Topography-Mediated Soil Moisture Controls Forest Hillslope-Scale NO and N2O Emissions.

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 (N2O); 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 N2O 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 + N2O losses. Annual emissions averaged 0.2 kg NO-N ha-1 (range: 0.1-0.3) and 1.0 kg N2O-N ha-1 (range: 0.7-4.9), with N2O 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 N2O 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.