Coupled Cycling of Dissolved Organic Nitrogen and Carbon in a Forest Stream

Abstract

Abstract. Dissolved organic nitrogen (DON) is an abundant but poorly understood pool of N in many ecosystems. We assessed DON cycling in a N-limited headwater forest stream via whole-ecosystem additions of dissolved inorganic nitrogen (DIN) and labile dissolved organic matter (DOM), hydrologic transport and biogeochemical modeling, and laboratory experiments with native sediments. We sampled surface and subsurface waters to understand how interaction among hydrologic exchange, DIN, DON, and dissolved organic carbon (DOC) influence stream N losses at summer baseflow. Added DON was taken up rapidly from the water column at rates exceeding DOC and DIN. A significant fraction of this DON was mineralized and nitrified. Combined DON and NO 3 -N uptake lengths resulted in spiraling lengths of ϳ210 m, suggesting the potential for multiple transformations of labile N loads within catchment boundaries. Simultaneous addition of DIN increased DOM uptake, but more so for C, resulting in an upward shift in the C:N ratio of uptake. Sediment incubations also showed a strong biotic influence on DOC and DON dynamics. Despite efficient uptake of added DOM, background DON and high molecular mass DOC concentrations increased downstream, resulting in higher DOM loads than could be accounted for by groundwater discharge and suggesting net release of less bioavailable forms from the channel/hyporheic zone. At the same time, subsurface DOM was characterized by very low C:N ratios and a disproportionately large DON pool despite rapid hydrologic mixing with dilute and high C:N ratio surface waters. Analysis of expected DON loads from conservative hyporheic fluxes indicated that watershed losses of DON would have been seven times greater in the absence of apparent benthic demand, suggesting tight internal cycling of subsurface DON. Our study further demonstrates the potential for significant transformation of N in headwater streams before export to downstream ecosystems.

Fluorescence spectroscopy opens new windows into dissolved organic matter dynamics in freshwater ecosystems: A review

“…changes indicate that certain fractions of DOM were selectively removed while others remain. Moreover, protein-like fluorescence was removed at a significantly greater rate than bulk concentrations of DOC and DON, consistent with the idea of different pools comprising DOM turnover at different rates (Brookshire et al 2005). Fluorescence characterization of DOM combined with traditional in-stream tracer releases to measure DOM uptake could therefore help elucidate the role of DOM pools of varying reactivity in supporting heterotrophic metabolism in aquatic ecosystems.…”

“…It is now clear that DON plays an important role in terrestrial N cycling (Neff et al 2003) and as new studies examine the cycling of DON in aquatic ecosystems, it will be important to develop a more complex view of DON as a mixture of labile and recalcitrant compounds that have substantial and very different roles (Brookshire et al 2005). In this context, tracing the dynamics of DOM fluorescence, particularly protein-like components (e.g., tyrosine-and tryptophan-like), could help elucidate the relative role of readily available and recalcitrant pools of both DOC and DON in supporting stream metabolism.…”

Source, biogeochemical cycling, and fluorescence characteristics of dissolved organic matter in an agro‐urban estuary

“…0.05) but negatively correlated with tyrosine-like fluorescence (r 2 5 0.50, p , 0.001). These results have important implications for DOM cycling in estuaries because they provide evidence that multiple pools of DON exist in estuaries, similar to fast and slow cycling pools of DON observed in freshwater ecosystems (Brookshire et al 2005). For instance, amino acids typically account for a small but dynamic fraction of DOM in surface waters (Volk et al 1997) and bulk analyses of DOC and DON, which reflect the combined result of both production and removal processes, cannot detect many C and N transformations.…”

Changes in discharge and solute dynamics between hillslope and valley-bottom intermittent streams

“…This lack of pattern complicates drawing any conclusion about factors driving stream DON dynamics between these two catchments. The DON pool in stream ecosystems is still poorly understood; yet, previous research acknowledges that it may be composed of a varying proportion of refractory and labile internally recycled DON (Brookshire et al, 2005;Lutz et al, 2012). The most recalcitrant fraction of DON may be intrinsically linked to DOC, so that C and N organic solutes may show similar patterns.…”