Variation in surface and subsurface nitrogen cycling in headwater floodplain wetlands due to soil type and wetland condition

Headwater wetlands intercept, store, and transform nitrogen inputs from the landscape. However, uncertainties in rate estimates for processes such as denitrification in surface and subsurface soils hinder our ability to quantitatively predict the downstream water quality benefits of these headwater systems. We measured nitrogen pools and fluxes from surface (0 to 10 cm) and subsurface soils (~ 50 cm) of six headwater wetlands of low and high ecological condition (based on anthropogenic stressors) across three hydric soil types (Atkins, Holly, Udifluvent-Dystrochrept Complex). We quantified net nitrification (− 0.027 to 0.113 mg N kg soil⁻¹ day⁻¹), net ammonification (− 0.159 to 1.683 mg N kg soil⁻¹ day⁻¹), and potential denitrification rates (0.12 to 17.05 mg N kg soil⁻¹ day⁻¹) in surface soils during the fall. We also quantified potential denitrification rates in subsurface soils at 4 of the 6 sites in fall and spring (0.005 to 0.177 mg N kg soil⁻¹ day¹). Potential denitrification rates in surface soils varied with soil type, and rates were positively correlated with ground cover and the percentage of time the water level was between 0 and 10 cm below the surface. Potential denitrification rates of subsurface soil did not vary between condition groups or seasons; however, nitrous oxide yield was higher in the spring compared to the fall. At the plot scale, potential denitrification rates in surface soils were correlated with ammonification rates, and rates in subsurface soils were correlated with conductivity and ammonium levels. This work demonstrates the variability in surface and subsurface soil nitrogen cycling and highlights the need to investigate nutrient dynamics at multiple soil depths to accurately quantify the role of headwater wetlands in nitrogen removal at regional scales where soil properties are highly variable.