Nitrous oxide dynamics across nitrogen and pH gradients in headwater streams

Abstract. 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.