Exploring subsurface heterogeneity of nitrogen and carbon cycling under a southeastern U.S. cattle grazing pasture

Subsurface cycling of nitrogen and carbon is central to our understanding of biosphere-atmosphere exchange and can strongly impact the ability of soil to be a greenhouse gas source or sink.  The underlying processes exhibit strong environmental dependence that can lead to substantial spatiotemporal heterogeneity across scales.  Here we present a study that explores how that variability manifests in subsurface greenhouse gases and their isotopes under a cattle grazing pasture in the southeastern U.S.  We used an automated array of 24 diffusive soil gas probes connected to a tunable infrared laser direct absorption spectrometer (TILDAS) to produce real-time maps of nitrous oxide isotopes, carbon dioxide, and oxygen with high spatial (meters) and temporal (hours) resolution.  We discuss ways to visualize and process the heterogeneity data, including using Lorenz plots and two-dimensional correlation, to reveal the magnitude and persistence of spatial heterogeneity of these gases.  CO₂ is found to be much more spatially homogeneous than N₂O, indicating a stronger dependence of N processing upon the local environment.  Analysis of N₂O isotopic signatures shows that the last step in subsurface N-cycling, N₂O reduction, is spatially heterogeneous and related to the magnitude of “hot spots” of N₂O production.