flooding-moisture (F-D-F-M), continuous flooding (F) greatly decreased N2O emissions in rice-growing season, but markedly enhanced N2O emissions in the following non-rice season (including following fallow period and wheat season). On the contrary, Midseason drainage and dry-wet episodes during rice season significantly increased N2O emissions in rice season, but cut down the N2O emissions in the following wheat season. However, under an identical N fertilizer application, annual total N2O emissions were generally comparable among the experimental plots experienced by various water regimes during the rice-growing season. The fertilizer-induced emission factor and background emission of N2O through the annual cropping rotation were closely correlated with the various water regimes practiced during the rice-growing season. Seasonal N2O emissions significantly increased with N inputs under each water regime except that the relationship between N2O emission and N input was not pronounced during the rice-growing season under continuous flooding. Over the whole annual cycle, however, the emission factor of N2O did not significantly differ among the plots under three water regime treatments, which was estimated to be0.87%,0.97%and0.85%for the plots experienced by the water regimes of F, F-D-F and F-D-F-M, respectively. Overall, the relationship between N2O emissions and N inputs estimated the emission factor and background emission of N2O to be, on average,0.89%and1.80kg N2O-N·hm-2in the rice-wheat rotation system experienced by various water regimes, respectively.5. The CO2, CH4and N2O emissions during the rice-growing season were significantly correlated to Fe (Ⅲ) fertilizer application. Fe (Ⅲ) fertilization slightly or significantly decreased soil total CO2emissions across the rice-wheat cropping rotation. Compared with the control, Fe (Ⅲ) fertilization decreased annual soil CO2emissions by26%and32%for the medium (Fe-M) and high (Fe-H) Fe (Ⅲ) fertilization applied plots, respectively. Net ecosystem exchange of CO2(NEE) was significantly affected by Fe (Ⅲ) fertilizer application. The NEE estimates indicated that rice production system sequestrated atmospheric CO2more efficiently than winter wheat cropping system. Compared with the control, Fe (Ⅲ) fertilization significantly benefited for ecosystem CO2sequestration (F2,6=14.5, P0.01), with19-21%greater in the rice-growing season and57-90%greater in the non-rice season. In comparision with the control, Fe (Ⅲ) fertilization significantly decreased CH4emissions from rice paddies by27%nd44%or the Fe-M and Fe-H plots, respectively (F2,6=22.7, P<0.01).Aone-way ANOVA showed that Fe (III) fertilization significantly increased N2O emissions during the rice-growing season (F2,6=8.4, P=0.02), non-rice season (F2,6=23.4, P<0.0\) and over the whole annual cycle (F2,6=28.6, PO.001). Over the whole annual paddy rice-winter wheat rotation cycle, N2O emissions were65-100%greater in the Fe (III) fertilizer applied plots than in the non-amendment control plots. The annual NGHGB were negative for all the field treatments, suggesting that atmospheric CO2sequestrated into the agroecosystems exceeded the CO2-equivalents due to annual CH4and N2O emissions from paddy rice-winter wheat rotation systems. Although Fe (III) fertilizer application increased annual N2O emissions, it remarkably decreased CH4emission and increasingly sequestrated atmospheric CO2into agroecosystems, which led to the decease in the NGHGB over the100-year time. In conclusion:First, the total CH4and N2O emissions from the rice seedling nurseries were significantly affected by water regime and fertilizer. The results suggest that moist irrigation instead of continuous waterlogging and decreased organic N fertilizer inputs that have been increasingly experienced in Chinese rice seedling nurseries would benefit for seedling growth while mitigating the combined global warming potentials of CH4and N2O in rice seedling nurseries in China. Second, seasonal CH4and N2O emissions differed significantly between conventional transplanted and direct seeded rice cultivation patterns. By integrating rice production benefits and the net GWPs from CH4and N2O, relative to direct seeded rice, all the fertilized plots under the transplanted rice cultivation mode exacerbated the net GWPs from CH4and N2O with different levels, particularly for the urea and urea formaldehyde applied plots. Moreover, transplanted rice vs. direct seeded rice greatly decreased grain yield in rice production. Third, relative to river water irrigation, sewage irrigation significantly intensified the radiative forcing derived from CH4and N2O emissions in rice production. Fourth, seasonal or annual N2O emissions differed greatly among various water regimes practiced during the rice-growing season. Continuous flooding greatly decreased N2O emissions in rice-growing season, but markedly enhanced N2O emissions in the following non-rice season (including following fallow period and wheat season); On the contrary, Midseason drainage and dry-wet episodes during rice season significantly increased N2O emissions in rice season, but cut down the N2O emissions in the following non-rice season. Finally, the CO2, CH4and N2O emissions across the annual rice-wheat cropping rotation were significantly correlated to Fe (Ⅲ) fertilizer application. The NEE and NGHGB estimates indicated that rice production system sequestrated atmospheric CO2more efficiently than winter wheat cropping system and Fe (Ⅲ) fertilization greatly benefited for ecosystem CO2sequestration. |
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