Nitrous oxide emissions from soils and tree stems of rainforests and cacao agroforests in the Congo basin, Cameroon

湿润的热带森林生物群落在温室气体一氧化二氮（N2O）的全球收支中起主导作用。然而，使用自下而上方法（测量/模拟土壤-大气通量数据外推）估算的热带雨林土壤N2O排放源强度在过去几十年中有所下降。这似乎与自上而下的研究相反，这些研究使用全球N2O大气观测和反演方法来估计N2O排放量，这些研究仍然指出热带是重要的N2O来源。这种自下而上和自上而下对一氧化二氮排放量的估计明显不匹配，可以用两种方式来解释。1)排水良好的土壤上的热带树木可能是土壤中产生的N2O的管道，这是一个在很大程度上被忽视的潜在排放途径。近年来的研究表明，无论是具有通气组织（作为土壤-大气气体交换渠道）的树木，还是没有通气组织的树木，都有N2O排放的证据。在一项初步研究中，我们能够证明喀麦隆热带树木从树干排放N2O的能力是广泛存在的。2)现有野外测量的地理偏差可能降低了对湿润热带森林生物群落N2O源强度的估计。尽管亚马逊河流域和刚果盆地的大部分热带雨林位于风化程度相对较重的富氮土壤上，但大多数微量气体研究都是在风化程度较低的土壤上进行的，这些土壤往往具有较低的氮有效性，因此排放较少的N2O。特别是对非洲刚果盆地的测量明显缺失。在初步研究中，我们发现我们计划进行研究的森林立地具有高氮有效性，这强烈暗示这些立地将有高的气态氮损失。结合室内法、气相色谱法、N2O的15N自然丰度特征和15N同位素示踪技术，我建议测量喀麦隆刚果盆地重度风化森林和农林业土壤以及树干的N2O通量。我的小组已经在热带森林站点测试了土壤N2O通量测量和15N追踪N2O的方法。我们在喀麦隆的两个地点测试了测量树干N2O排放量的方法。这项拟议的研究将首次在非洲一个从未测量过的重要生物群落（刚果盆地的雨林）中提供土壤N2O通量的可靠测量，并将提供第一个证据，证明排水良好的土壤上的热带树木是否作为土壤中N2O的重要管道。

The humid tropical forest biome plays a dominant role in the global budget of the greenhouse gas nitrous oxide (N2O). However, the estimated source strength of N2O emissions from tropical rainforest soils using a bottom-up approach (extrapolation of measured/modeled soil-atmosphere flux data) has decreased in the past decades. This appears to be in contrast with top-down studies that used global N2O atmospheric observations and inversion methods to estimate N2O emissions, which continue to point at the tropics as an important N2O source. This apparent mismatch of the bottom-up and top-down estimates of N2O emissions may be explained in two ways. 1) Tropical trees on well drained soils may serve as a conduit for N2O produced in the soil, a potential emission pathway that has been largely ignored. Recent studies have shown that not only trees with aerenchyma (serving as channel for soil-atmosphere gas exchange) but also trees without this feature showed evidence of N2O emission. In a preliminary study we were able to show that the ability to emit N2O from tree stems is widespread in tropical trees in Cameroon. 2) Geographic bias of the existing field measurements may have reduced estimates of the N2O source strength of the humid tropical forests biome. Even though the majority of remaining tropical rainforests in the Amazon and Congo basins are located on relatively heavily weathered nitrogen-rich soils, most trace gas studies were conducted on less weathered soils which tend to have lower N availability and consequently emit less N2O. Especially measurements from the Congo basin in Africa are notably absent. In the preliminary study we showed that the forest sites where we plan to conduct our study have a high N availability which strongly hints that these sites will have high gaseous nitrogen losses. Using a combination of chamber methods, gas chromatography, 15N natural abundance signatures of N2O and 15N isotope tracing techniques, I propose to measure N2O fluxes from heavily weathered forest and agroforestry soils and from tree stems in the Congo Basin of Cameroon. The methods for soil N2O flux measurements and 15N tracing to N2O have been tested in tropical forest sites by my group. The method for measuring N2O emissions from tree stems we tested at two sites in Cameroon. This proposed study will provide the first ever robust measurements of soil N2O fluxes in a never-before measured important biome in Africa (rainforest in the Congo Basin), and will also provide the first evidence of whether or not tropical trees on well-drained soils serve as important conduits of N2O from the soil.