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，其在大气中的寿命约为114年，每分子对全球变暖的影响约为二氧化碳的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，据认为，人类活动向大气中排放的一氧化二氮的总量约有一半是由农业产生的。目前土地利用变化和农业集约化的趋势（特别是化肥使用的增加）表明，到2030年，全球一氧化二氮排放量将再增加20%。然而，由于难以测量生态系统产生的一氧化二氮，因此难以管理和预测主要人类来源产生的一氧化二氮。在这个项目中，我们将测试一种很有前途的新技术，即使用激光传感器测量生态系统水平上氧化亚氮与大气的交换。这种新颖的方法将提供机会，在每小时到每天的时间尺度上测量整个系统的氧化亚氮动力学，这些区域太大，无法用传统技术有效地采样。有了这种新颖的方法，对控制森林、草原和农业用地一氧化二氮生产和消耗的因素的新理解可能成为可能。这将大大提高管理和减少这一重要温室气体产生的能力，因为目前管理工作受到一氧化二氮测量中的重大不确定性的阻碍。了解和减轻农业和其他管理土壤一氧化二氮排放的一个主要障碍是难以测量通量，因为在米和小时的尺度上，时空变化往往超过一个数量级。在这个项目中，开放路径量子级联激光传感器将与标准微气象测量相结合，开发一个太阳能测量系统来量化一氧化二氮的通量。这种新颖的方法将提供机会在多公顷的区域内以小时到每日的时间尺度测量整个系统的氧化亚氮通量，从而解决和整合使该通量难以量化和就地建模的空间和时间变动性。一种新开发的开放路径量子级联激光（OP-QCL）传感器将用于现有的两个二氧化碳微气象涡动相关塔，以测量生态系统水平的氧化亚氮交换。除了OP-QCL传感器外，研究区域内还将部署多个标准静态室，以执行OP-QCL测量结果的地面验证。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年没有耕种的草原上。除了验证微气象方法外，该项目还将检验与日至季节尺度上通量的时间变化有关的假设，没有连续的观测就无法回答这些假设。具体项目目标包括：1)开发、测试和验证一种OP-QCL传感器，用于利用微气象塔测量生态系统层面的氧化亚氮通量；2)将氧化亚氮通量变化与时间环境变率（包括日、季节和偶发事件，如暴雨事件和管理活动，如耕作和施肥）联系起来并评估其重要性。

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.