大气硫沉降对稻田土壤甲烷排放的影响及其机制

Acid rain and climate change induced by increasing concentrations of atmospheric greenhouse gases are among the current most important global environmental problems. China has been suffering from serious pollution of sulfuric acid rain, with occurrence area covering a vast of rice-growing regions including the Yangtze River watershed and Pearl River Delta. Rice paddies are recognized as an important source of methane, a potent greenhouse gas. But up to now, very limited information has been available on whether the change of soil properties and sulfur contents under long-term pollution of acid rain influences soil carbon cycling processes and thereby affects the methane fluxes from rice paddies. This project is thus proposed to explore the relevance of acid rain with methane fluxes from rice paddies. Multi-year in situ monitoring campaign is planned for a series of paddy fields distributed across an urban to rural transect of Chongqing subjected to a gradient of acid rain sulfate deposition to investigate the actual impact of long-term acid rain on methane emission from rice paddies. Field micro-plot experiments with simulated acid rain will also be carried out to identify the short-term effect of atmospheric sulfur deposition on methane emission from rice paddies and their dose-effect relationship. Soil incubation experiments will be conducted to further study the sulfate deposition-induced changes of carbon/sulfur transformation processes and microbial diversity in paddy soils and their coupling relationship with methane emission. The interactive effects between sulfate deposition and other factors such as fertilization and soil drying-rewetting cycles on methane emission from rice paddies will also be investigated. The results from this study can be expected: 1)to expand the knowledge on the eco-environmental implications of acid rain; 2)to clarify the effects and underlying mechanisms of acid rain on methane fluxes from rice paddies; and 3)to contribute to evaluating the magnitude of the effect of atmospheric sulfur deposition on greenhouse gases emissions from rice paddies in China and other regions.

酸雨和大气温室气体浓度上升引起的气候变化是全球关注的环境问题。我国是硫酸型酸雨严重的国家，污染范围现已扩展到长江流域和珠江三角洲等广大植稻地区，而稻田则是温室气体甲烷的重要排放源。在长期酸雨硫沉降下土壤性质的变化和硫含量的增加是否改变了碳循环过程，进而影响稻田甲烷的排放，目前尚不清楚。本项目拟在大气硫沉降梯度长期稳定存在的重庆主城附近、郊区和偏远农村选择代表性稻田，进行多年连续田间原位观测,考察长期酸雨硫沉降对稻田甲烷排放的影响；开展模拟酸雨控制性微区试验，探讨大气硫沉降对稻田甲烷排放的短期影响和剂量-效应关系；采用室内土培实验等方法，研究硫沉降对土壤碳硫转化过程和微生物多样性的影响及其与甲烷排放的耦合关系，分析施肥和土壤干湿交替等因素与硫沉降对稻田甲烷排放的交互效应。结果将为阐明酸雨对稻田甲烷产生与排放的影响与机制、评估大气硫沉降对我国及其它地区稻田CH4排放的影响程度提供重要科学依据。

为揭示酸雨硫沉降对稻田甲烷排放的影响及其机制，在大气硫沉降梯度长期稳定存在的重庆主城附近和郊区选择代表性稻田，进行了长期连续田间原位观测；开展了模拟酸雨控制性微区试验，以研究大气硫沉降与稻田甲烷排放的剂量-效应关系，并探讨外源硫输入对土壤微生物多样性的影响及其与甲烷排放的耦合关系。结果表明：. 1）酸雨重污染区（九龙坡）试验田在稻季、休闲期和麦季的CH4排放量都略低于轻污染区（北碚），但差异并不显著（p>0.05）；2）当土壤人为添加不同水平（25100mg SO42- kg-1）的硫酸盐（Na2SO4）时，土壤CH4排放量较对照降低了21%45%，并且CH4排放量与间隙水硫酸盐浓度间呈显著负相关（p<0.0001）；3）土壤一次性大剂量撒施硫酸盐对CH4排放的抑制率比同量硫酸持续低剂量分施高约13%；4）土壤添加硫酸盐对CH4排放有明显抑制作用，但历史上的酸雨重污染区（九龙坡）与轻污染区（北碚）稻田原位监测的CH4排放量并无显著差异，究其原因，主要是近二十年来严厉实施的大气SO2减排措施，缓解了酸雨污染，导致两地土壤中的硫酸盐含量已无显著差异（p>0.05）；5) 脱硫石膏（FGD）是钙基湿法技术控制燃煤发电烟气SO2排放生成的副产物，当其施用量为216 t ha-1时，稻季CH4排放量比照降低了29%91%，并且排放量与施用量间呈显著的负指数函数关系；6）施加FGD降低了稻田土壤产甲烷菌mcrA基因丰度，提高了硫酸盐还原菌属相对丰度，两者与甲烷排放通量分别呈显著正相关与负相关（p<0.05）；7）就本研究而言，没有发现施用FGD对土壤—水稻系统产生重金属富积效应，相反，土壤中镉、铅等重金属的可交换态/有效态因FGD的添加而降低了。研究结果为评价大气硫沉降对我国稻田CH4 排放的影响提供了重要的实测数据和科学依据，也显示了施用脱硫石膏控制稻田甲烷排放的可能性。.