Small headwater floodplain wetlands are thought to have a disproportionate effect on controlling nitrogen inputs to downstream water bodies but quantifying their effect at a regional scale remains challenging due to wetland-to-wetland variation in both geophysical form and level of anthropogenic stress. In this study, we investigate variation in nitrogen cycling of headwater floodplain wetlands based on soil type and relative wetland condition, representing these two factors. We ask whether nitrogen pools and fluxes from surface (0 to 10 cm) and subsurface (~ 50 cm) soils vary across headwater wetlands of low and high condition and across three hydric soil types(Atkins, Holly, Udifluvent-Dystrochrepts)in a Latin square design. We quantified net nitrification (-1 to 5 µg N· kg soil-1· day-1), net ammonification (-7 to 70 µg N · kg soil-1· day-1), and potential denitrification rates in surface soils during the fall (119 to 17,046 µg N · kg soil-1· day-1). We also quantified potential denitrification rates in subsurface soils at 4 of the 6 sites in fall and spring (5 to 177 µg N · kg soil-1· day-1). Supplemental data included a site-level physical characterization and plot-level vegetation surveys.
Results/Conclusions
Soil TN was significantly higher in high-condition sites, and the high-condition Holly site had significantly higher soil TN than the high-condition Atkins and UD sites. Potential denitrification in surface soils did not vary with condition but did vary among soil types with higher mean denitrification rates at Holly sites compared to the Atkins sites and the low condition UD site. Potential denitrification in surface soils were also positively correlated with the amount of 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 didn’t 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 in surface and subsurface soils were not correlated with each other but were correlated with their respective ammonium pools. Given the importance of soil type and hydrology, future studies are needed to evaluate denitrification rates across a wider range of soil physical conditions. This work also highlights the need to characterize both surface and subsurface denitrification rates to accurately quantify the role of headwater wetlands in nitrogen removal at regional scales where soil properties greatly vary.