Hydrology and riparian forests drive carbon and nitrogen supply and DOC : NO₃⁻ stoichiometry along a headwater Mediterranean stream

Abstract

Abstract. In forest headwater streams, metabolic processes are predominately heterotrophic and depend on both the availability of carbon (C) and nitrogen (N) and a favourable C:N stoichiometry. In this context, hydrological conditions and the presence of riparian forests adjacent to streams can play an important, yet understudied role in determining dissolved organic carbon (DOC) and nitrate (NO3-) concentrations and DOC:NO3- molar ratios. Here, we aimed to investigate how the interplay between hydrological conditions and riparian forest coverage drives DOC and NO3- supply and DOC:NO3- stoichiometry in an oligotrophic headwater Mediterranean stream. We analysed DOC and NO3- concentrations and DOC:NO3- molar ratios during both base flow and storm flow conditions at three stream locations along a longitudinal gradient of increased riparian forest coverage. Further, we performed an event analysis to examine the hydroclimatic conditions that favour the transfer of DOC and NO3- from riparian soils to the stream during storms. Stream DOC and NO3- concentrations were generally low (overall averages ± SD were 1.0±0.6 mg C L−1 and 0.20±0.09 mg N L−1), although significantly higher during storm flow compared to base flow conditions in all three stream sites. Optimal DOC:NO3- stoichiometry for stream heterotrophic microorganisms (corresponding to DOC:NO3- molar ratios between 4.8 and 11.7) was prevalent at the midstream and downstream sites under both flow conditions, whereas C-limited conditions were prevalent at the upstream site, which had no surrounding riparian forest. The hydroclimatic analysis of storms suggested that large and medium storm events display a distinct mechanism of DOC and NO3- mobilization. In comparison to large storms, medium storm events showed limited hydrological responses that led to significantly lower increases in stream DOC and NO3- concentrations. During large storm events, different patterns of DOC and NO3- mobilization arise, depending on antecedent soil moisture conditions: drier antecedent conditions promoted rapid elevations of the riparian groundwater table, hydrologically activating a wider and shallower soil layer, and leading to relatively higher increases in stream DOC and NO3- concentrations compared to large storm events preceded by wet conditions. Our results suggest that (i) increased supply of limited resources during storms can potentially sustain in-stream heterotrophic activity during high flows, especially during large storm events preceded by dry conditions, and (ii) C-limited conditions upstream were overcome downstream, likely due to higher C inputs from riparian forests present at lower elevations. The contrasting spatiotemporal patterns in DOC and NO3- availability and DOC:NO3- stoichiometry observed at the studied stream suggest that groundwater inputs from riparian forests are essential for maintaining in-stream heterotrophic activity in oligotrophic, forest headwater catchments.

Hydrology Controls Dissolved Organic Carbon and Nitrogen Export and Post‐Storm Recovery in Two Arctic Headwaters

“…Across regions, most C‐Q studies describe the responses of individual constituents, such as DOC, dissolved inorganic nitrogen (DIN), or reactive phosphorus in temperate rivers (Fazekas et al., 2020; Khamis et al., 2021; Raymond & Saiers, 2010; Vaughan et al., 2017; Wagner et al., 2019; Zarnetske et al., 2018), as well as in permafrost‐influenced landscapes (Shogren et al., 2021; Webster et al., 2021). Efforts to capture multi‐solute response across seasons (Kincaid et al., 2020), land use and land cover (Fazekas et al., 2021; Wymore et al., 2021), and antecedent hydrologic conditions (Gorski & Zimmer, 2021; Ledesma et al., 2022) have significantly advanced applications of the C‐Q method by acknowledging that the transport of reactive solutes during storm events is inherently linked with the availability of other solutes and should be assessed in unison (Ledesma et al., 2022; Marcé et al., 2018; Wymore et al., 2021). For example, many studies in temperate regions recognize that the instream transport and transformation of C and N are inherently coupled (Plont et al., 2020), both directly through processes such as denitrification and assimilatory N processes (Burgin & Hamilton, 2007; Helton et al., 2015; Rodríguez‐Cardona et al., 2020) and indirectly through stoichiometric constraints (Frei et al., 2020; Taylor & Townsend, 2010).…”

Dissolved organic carbon bioreactivity and DOC:DIN stoichiometry control ammonium uptake in an intermittent Mediterranean stream

“…Moreover, Mediterranean streams show a marked seasonality in both DOC and DIN concentrations, with peaks during leaf litter fall and marked declines, especially of DIN, during the vegetative period (Bernal et al, 2005; Vázquez et al, 2011, 2013). Stoichiometric ratios also can vary greatly among seasons because the range of variability in concentration is usually higher for DIN than for DOC (Bernal et al, 2005; Ledesma et al, 2022). Likewise, the composition of dissolved organic matter (DOM), and especially the amount of aromatic and high molecular weight compounds, can vary largely over time in this type of stream (von Schiller et al, 2015).…”

“…dissolved organic carbon availability, heterotrophic activity, in-stream nitrogen uptake, stream biofilms, stream metabolism Ledesma et al, 2022). