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Abstract Soil nitrous oxide (N2O) emissions are highly variable in space and time, making it difficult to estimate ecosystem level fluxes of this potent greenhouse gas. While topographic depressions are often evoked as permanent N2O hot spots and rain events are well‐known triggers of N2O hot moments, soil N2O emissions are still poorly predicted. Thus, the objective of this study was to determine how to best use topography and rain events as variables to predict soil N2O emissions at the field scale. We measured soil N2O emissions 11 times over the course of one growing season from 65 locations within an agricultural field exhibiting microtopography. We found that the topographic indices best predicting soil N2O emissions varied by date, with soil properties as consistently poor predictors. Large rain events (>30 mm) led to an N2O hot moment only in the early summer and not in the cool spring or later in the summer when crops were at peak growth and likely had high evapotranspiration rates. In a laboratory experiment, we demonstrated that low heterotrophic respiration rates at cold temperatures slowly depleted soil dissolved O2, thus suppressing denitrification over the 2–3 day timescale typical of field ponding. Our findings show that topographic depressions do not consistently act as N2O hot spots and that rainfall does not consistently trigger N2O hot moments. We assert that the spatiotemporal variation in soil N2O emissions is not always characterized by predictable hot spots or hot moments and that controls on this variation change depending on environmental conditions.
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Modeling Hydrologically Mediated Hot Moments of Transient Anomalous Diffusion in Aquifers Using an Impulsive Fractional‐Derivative Equation
Abstract Hydrologically mediated hot moments (HM‐HMs) of transient anomalous diffusion (TAD) denote abrupt shifts in hydraulic conditions that can profoundly influence the dynamics of anomalous diffusion for pollutants within heterogeneous aquifers. How to efficiently model these complex dynamics remains a significant challenge. To bridge this knowledge gap, we propose an innovative model termed “the impulsive, tempered fractional advection‐dispersion equation” (IT‐fADE) to simulate HM‐HMs of TAD. The model is approximated using an L1‐based finite difference solver with unconditional stability and an efficient convergence rate. Application results demonstrate that the IT‐fADE model and its solver successfully capture TAD induced by hydrologically trigged hot phenomena (including hot moments and hot spots) across three distinct aquifers: (a) transient sub‐diffusion arising from sudden shifts in hydraulic gradient within a regional‐scale alluvial aquifer, (b) transient sub‐ or super‐diffusion due to convergent or push‐pull tracer experiments within a local‐scale fractured aquifer, and (c) transient sub‐diffusion likely attributed to multiple‐conduit flow within an intermediate‐scale karst aquifer. The impulsive terms and fractional differential operator integrated into the IT‐fADE aptly capture the ephemeral nature and evolving memory of HM‐HMs of TAD by incorporating multiple stress periods into the model. The sequential HM‐HM model also characterizes breakthrough curves of pollutants as they encounter hydrologically mediated, parallel hot spots. Furthermore, we delve into discussions concerning model parameters, extensions, and comparisons, as well as impulse signals and the propagation of memory within the context of employing IT‐fADE to capture hot phenomena of TAD in aquatic systems.
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- Award ID(s):
- 2305141
- PAR ID:
- 10492792
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 60
- Issue:
- 3
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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