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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.

 

Introduction Alliaria petiolata (garlic mustard), an invasive forest herb in North America, often alters nutrient availability in its non-native ecosystems, but the mechanisms driving these changes have yet to be determined. We hypothesized three potential mechanisms through which garlic mustard could directly influence soil nitrogen (N) cycling: by increasing soil pH, by modifying soil microbial community composition, and by altering nutrient availability through litter inputs. Materials and methods To test these hypotheses, we evaluated garlic mustard effects on soil pH and other soil properties; fungal and prokaryotic (bacterial and archaeal) community composition; and soil N cycling rates (gross N mineralization and nitrification rates, microbial N assimilation rates, and nitrification- versus denitrification-derived nitrous oxide fluxes); and we assessed correlations among these variables. We collected soil samples from garlic mustard present, absent, and removed treatments in eight forests in central Illinois, United States, during the rosette, flowering, and senescence phenological stages of garlic mustard life cycle. Results We found that garlic mustard increased soil pH, altered fungal and prokaryotic communities, and increased rates of N mineralization, nitrification, nitrification-derived net N2O emission. Significant correlations between soil pH and microbial community composition suggest that garlic mustard effects on soil pH could both directly and indirectly influence soil N cycling rates. Discussion Correspondence of gross rates of N mineralization and nitrification with microbial community composition suggest that garlic mustard modification of soil microbial communities could directly lead to changes in soil N cycling. We had expected that early season, nutrient-rich litter inputs from mortality of young garlic mustard could accelerate gross N mineralization and microbial N assimilation whereas late season, nutrient poorer litter inputs from senesced garlic mustard could suppress N mineralization, but we did not observe these patterns in support of the litter input mechanism. Together, our results elucidate how garlic mustard effects on soil pH and microbial community composition can accelerate soil N cycling to potentially contribute to the invasion success of garlic mustard. 

Topographic depressions in upland soils experience anaerobic conditions conducive for iron (Fe) reduction following heavy rainfall. These depressional areas can also accumulate reactive Fe compounds, carbon (C), and nitrate, creating potential hot spots of Fe-mediated carbon dioxide (CO2) and nitrous oxide (N2O) production. While there are multiple mechanisms by which Fe redox reactions can facilitate CO2 and N2O production, it is unclear what their cumulative effect is on CO2 and N2O emissions in depressional soils under dynamic redox. We hypothesized that Fe reduction and oxidation facilitate greater CO2 and N2O emissions in depressional compared to upslope soils in response to flooding. To test this, we amended upslope and depressional soils with Fe(II), Fe(III), or labile C and measured CO2 and N2O emissions in response to flooding. We found that depressional soils have greater Fe reduction potential, which can contribute to soil CO2 emissions during flooded conditions when C is not limiting. Additionally, Fe(II) addition stimulated N2O production, suggesting that chemodenitrification may be an important pathway of N2O production in depressions that accumulate Fe(II). As rainfall intensification results in more frequent flooding of depressional upland soils, Fe-mediated CO2 and N2O production may become increasingly important pathways of soil greenhouse gas emissions.