Joachim Audet

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.

Fresh waters in general and ponds in particular are biogeochemical hot spots and as such are sources of greenhouse gas (GHG) emissions, especially methane (CH₄) and carbon dioxide (CO₂). Several studies have shown that thermal stratification of the water column is a common characteristic of ponds, but how this process influences GHG dynamics is less well studied. We tracked thermal stratification and the associated hypoxia in six ponds across the growing season and measured dissolved CH₄ and CO₂ in surface and bottom waters, along with CH₄ ebullition, every two weeks. The results showed that all ponds stratified during the growing season and that thermal stratification often, but not always, led to bottom-water hypoxia. Where hypoxia occurred, bottom-water CH₄ was approximately 16 times higher (median 3.05 vs 0.19 mg C L⁻¹), CO₂ about 3 times higher (11.6 vs 4.03 mg C L⁻¹) and CH₄ ebullition was about 3.4 times higher than in oxic conditions. In contrast, surface-water GHG concentrations showed no significant difference between periods with and without bottom-water hypoxia. Bottom-water temperatures were not significantly different between hypoxic and oxic states, indicating oxygen availability, rather than temperature, dominated GHG buildup and ebullition. Stratification and the resulting hypoxia enhance CH₄ ebullition and promote high concentrations of GHGs in bottom waters, which may be released in small or larger pulses as the water column mixes partially or fully. In summary this study demonstrates that thermal stratification, by inducing bottom-water hypoxia, strongly influences the GHG dynamics across a range of timescales in ponds.

Fresh waters in general and ponds in particular are biogeochemical hot spots and as such are sources of greenhouse gas (GHG) emissions, especially methane (CH₄) and carbon dioxide (CO₂). Several studies have shown that thermal stratification of the water column is a common characteristic of ponds, but how this process influences GHG dynamics is less well studied. We tracked thermal stratification and the associated hypoxia in six ponds across the growing season and measured dissolved CH₄ and CO₂ in surface and bottom waters, along with CH₄ ebullition, every two weeks. The results showed that all ponds stratified during the growing season and that thermal stratification often, but not always, led to bottom-water hypoxia. Where hypoxia occurred, bottom-water CH₄ was approximately 16 times higher (median 3.05 vs 0.19 mg C L⁻¹), CO₂ about 3 times higher (11.6 vs 4.03 mg C L⁻¹) and CH₄ ebullition was about 3.4 times higher than in oxic conditions. In contrast, surface-water GHG concentrations showed no significant difference between periods with and without bottom-water hypoxia. Bottom-water temperatures were not significantly different between hypoxic and oxic states, indicating oxygen availability, rather than temperature, dominated GHG buildup and ebullition. Stratification and the resulting hypoxia enhance CH₄ ebullition and promote high concentrations of GHGs in bottom waters, which may be released in small or larger pulses as the water column mixes partially or fully. In summary this study demonstrates that thermal stratification, by inducing bottom-water hypoxia, strongly influences the GHG dynamics across a range of timescales in ponds.

Fresh waters in general and ponds in particular are biogeochemical hot spots and as such are sources of greenhouse gas (GHG) emissions, especially methane (CH4) and carbon dioxide (CO2). Several studies have shown that thermal stratification of the water column is a common characteristic of ponds, but how this process influences GHG dynamics is less well studied. We tracked thermal stratification and the associated hypoxia in six ponds across the growing season and measured dissolved CH4 and CO2 in surface and bottom waters, along with CH4 ebullition, every two weeks. The results showed that all ponds stratified during the growing season and that thermal stratification often, but not always, led to bottom-water hypoxia. Where hypoxia occurred, bottom-water CH4 was approximately 16 times higher (median 3.05 vs 0.19 mg C L−1), CO2 about 3 times higher (11.6 vs 4.03 mg C L−1) and CH4 ebullition was about 3.4 times higher than in oxic conditions. In contrast, surface-water GHG concentrations showed no significant difference between periods with and without bottom-water hypoxia. Bottom-water temperatures were not significantly different between hypoxic and oxic states, indicating oxygen availability, rather than temperature, dominated GHG buildup and ebullition. Stratification and the resulting hypoxia enhance CH4 ebullition and promote high concentrations of GHGs in bottom waters, which may be released in small or larger pulses as the water column mixes partially or fully. In summary this study demonstrates that thermal stratification, by inducing bottom-water hypoxia, strongly influences the GHG dynamics across a range of timescales in ponds.

