Topography‐Mediated Soil Moisture Controls Forest Hillslope‐Scale NO and N 2 O Emissions

Abstract

Temperate forests are key terrestrial carbon sinks, yet their capacity to sequester carbon is often limited by nitrogen (N), a nutrient whose availability is declining in many ecosystems. Among factors contributing to declining N availability, climate‐driven changes in soil hydrology could force N loss via the emission of both nitric oxide (NO) and nitrous oxide (N 2 O); however, forecasting these losses is challenging because they are regulated by soil moisture, a factor that regulates microbial activity and substrate availability but varies significantly across space and time. Here, we ask: how do topography‐driven soil moisture gradients and changes in seasonality (e.g., spring–thaw cycles) mediate hillslope‐scale N emissions? We measured over 2 years of high‐resolution in situ NO and N 2 O fluxes from 16 automated chambers deployed along a topographic gradient in a temperate forest to show that soil moisture gradients governed spatial and temporal patterns of soil N emissions. These gradients produced tradeoffs in process controls, whereby temperature regulated N emissions in drier upper positions, giving way to soil moisture regulating microbial pathways and emissions in wetter downslope positions. We then used these relationships among soil moisture, temperature, and N availability to develop models for predicting hillslope‐scale NO + N 2 O losses. Annual emissions averaged 0.2 kg NO‐N ha −1 (range: 0.1–0.3) and 1.0 kg N 2 O‐N ha −1 (range: 0.7–4.9), with N 2 O showing a stronger response to moisture‐driven changes than NO. The spring freeze–thaw period accounted for 15%–26% of NO and 24%–58% of N 2 O annual emissions, with the highest emissions measured consistently at lower topographic positions. These findings establish topography‐mediated hydrology as a primary control of forest soil N dynamics and gaseous N emissions, reducing uncertainty in forecasts of hillslope‐scale N losses under a changing climate.

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