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.
