Yearlong Impact of Buried Organic Carbon on Nitrate Retention in Stream Sediments

Carbon supply influences nitrogen transformation in ecosystems, but the longer‐term effects of buried organic carbon on nitrogen processing in stream sediments have been rarely addressed. The effects of buried particulate organic carbon (red maple leaves) on net nitrogen retention, net dissolved organic carbon (DOC) production, and pore water dissolved oxygen concentration were assessed for 1 yr in a nitrogen‐rich gaining stream (Emmons Creek, WI). Retention of nitrate and total dissolved nitrogen (TDN) and production of DOC were measured by comparing groundwater fluxes of nutrients at shallow and deeper depths in mesocosms inserted in the sediments. Buried leaves caused large increases in nitrate and TDN retention and DOC production relative to the control (combusted sand) that decreased in magnitude throughout the year. Nitrate and TDN retention occurred throughout the year in unmanipulated (ambient) sediments. Dissolved oxygen approached anoxia in most mesocosms containing buried leaves and ambient sediments, particularly early in the experiment. Collectively, these results indicate that buried leaves had persistent, diminishing effects on nitrate and TDN retention throughout a 1‐yr period. The TDN retention in the ambient sediments throughout the year suggests that the deep sediments in Emmons Creek are a nitrogen sink. Core Ideas Buried POC resulted in N retention in stream sediments in a yearlong experiment. N retention occurred throughout the year in unmanipulated (ambient) sediments. Nitrate retention was nonlinearly related to DO concentration. The results suggest that the deep sediments served as a nitrogen sink. Carbon addition to sediments may improve downstream water quality.