Nicole Wrage-Mönnig

Headwater streams in agricultural landscapes can contribute substantially to nitrous oxide (N ₂ O) emissions, yet the environmental controls on stream N ₂ O dynamics remain poorly resolved, particularly in systems with low pH. We investigated 72 Danish headwater streams spanning broad gradients in pH (5.0–8.8), land use, and soil type to identify the main drivers of N ₂ O variability. Nitrate (NO3-) was the strongest predictor of N ₂ O saturation, and its positive association with N ₂ O intensified under acidic conditions according to linear mixed models. In the same model, ammonium, dissolved organic carbon, and stream depth also showed significant but weaker positive relationships with N ₂ O. Spatial differences among streams explained considerably more variation than seasonal or regional patterns, underscoring the dominance of local factors. Streams with pH < 6 consistently exhibited higher N ₂ O saturation, and generalized additive modelling indicated a marked decline in N ₂ O levels beginning near pH 6. Despite generally high N ₂ O saturation, approximately 9 % of observations displayed undersaturation, which occurred mainly in streams with low NO3- concentrations and across all seasons. Our results indicate that acidic, weakly buffered catchments may enhance in-stream N ₂ O accumulation even at moderate nitrogen levels. These findings highlight the need to consider pH-related controls when assessing N ₂ O dynamics in freshwater networks and when designing mitigation strategies for agricultural landscapes.

Headwater streams in agricultural landscapes can contribute substantially to nitrous oxide (N₂O) emissions, yet the environmental controls on stream N₂O dynamics remain poorly resolved, particularly in systems with low pH. We investigated 72 Danish headwater streams spanning broad gradients in pH (5.0 - 8.8), land use, and soil type to identify the main drivers of N₂O variability. Nitrate (NO₃⁻) was the strongest predictor of N₂O saturation, and its positive association with N₂O intensified under acidic conditions according to linear mixed models. Ammonium, dissolved organic carbon, and stream depth also showed significant but weaker positive relationships with N₂O. Spatial differences among streams explained considerably more variation than seasonal or regional patterns, underscoring the dominance of local factors. Streams with pH < 6 consistently exhibited higher N₂O saturation, and generalized additive modelling indicated a marked decline in N₂O levels beginning near pH 6. Despite generally high N₂O saturation, approximately 9 % of observations displayed undersaturation, which occurred mainly in streams with low NO₃⁻ concentrations and across all seasons. Our results indicate that acidic, weakly buffered catchments may enhance in‑stream N₂O accumulation even at moderate nitrogen levels. These findings highlight the need to consider pH‑related controls when assessing N₂O dynamics in freshwater networks and when designing mitigation strategies for agricultural landscapes.

Headwater streams in agricultural landscapes can contribute substantially to nitrous oxide (N₂O) emissions, yet the environmental controls on stream N₂O dynamics remain poorly resolved, particularly in systems with low pH. We investigated 72 Danish headwater streams spanning broad gradients in pH (5.0 - 8.8), land use, and soil type to identify the main drivers of N₂O variability. Nitrate (NO₃⁻) was the strongest predictor of N₂O saturation, and its positive association with N₂O intensified under acidic conditions according to linear mixed models. Ammonium, dissolved organic carbon, and stream depth also showed significant but weaker positive relationships with N₂O. Spatial differences among streams explained considerably more variation than seasonal or regional patterns, underscoring the dominance of local factors. Streams with pH < 6 consistently exhibited higher N₂O saturation, and generalized additive modelling indicated a marked decline in N₂O levels beginning near pH 6. Despite generally high N₂O saturation, approximately 9 % of observations displayed undersaturation, which occurred mainly in streams with low NO₃⁻ concentrations and across all seasons. Our results indicate that acidic, weakly buffered catchments may enhance in‑stream N₂O accumulation even at moderate nitrogen levels. These findings highlight the need to consider pH‑related controls when assessing N₂O dynamics in freshwater networks and when designing mitigation strategies for agricultural landscapes.

