Reach-scale Denitrification and Nitrogen Concentrations in Six Streams Draining Grassland and Cropland Landscapes

Abstract

Denitrification is a critical microbial process that removes bioavailable nitrogen (N) in natural and human-altered ecosystems. Methods for measuring denitrification in aquatic ecosystems are fairly studied. However, the understanding of ecosystem metabolism and denitrification factors in agriculturally influenced streams is limited. We wanted to evaluate the connection between nitrogen (N) cycling and ecosystem metabolism in headwater streams affected by land use due to the increase of agricultural N pollution and its negative impact on downstream water quality. Here, we quantify N concentrations and denitrification in six headwater streams draining contrasting land use watersheds using the open-channel single station approach. We compared two contrasting watersheds; (a) cropland land use, representing the excessive agricultural N pollution to streams, and (b) grassland land use, representing original land use type prior to agricultural N pollution. To understand in-stream N processes and ecosystem metabolism, we analyzed diel patterns of nitrate (NO3-) concentrations and dissolved gasses (O2, Ar, N2, N2O) during the summer of 2019. We estimated open-channel reach-scale denitrification, gross primary production (GPP), and ecosystem respiration (ER). Streams draining primarily cropland had higher NO3- concentrations (4.2 NO3--N mg L-1 +/- 0.11) associated with higher metabolism rates. Higher N concentrations drove higher denitrification rates among cropland-dominated streams (mean denitrification rates [0.31 g/m2 day +/- 0.4] in cropland-dominated streams vs. [0.01 g/m2 day +/- 0.01] in grassland-dominated streams). Overall, the excess nutrients from the agricultural landscapes resulted in higher nitrate concentrations and rates of metabolism compared to the grassland landscapes. Estimates of denitrification and stream metabolism are needed from underrepresented systems to aid in the quantification of nitrogen fluxes, locally and globally.