EAGER: Collaborative Research: Development of a New Technique to Measure Ecosystem-level Soil Nitrous Oxide Fluxes using Micrometeorological Towers

一氧化二氮是大气中第三大温室气体，大气寿命约为114年，每个分子对全球变暖的影响大约是二氧化碳的300倍。大气中一氧化二氮的浓度正在增加，这主要是由于农业，农业被认为占可归因于人类活动的大气总排放量的大约一半。目前土地利用变化和农业集约化(特别是增加化肥使用)的趋势表明，到2030年，全球一氧化二氮排放量将再增加20%。然而，测量生态系统一氧化二氮产量的困难使得管理和预测主要人类来源的一氧化二氮产量变得困难。在这个项目中，我们将测试一种很有前途的新技术，使用激光传感器来测量一氧化二氮与大气在生态系统层面的交换。这一新的方法将提供机会，在每小时到每天的时间尺度上测量整个系统的一氧化二氮动态，范围太大，无法用传统技术有效采样。通过这种新的方法，有可能对控制森林、草原和农田中一氧化二氮的生产和消费的因素有新的理解。这将导致更好地管理和最大限度地减少这种重要温室气体的产生，因为管理目前受到一氧化二氮测量的重大不确定性的阻碍。了解和减少农业和其他管理土壤的一氧化二氮排放的一个主要障碍是测量通量的困难，因为时间和空间变异性往往超过米和小时尺度的数量级。在这个项目中，开路量子级联激光传感器将与标准的微气象测量相结合，以开发一种太阳能测量系统来量化一氧化二氮通量。这一新的方法将提供机会，在每小时到每天的时间尺度上测量多公顷区域内的整个系统的一氧化二氮通量，从而解决和整合使该通量如此难以在现场量化和建模的空间和时间变化。新开发的开路量子级联激光器(OP-QCL)传感器将分别用于现有的两个二氧化碳微气象涡旋协方差塔，以测量生态系统级别的一氧化二氮交换。除了OP-QCL传感器外，还将在研究区域内部署多个标准静态室，以执行OP-QCL测量的地面验证。其中一个传感器将部署在高排放的连续施肥玉米系统中；另一个将部署在25年未耕种的草原上。除了验证微气象方法外，该项目还将测试与通量在日尺度到季节尺度上的时间变化有关的假设，如果没有持续的观察，这些假设是无法回答的。具体的项目目标包括：1)开发、测试和验证OP-QCL传感器，用于使用微气象塔测量生态系统一级的一氧化二氮通量；以及2)将一氧化二氮通量的变化与时间环境变异性联系起来并评估其意义，包括每日、季节性和间歇性事件，例如大雨事件和管理活动，如种植系统中的耕作和施肥。

Nitrous oxide is the third most important greenhouse gas in the atmosphere, with an atmospheric lifetime of about 114 years and a global warming impact per molecule that is about 300 times greater than that of carbon dioxide. Atmospheric concentrations of nitrous oxide are increasing, primarily due to agriculture, which is thought to be responsible for about half of the total output to the atmosphere that can be attributed to human activities. Current trends in land-use change and agricultural intensification (in particular increasing fertilizer use) suggest that an additional 20% increase in global nitrous oxide emissions will occur by 2030. However, difficulties in measuring the output of nitrous oxide from ecosystems make it hard to manage and predict nitrous oxide production from major human sources. This project we will test a promising new technique to measure the ecosystem-level exchange of nitrous oxide with the atmosphere using a laser sensor. This novel approach will provide opportunities to measure whole system nitrous oxide dynamics at hourly to daily time scales over areas that are too large to effectively sample with traditional techniques. With this novel approach, a new understanding of the factors that control the production and consumption of nitrous oxide in forests, grasslands, and agricultural lands may be possible. This would lead to much better ability to manage and minimize the production of this important greenhouse gas, because management is currently hampered by significant uncertainties in the measurement of nitrous oxide. A major barrier to understanding and mitigating emissions of nitrous oxide from agricultural and other managed soils is the difficulty with which fluxes are measured, given temporal and spatial variability that often exceeds an order of magnitude at scales of meters and hours. In this project, open-path quantum cascade laser sensors will be integrated with standard micrometeorological measurements to develop a solar-powered measurement system to quantify nitrous oxide fluxes. This novel approach will provide opportunities to measure whole system nitrous oxide fluxes at hourly to daily time scales over multi-hectare areas, and thereby resolve and integrate the spatial and temporal variability that makes this flux so difficult to quantify and model in situ. A newly developed open-path quantum cascade laser (OP-QCL) sensor will be used on each of two existing carbon dioxide micrometeorological eddy covariance towers to measure ecosystem-level nitrous oxide exchange. In addition to the OP-QCL sensors, multiple standard static chambers will be deployed within the study area to perform ground based validation of the OP-QCL measurements. One of the sensors will be deployed in a high-emission fertilized continuous corn system; the other will be deployed in grassland that has not been farmed for 25 years. In addition to validating the micrometeorological method, the project will test hypotheses related to the temporal variability of fluxes at diurnal to seasonal scales, which cannot be answered without continuous observations. Specific project objectives include 1) to develop, test, and validate an OP-QCL sensor for ecosystem-level measurements of nitrous oxide fluxes using micrometeorological towers; and 2) to relate and assess the significance of changes in nitrous oxide flux with temporal environmental variability, including daily, seasonal, and episodic events such as large rain events and management activities such as tillage and fertilization in cropped systems.