Headwater streams in agricultural landscapes can contribute substantially to nitrous oxide (N₂O) emissions, yet the environmental controls on stream N₂O dynamics remain poorly resolved, particularly in systems with low pH. We investigated 72 Danish headwater streams spanning broad gradients in pH (5.0 - 8.8), land use, and soil type to identify the main drivers of N₂O variability. Nitrate (NO₃⁻) was the strongest predictor of N₂O saturation, and its positive association with N₂O intensified under acidic conditions according to linear mixed models. Ammonium, dissolved organic carbon, and stream depth also showed significant but weaker positive relationships with N₂O. Spatial differences among streams explained considerably more variation than seasonal or regional patterns, underscoring the dominance of local factors. Streams with pH < 6 consistently exhibited higher N₂O saturation, and generalized additive modelling indicated a marked decline in N₂O levels beginning near pH 6. Despite generally high N₂O saturation, approximately 9 % of observations displayed undersaturation, which occurred mainly in streams with low NO₃⁻ concentrations and across all seasons. Our results indicate that acidic, weakly buffered catchments may enhance in‑stream N₂O accumulation even at moderate nitrogen levels. These findings highlight the need to consider pH‑related controls when assessing N₂O dynamics in freshwater networks and when designing mitigation strategies for agricultural landscapes.

Headwater streams in agricultural landscapes can contribute substantially to nitrous oxide (N₂O) emissions, yet the environmental controls on stream N₂O dynamics remain poorly resolved, particularly in systems with low pH. We investigated 72 Danish headwater streams spanning broad gradients in pH (5.0 - 8.8), land use, and soil type to identify the main drivers of N₂O variability. Nitrate (NO₃⁻) was the strongest predictor of N₂O saturation, and its positive association with N₂O intensified under acidic conditions according to linear mixed models. Ammonium, dissolved organic carbon, and stream depth also showed significant but weaker positive relationships with N₂O. Spatial differences among streams explained considerably more variation than seasonal or regional patterns, underscoring the dominance of local factors. Streams with pH < 6 consistently exhibited higher N₂O saturation, and generalized additive modelling indicated a marked decline in N₂O levels beginning near pH 6. Despite generally high N₂O saturation, approximately 9 % of observations displayed undersaturation, which occurred mainly in streams with low NO₃⁻ concentrations and across all seasons. Our results indicate that acidic, weakly buffered catchments may enhance in‑stream N₂O accumulation even at moderate nitrogen levels. These findings highlight the need to consider pH‑related controls when assessing N₂O dynamics in freshwater networks and when designing mitigation strategies for agricultural landscapes.

Headwater streams in agricultural landscapes can contribute substantially to nitrous oxide (N₂O) emissions, yet the environmental controls on stream N₂O dynamics remain poorly resolved, particularly in systems with low pH. We investigated 72 Danish headwater streams spanning broad gradients in pH (5.0 - 8.8), land use, and soil type to identify the main drivers of N₂O variability. Nitrate (NO₃⁻) was the strongest predictor of N₂O saturation, and its positive association with N₂O intensified under acidic conditions according to linear mixed models. Ammonium, dissolved organic carbon, and stream depth also showed significant but weaker positive relationships with N₂O. Spatial differences among streams explained considerably more variation than seasonal or regional patterns, underscoring the dominance of local factors. Streams with pH < 6 consistently exhibited higher N₂O saturation, and generalized additive modelling indicated a marked decline in N₂O levels beginning near pH 6. Despite generally high N₂O saturation, approximately 9 % of observations displayed undersaturation, which occurred mainly in streams with low NO₃⁻ concentrations and across all seasons. Our results indicate that acidic, weakly buffered catchments may enhance in‑stream N₂O accumulation even at moderate nitrogen levels. These findings highlight the need to consider pH‑related controls when assessing N₂O dynamics in freshwater networks and when designing mitigation strategies for agricultural landscapes.

