RAPID: Collaborative Research: ENSO and Tropical Rain Forest Soil Carbon (CH4, CO2) Fluxes

碳管理对于降低气候变化的强度和影响至关重要。虽然高纬度气候显示出最大的温度变化，但热带森林吸收并封存了与大气交换的40%的人为和自然碳。这些森林中的土壤是最大的陆地碳库。在高水、低氧条件下，分解导致甲烷气体（CH4）的产生和释放，被称为产甲烷菌的微生物。在旱季，雨林深处的土壤保持湿润，但表层是干燥的。当这种情况发生时，另一组微生物会消耗甲烷并将其转化为二氧化碳。与二氧化碳（CO2）相比，甲烷气体的温室效应要大10到100倍。因此，这两种气体在热带雨林中的相对释放对气候变暖有很大的影响。2015年和2016年，异常强烈的厄尔尼诺Niño南方涛动（ENSO）事件开始影响北美。据预测，这次ENSO将导致中美洲热带森林的严重干旱。考虑到降水在土壤碳循环中的控制作用，ENSO发展带来的干旱影响可能是巨大的。这个RAPID项目将测试ENSO循环控制全球二氧化碳和甲烷循环的年年化变化的想法，通过将热带森林土壤从正常雨季的强大甲烷来源转变为在El Niño-induced干旱事件期间全年的甲烷汇和二氧化碳来源。土壤碳动态已经通过根系和微生物生产和周转成像以及二氧化碳生产和外排的直接测量来测量。然而，即使对产出进行了详细的测量，为La Selva生物站附近的哥斯达黎加雨林开发的碳预算也未能占到固定碳总量的近10%。这些测量中可能缺少的一个成分是甲烷（CH4）。热带森林既是CH4的消费者又是生产者。在正常湿润的年份，在缺氧、潮湿的土壤条件下，产甲烷菌相对于养甲烷菌的活性增加，导致甲烷释放。然后，当土壤在风暴之间变干时，甲烷氧化菌可能会大大增加甲烷氧化的速度，导致更多的土壤二氧化碳被释放出来。2015年至2016年在北美发现了异常强烈的ENSO事件。中美洲热带森林与enso相关的干旱在La Selva地区可能是极端的。具体来说，假设：(1)在正常潮湿的雨季，地下产生的大量CO2被转化为CH4，最终释放到大气中；(2)湿雨季厌氧土壤条件通过产甲烷和分解释放更多的CH4，(3)在强El Niño事件期间，干燥条件将导致这些土壤中甲烷氧化菌的甲烷氧化增加。为了验证这些问题，将通过ENSO前、ENSO后和ENSO后的气候条件连续测量CO2和CH4。qPCR和RT-qPCR将用于量化产甲烷菌（产甲烷菌）和消费甲烷菌（产甲烷菌）微生物的丰度和活性。这些措施将与根系、林下植被和凋落物调查相结合，以比较为这些微生物提供C的活的和死的植物生物量。

Carbon management is essential to reduce the intensity and impact of climate change. While high-latitude climates show the largest temperature changes, tropical forests take up and sequester forty percent of the anthropogenic and natural carbon exchanged with the atmosphere. Soils in these forests are the largest pool of terrestrial carbon. Under high water, low oxygen conditions, decomposition results in the production and release of methane gas (CH4)by microorganisms known as methanogens. During dry seasons, deeper rainforest soils remain wet, but dry at the surface. When that happens, a different group of microorganisms consume the methane and convert it to carbon dioxide. Methane gas has a ten to one hundred-fold greater greenhouse warming effect compared to that of carbon dioxide (CO2). Consequently, the relative release of these two gases in tropical rainforests has a large impact on the warming of climate. An unusually strong El Niño Southern Oscillation (ENSO) event is beginning to impact North America in 2015 and 2016. It is predicted that this ENSO will result in a major drought in tropical forests of Central America. Given the governing role of precipitation in soil carbon cycle, the impact of the drought from the developing ENSO is likely to be substantial. This RAPID project will test the idea that the ENSO cycle controls much of the year-to-year variability in the global carbon dioxide and methane cycle, by turning tropical forest soils from a strong source for methane during the normal rainy season, to a year-round sink for methane and source of carbon dioxide during El Niño-induced drought events.Soil carbon dynamics have been measured through imaging of root and microbial production and turnover, and direct measurements of CO2 production and efflux. However, even with detailed measurements of outputs, the carbon budget developed for the Costa Rican rain forest near La Selva Biologcial Station fails to account for nearly ten percent of the total fixed C. One potential missing component in these measurements is methane (CH4). Tropical forests are both consumers and producers of CH4. During normal wet years, increased activity of methanogens relative to methanotrophs in anoxic, wet soil conditions results in methane release. Then as soils dry between storms, methanotrophs may dramatically increase rates of methane oxidation, resulting in the release of higher amounts of respired soil CO2. An unusually strong 2015-16 ENSO event has been detected in North America. ENSO-associated drought in tropical forests of Central America are likely to be extreme at the La Selva site. Specifically, it is hypothesized that: (1) large amounts of CO2 produced underground during normal, wet rainy seasons are converted to CH4, which is eventually released to the atmosphere; (2) the anaerobic soil conditions of wet rainy seasons cause even greater CH4 release through methanogenesis coupled to decomposition, and (3 during a strong El Niño event, drier conditions will result in an increase in methane oxidation by methanotrophs in these soils. To test these questions, CO2 and CH4 will be continuously measured, through the pre-ENSO, ENSO, and post-ENSO climate conditions. qPCR and RT-qPCR will be used to quantify the abundance and activity of both methane-producing (methanogens) and -consuming (methanotrophs) microorganisms. These measures will be coupled with root, understory and litter surveys to compare living and dead plant biomass that contributes the C to these microbes.