Peatland drainage for agricultural purposes has transformed wetlands into major nutrient sources, but rewetting offers potential for nutrient retention and climate mitigation. We quantified hydrological and nutrient fluxes in a 14.5 ha former pump-drained fen (Strande Enge, Denmark) across 1 year before and 2 years after rewetting. Restoration involved removing pumping infrastructure and reconnecting upstream catchments with the aim to restore natural flow paths. Prior to rewetting, the site exhibited high hydraulic loading (5904 mm yr ⁻¹ ) dominated by groundwater inputs and acted as a strong nutrient source, with net losses of total nitrogen (TN: 123 kg ha ⁻¹ yr ⁻¹ ), total phosphorus (TP: 23 kg ha ⁻¹ yr ⁻¹ ), and total organic carbon (TOC: 1336 kg ha ⁻¹ yr ⁻¹ ). Following rewetting, hydraulic loading increased more than fourfold (24,258–27,160 mm yr ⁻¹ ), and the system shifted to a flow-through wetland dominated by surface water inputs. Despite substantially higher nutrient loads after rewetting, the fen became a large sink for TN (209–242 kg ha ⁻¹ yr ⁻¹ ; 21% retention) and retained 44–72% of soluble reactive phosphorus. However, although their losses declined, particulate phosphorus and organic matter were still exported, particularly during high-flow events. At catchment scale, rewetting reduced the yearly TN export to the downstream lake by 27–38%, while the annual TP export varied strongly after rewetting (12% reduction in year 1 and 74% increase in year 2). These findings demonstrate that rewetting of pumped fens can rapidly transform nutrient source hotspots into effective nitrogen sinks, but additional measures may be needed to control particulate losses of nutrients and organic carbon, if these prove to persist over time. Overall, our results show that rewetting can reduce nutrient losses, including TOC losses, thereby contributing to mitigation of aquatic pollution and helping preserve peatland carbon stocks.

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.

Peatlands store a substantial fraction of the global soil carbon pool. Widespread drainage and land-use change have accelerated peat decomposition, while current restoration efforts aim to slow or reverse peat carbon oxidation and associated greenhouse gas emissions through rewetting. Understanding when and to what extent rewetting restores carbon sequestration and long-term peat accumulation remains a key scientific and management challenge.Over recent decades, substantial progress has been made in identifying the biogeochemical controls on peat carbon turnover in rewetted systems. Water table dynamics, hydrological connectivity, redox conditions, substrate availability, nutrient status, and vegetation composition jointly regulate microbial processes driving organic matter decomposition and carbon fluxes. Yet, key questions remain: what really drives carbon turnover in peatlands? Is it “the overriding role of the water table”, the “iron gate” of mineral interactions, or the “iron wheel”? Could there even be a single enzyme controlling global carbon storage (Wen et al., 2019)? These questions highlight how peat carbon cycling defies simple explanations and directly challenges long-standing paradigms. Classical concepts emphasizing intrinsic substrate recalcitrance, single inhibitory controls (e.g., phenolics or water table position), or strictly separated aerobic–anaerobic microbial pathways fail to capture the complexity of carbon stabilization and turnover observed in rewetted peatlands (Zak et al., 2019). Instead, emerging evidence points to a network of interacting, context-dependent processes, including microbial community turnover, mineral–organic interactions, and dynamic redox condition changes, underpinning peat carbon persistence.Yet, these mechanisms are typically studied at micro- to plot scales, while restoration success and climate feedbacks are evaluated at ecosystem to landscape scales, posing persistent challenges for upscaling. In this contribution, we synthesize current insights into carbon cycling in rewetted riparian peatlands by explicitly linking microbial and biogeochemical controls on carbon decomposition with restoration approaches aimed at managing carbon fluxes. Emphasis is placed on spatial and temporal heterogeneity in peat properties, hydrology, and microbial functioning, and on how this variability propagates uncertainty in carbon balance assessments and model predictions.By integrating process-based understanding with measurements and modeling perspectives, both recent advances and remaining knowledge gaps in predicting peat carbon cycling under restoration will be highlighted. Re-assessing prevailing paradigms and strengthening cross-scale linkages are essential for designing effective rewetting strategies. ReferencesWen, Y., Zang, H., Ma, Q., Evans, C. D., Chadwick, D. R., & Jones, D. L. (2019). Is the ‘enzyme latch’or ‘iron gate’the key to protecting soil organic carbon in peatlands?. Geoderma, 349, 107-113.Zak, D., Roth, C., Unger, V., Goldhammer, T., Fenner, N., Freeman, C., & Jurasinski, G. (2019). Unraveling the importance of polyphenols for microbial carbon mineralization in rewetted riparian peatlands. Frontiers in Environmental Science, 7, 147.

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.