Denitrification is the key process leading to production and loss of nitrogen gases from soils. Its main drivers are N availability and soil water content, but interactions with other elements, such as carbon and phosphorus, can also influence N₂O formation. So far, robust information on the effects of P and the historical context of P addition on N₂O sources remains limited. To address this knowledge gap, we conducted a mesocosm chamber experiment using isotopic approaches to investigate N transformations and N₂O sources following P fertilizer addition in soils with varying histories of P fertilization (low and high P). Differences in long-term fertilization affected C, N, and P availability as well as microbial community composition and nutrient cycling processes. Initially, microbes in both soils were C-limited with slightly higher C availability and microbial respiration in high P soils. Low P availability in low P soil did not restrict denitrification. In contrast, long-term P-unfertilized soil had higher N₂O losses compared to high P soil, which were further stimulated with P addition. Glucose addition alleviated C limitation and strongly promoted microbial growth and respiration, but did not affect N₂O emissions among treatments. Bacterial denitrification and nitrifier denitrification were the main N₂O forming processes, while dissimilatory nitrate reduction to ammonium (DNRA) contributed to NO₃ ⁻ reduction, but only slightly to N₂O formation.

Measuring soil dinitrogen (N2) emissions is notoriously challenging under field conditions. Hence, N2 emissions represent a significant uncertainty in the nitrogen mass balance of terrestrial ecosystems. The 15N gas flux (15NGF) method is the only method currently available for directly quantifying N2 emissions in situ. However, this method has rarely undergone independent validation under field conditions. In this study, our objectives were to: (1) Quantify N2 emissions and their role in the fertilizer N mass balance of a wheat rotation using the 15NGF method (2) Verify the obtained quantities of N2 emissions using a mass balance approach and (3) Verify the temporal N2 emission dynamics at the soil-atmosphere interface using vertical soil profiles of 15N2 enrichment.To achieve these objectives, we grew winter wheat in lysimeters and applied 15N enriched mineral fertilizers via fertigation in three doses (sum 170 kg N ha-1). We then analyzed gaseous (NH3, N2O, N2) and hydrological N losses, as well as fertilizer N fates in plant and soil, and 15N2 enrichment in soil air.Our results showed that N2 emissions directly measured using the 15NGF method amounted to 30 ± 4 kg N ha-1, which was equivalent to 18 ± 3 % of the applied fertilizer N. These measurements agreed with unrecovered fertilizer N obtained from the 15N fertilizer mass balance, although the latter had large inherent uncertainty (21 ± 21 kg N ha-1). N2O emissions, however, were negligible (0.14 ± 0.02 kg N ha-1). The temporal variability of measured N2 emissions after fertilizer additions was generally well explained by 15N2 enrichment in soil gas.Overall, we provide independent validation of the 15NGF method in measuring N2 emissions in the field and highlight the significant role of these emissions in the nitrogen balance of crop systems. Our data also suggest that soil gas measurements in combination with diffusion modeling could serve as an alternative method for quantifying N2 emissions. These results should encourage a wider application of the 15NGF method in order to improve our understanding of N2 emissions and reduce the current uncertainties in estimates of these emissions.

Streams and rivers have been highlighted as significant but poorly constrained sources of nitrous oxide (N2O), a greenhouse gas ≈300 times more potent than carbon dioxide. A large share of stream N2O emissions arises from the use of nitrogen (N) fertilizers by agriculture and therefore most of the research on N2O emissions from streams has focused on agricultural areas, especially on fertile calcareous soils having near-neutral pH.However, recent research suggests that streams located in regions having low pH (<5.5) and high iron content in soils may promote disproportionally high N2O emissions. We tested this hypothesis by investigating the drivers of N2O emissions in agricultural and natural streams located in regions of Denmark where soils with low pH and high iron contents are prevalent.We measured N2O emissions monthly for a year in 10 streams located in agricultural and natural areas. Furthermore, we also measured N2O emissions four times a year in 80 streams covering a broad gradient of land-use and soil properties.The preliminary results indicate that, within agricultural and natural areas, streams with low pH have higher emissions of N2O than those with higher pH. We will compare our results with the estimates calculated using the IPCC methodology and discuss the implications of our findings for national greenhouse gas inventories.