DISSERTATION RESEARCH: Nitrous Oxide and Dinitrogen Production from Tropical Forest Soils Under Ambient and Fertilized Conditions

一氧化二氮是一种温室气体，从含氮丰富的土壤中排放出来。在人类主导的景观中，肥沃的农田是最重要的氮源，而在自然景观中，通常富含氮的完整低地热带森林是最大的氮源。这项研究的目的是测量热带森林中一氧化二氮的排放量，目前尚不清楚氮污染是以一氧化二氮还是以氮气的形式排放的（氮气是惰性的，已经占大气的78%）。虽然一氧化二氮的排放相对容易测量，但众所周知，氮的排放很难测量。为了克服这个问题，研究人员将使用一种新开发的技术来直接测量土壤中产生的氮气和一氧化二氮。通过比较波多黎各Luquillo一个完整的热带森林中施肥和未施肥的土壤，他们将提供有关氧化亚氮排放和氮污染对热带森林影响的信息。研究人员将通过他们建立的教育网站与其他学者分享他们的发现。研究人员将使用无氮空气再循环法（N- farm）来量化在环境和富氮条件下，从具有不同天然氧状态（山脊、斜坡和山谷）的土壤中提取的完整土芯中产生的二氮（N2）和一氧化二氮（N2O）。由于N-FARM允许在不同的大气氧条件下测量气体通量，将这些测量结果与基于现场的土壤氧连续测量相结合，将允许对结果进行缩放，以产生每年景观尺度的N2和N2O产量估算。具体来说，研究人员将测试两个假设：H1: a)山谷中较低的土壤氧气将导致比更多高地地区更高的总氮气体损失；b)氮气体损失将使山谷中的N2O:N2比更低，因为较低的土壤氧气有利于将硝酸盐还原为N2，而不是N2O。H2：施氮将a)通过硝化作用和反硝化作用导致更大的氮气损失，b)由于硝化作用不产生N2，气体损失将具有更高的N2O:N2比。为了验证这些假设，研究人员将在Luquillo山脉的山脊/斜坡/山谷横断面上检查重复施肥的地块。他们将从控制下和施肥的地块收集土壤核，并在收集后的一周内在NFARM设备上运行。这些数据将提供热带土壤中第一个直接的氮排放测量，并将增加对热带森林今天和未来可能发生的氮损失方式的了解。

Nitrous oxide is a greenhouse gas that is emitted from soils where nitrogen is abundant. In human dominated landscapes, fertilized farm fields are the most important source, and in natural landscapes, intact lowland tropical forests, which are often very nitrogen rich, are the greatest source. This research aims to measure emissions of nitrous oxide in tropical forests, where it is not clear if nitrogen pollution is emitted as nitrous oxide or as nitrogen gas (which is inert and already makes up 78% of the atmosphere). While nitrous oxide emissions are relatively easy to measure, nitrogen gas emissions are notoriously difficult to measure. To overcome this problem, the researchers will use a newly developed technique to directly measure nitrogen gas and nitrous oxide production from soils. By comparing fertilized and unfertilized soils in an intact tropical forest in Luquillo, Puerto Rico, they will provide information on nitrous oxide emissions, and the impact of nitrogen pollution on tropical forests. The researchers will share their findings with the general audience through an education website that they have established, and with other academics.The researchers will use the Nitrogen Free Air Recirculation Method (N-FARM) to quantify dinitrogen (N2) and nitrous oxide (N2O) production from intact soil cores taken from soils with different native oxygen status (ridges, slopes and valleys) under ambient and N enriched conditions. Because N-FARM allows gas fluxes to be measured under different atmospheric oxygen conditions, coupling these measurements with field-based continuous measurements of soil oxygen will allow results to be scaled to generate annual landscape scale estimates of N2 and N2O production. Specifically, the researchers will test two hypotheses: H1: a) lower soil oxygen in valleys will result in higher total N gas loss than on more upland portions of the landscape and b) N gas losses will have a lower N2O:N2 ratio in valleys, since lower soil oxygen favors reduction of nitrate to N2, rather than N2O. H2: Nitrogen fertilization will a) induce greater nitrogen gas loss, via both nitrification and denitrification, and b) gas losses will have a higher N2O:N2 ratio since nitrification does not produce N2. To test these hypotheses, the researchers will examine replicated fertilized plots on ridge/slope/valley transects in the Luquillo Mountains. They will collect soil cores from beneath control and fertilized plots and run them on the NFARM apparatus within a week of collection. These data will provide the first direct N2 emissions measurements from tropical soils, and will add much needed understanding to the way N losses occur from tropical forest today, and potentially in the future.