N20 and N0 Flux from Tropical Pastures and Forests, Atmospheric Inputs, Seasonality, and Controlling Mechanisms

自然排放的氮氧化物在当地氮收支和对大气化学的影响方面具有生物地球化学意义。在这种情况下，两种最重要的氮氧化物是氧化亚氮（N2O）和一氧化氮（NO）。目前的证据表明，在全球范围内，热带土壤可能是N2O的主要来源，而由热带森林转化而来的牧场可能是N2O的特别活跃的来源。由于新热带森林正在大规模地转变为牧场，因此很明显，新热带森林到牧场的转变是最重要的一种生态系统，其中应该进行N2O通量测量。对NO通量的了解比N2O少得多，但现有数据表明，作为热带土壤自然排放的氮氧化物，NO可能比N2O更重要。数据有力地表明，如果要评价热带土壤是大气中的氮氧化物来源，则应同时测量一氧化氮通量和氧化亚氮通量。本研究需要实地测量哥斯达黎加加勒比低地相对肥沃的冲积土和相对贫瘠的残余土上的原生林、次生林和牧场的N2O和NO通量。这些测量与同时进行的土壤特性测量（如H2O含量和NH4+浓度）将不仅提供从土壤到大气的年通量的估计，而且将提供筛选潜在因果关系的统计手段。这种多变量分析将与现场和实验室实验有关，以确定驱动这些通量的因果因素。这项研究的总体重要性来自两个主要来源。首先，所研究的氮氧化物被认为是导致全球气候变化的一系列大气“温室”气体的主要组成部分。此外，这项研究将产生关于热带环境中土地利用变化的系统影响的重要基本信息。在这种情况下，它应该产生具有广泛效用的重大结果。

Naturally emitted nitrogen oxides are of biogeochemical significance in terms of local nitrogen budgets and influences on atmospheric chemistry. Two of the most important nitrogen oxides in the context are nitrous oxide (N2O) and nitric oxide (NO). Present evidence suggests that tropical soils may be primary source of N2O on a global basis and that pastures converted from tropical forests may be especially active sources of N2O. Because neotropical forests are being converted to pastures on such a large scale, it is evident that the neotropical forest- to-pasture transition is the most important kind of ecosystem in which N2O flux measurements should be made. Much less is known about NO flux than about N2O, but available data suggest that NO may be more important as a naturally emitted nitrogen oxide from tropical soils than is N2O. Data strongly indicate that NO fluxes should be measured along with N2O fluxes if tropical soils are to be evaluated as an NOx source to the atmosphere. This research entails field measurements of N2O and NO flux from replicated plots representing primary forest, secondary forest, and pastures on relatively fertile alluvial soils and relatively infertile residual soils in the Caribbean Lowlands of Costa Rica. These measurements together with concurrent measurements of soil properties such as H2O content and NH4+ concentration will not only provide an estimate of annual fluxes from soils to the atmosphere, but will provide a statistical means to screen for potential causal relations. Such multivariate analyses will be related to field and laboratory experiments to define causal factors driving these fluxes. The overall importance of this research derives from two primary sources. First, the oxides of nitrogen under study are felt to the major components of the array of atmospheric "greenhouse" gases that contribute to global climate changes. As well, the study will yield important fundamental information regarding the system effects of land use changes in tropical environments. In this context it should produce significant results of wide utility.