RAPID: COLLABORATIVE RESEARCH: ENSO and Tropical Rain Forest Soil Carbon (CH4, CO2) Fluxes

碳管理对于减少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，但热带森林吸收并固定了与大气交换的人为和自然碳的40%。这些森林中的土壤是最大的陆地碳库。在高水分、低氧气的条件下，分解会导致甲烷气体(CH4)的产生和释放，这些微生物被称为产甲烷菌。在旱季，深层热带雨林土壤保持湿润，但地表干燥。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十到一百倍。因此，热带雨林中这两种气体的相对释放对气候变暖有很大影响。2015年和2016年，一场异常强烈的厄尔尼诺南方涛动(ENSO)事件开始影响北美。据预测，这次厄尔尼诺现象将导致中美洲热带森林出现大干旱。考虑到降水在土壤碳循环中的主导作用，来自发展中的ENSO的干旱的影响可能是巨大的。这个快速项目将测试ENSO循环控制全球二氧化碳和甲烷循环年际变化的大部分的想法，方法是将热带森林土壤从正常雨季的甲烷强来源转变为厄尔尼诺引起的干旱事件期间全年的甲烷汇和二氧化碳来源。通过成像根系和微生物的生产和周转，以及直接测量二氧化碳的产生和排放，测量了土壤碳动态。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热带雨林制定的碳收支也未能占到固定碳总量的近10%。这些测量中一个潜在的缺失成分是甲烷(CH4)。热带森林既是甲烷的消费者，也是甲烷的生产者。在正常的潮湿年份，在缺氧、潮湿的土壤条件下，甲烷菌相对于甲烷氧化菌的活性增加会导致甲烷的释放。然后，当土壤在暴风雨之间变干时，甲烷氧化菌可能会极大地增加甲烷的氧化速率，导致土壤中呼吸的二氧化碳释放更多。在北美检测到了2015-16年异常强烈的ENSO事件。中美洲热带森林中与ENSO有关的干旱在拉塞尔瓦地区可能是极端的。具体地说，假设：(1)在正常的雨季，地下产生的大量二氧化碳被转化为甲烷，最终释放到大气中；(2)雨季的厌氧土壤条件通过甲烷生成和分解导致更大的甲烷释放，以及(3)在强烈的厄尔尼诺事件期间，较干燥的条件将导致这些土壤中甲烷氧化菌对甲烷的氧化增加。为了测试这些问题，将通过ENSO前、ENSO和ENSO后的气候条件连续测量二氧化碳和CH4。定量聚合酶链式反应和逆转录定量聚合酶链式反应将被用来量化甲烷产生菌(产甲烷菌)和消耗甲烷菌(甲烷氧化菌)的丰度和活性。这些措施将与根、林下和凋落物调查相结合，以比较活的和死的植物生物量对这些微生物的C的贡献。

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.