Micro-Scale Mechanisms of N2O Production in Soil

一氧化二氮（N2O）是一种强效温室气体，其一半以上的人为排放来自农业土壤。因此，对一氧化二氮排放的准确评估和建模非常重要，但目前仍难以实现。一个主要原因是N2O生成速率的高时空变异性。“热点”是土壤剖面中在时间和空间上可变的微点，在给定的时间点上，它们可能负责大部分土壤N2O的产生。土壤中热点的出现需要一套最佳的物理、化学和生物条件。然而，这些情况在很大程度上是未知的。热点发生的极高的时间变异性，通常被称为“热点时刻”，使得它们的识别更加困难。该提案将在小空间尺度（1-1000µm）上实时测量N2O的产生，并将回答以下问题：1)N2O热点出现需要什么样的物理条件；2)什么微生物需要在正确的地点和时间出现才能使热点发挥作用；3)微尺度热点的N2O产生的知识是否有助于预测更大尺度的土壤N2O排放。该项目的方法解决了关于微尺度环境对热点/热时刻N2O生产影响的关键知识差距。土壤N2O生产将使用一种新的策略来研究-考虑N2O生产发生地点土壤微环境的异质性。为此，该项目将依赖于先进工具的新组合，包括基于同步加速器的x射线计算机微断层扫描（x射线微ct）信息与同位素源追踪，微尺度O2测绘和微生物群落分析。这一组合将有助于识别N2O热点，并描述其物理特性和微生物群落组成。该项目将验证这样一个假设，即土壤孔隙分布和特征的微观尺度模式是确定特定有机基质丰富地区是否会成为N2O生产热点的主要驱动力。孔隙的影响是通过影响物理微环境条件和活性微生物群落组成来实现的。该项目将提供1)估算相关运输过程的能力；2)对硝化和反硝化过程微观尺度时空耦合的新认识；3)分离已知驱动N2O排放的两个关键要素的独特可能性，即：(i)土壤孔隙作为具有影响输送现象潜力的物理途径，以及（ii）实现这种潜力的孔隙的水/空气填充状态；4)开发基于x射线微ct信息的N2O排放野外预测物理措施。博士后、研究生和本科生将参与该项目。3D计算机断层扫描图像将用于开发一套交互式计算机工具，这些工具将在该项目的网站上向公众提供。这些工具还将作为课程改进工具提供给K-12教师，并包括在向农民和作物顾问介绍氮循环的推广报告中。

Nitrous oxide (N2O) is a potent greenhouse gas and more than half of its anthropogenic production is from agricultural soils. Accurate assessments and modeling of N2O emissions are thus important but have remained elusive. A major reason is the high temporal and spatial variability in N2O production rates. "Hot spots" are temporally and spatially variable micro-sites within a soil profile that at a given point in time might be responsible for the majority of soil N2O production. Occurrence of a hot spot in soil requires an optimal set of physical, chemical and biological conditions. However, these conditions are largely unknown. Extremely high temporal variability in hot spot occurrence, often referred to as "hot moments", makes their identification even more difficult. This proposal will measure N2O production in real time at small spatial scale (1-1000 µm) and will answer the following questions 1) what physical conditions are needed for a N2O hot spot to emerge, 2) what microorganisms need to be present at the right place and time to enable the hot spot's functioning, and 3) can knowledge of N2O production in micro-scale hot spots be useful in predictions of soil N2O emissions at larger scales.The project's approach addresses the critical knowledge gap regarding the influence of micro-scale environments on hot spot/hot moment N2O production. Soil N2O production will be studied using a new strategy - accounting for heterogeneity in soil micro-environments at the locations where N2O production takes place. For that the project will rely on a novel combination of advanced tools, including synchrotron based X-ray computed micro-tomography (X-ray micro-CT) information with isotope source tracing, micro-scale O2 mapping, and microbial community analyses. This combination will enable identification of N2O hot spots and descriptions of their physical characteristics and microbial community compositions. The project will test the hypothesis that the micro-scale patterns in distribution and characteristics of soil pores act as the main driving force defining whether a particular organic-substrate-rich area will become a hot spot of N2O production. The influence of pores takes place via impacts on physical micro-environmental conditions as well as on composition of active microbial communities. The project will provide 1) ability to estimate the relevant transport processes; 2) new insights into micro-scale spatial/temporal coupling of nitrification and denitrification processes; 3) unique possibility to separate the contributions of the two key elements known to drive N2O emissions, that is, (i) soil pores as physical avenues with a potential to affect transport phenomena and (ii) water/air filled status of the pores through which that potential is realized; and 4) development of physical measures for field-scale predictions of N2O emissions based on X-ray micro-CT information.Post-doc, graduate and undergraduate students will be involved in the project. The 3D computed tomography images will be used to develop a set of interactive computer tools, which will be made available to general public at the project's website. The tools will also be presented to K-12 teachers as a curriculum enhancement instrument and included in extension presentations to farmers and crop consultants on nitrogen cycling.