Dynamic Controls on Field‐Scale Soil Nitrous Oxide Hot Spots and Hot Moments Across a Microtopographic Gradient

Abstract

AbstractSoil 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.