Small, Older Water Fraction Controls Dissolved Inorganic Nitrogen Concentrations in Stream Water during the Non-Growing Season in a Forested Headwater Catchment

Recent work has shown that stream water dissolved inorganic nitrogen (DIN) concentrations in agricultural catchments are linked to the fraction of discharge younger than some threshold (young water). Whether a similar linkage exists in forested headwater catchments remains an open question. To test this, we paired weekly stream water DIN concentration data from Watershed 3 (WS 3) in Hubbard Brook Experimental Forest in New Hampshire, USA with predicted dynamic daily transit time distributions from the output of a published model for WS 3 (Benettin et al., 2015) for the time period November 2006 to November 2010. Given that stream water DIN concentrations are typically below the minimum detection limit during the growing season, we limited our analysis to the non-growing season (October 15 to May 15). Using an iterative approach, we calculated the Spearman rank correlation coefficient () between stream water DIN concentration and the fraction of water between variable upper and lower limits of the transit time distributions. We found that stream water DIN concentration had the strongest overall correlation with the fraction of discharge between 10 and 230 days old ( = -0.606, P < 0.001), which constituted approximately 60 to 80% of discharge. This result is in contrast to recent work in agricultural catchments, which have generally shown a positive relationship between younger water and stream water DIN concentrations. Stream water DIN concentration in WS 3 had the strongest positive correlation with the fraction of discharge between 232 and 358 days old ( = 0.552, P < 0.001), despite this fraction only constituting between approximately 3 and 16% of discharge. This suggests that a small amount of older water plays a key role in controlling stream water DIN concentrations in WS 3 during the non-growing season. Future work will explore whether other forested headwater catchments exhibit similar behavior.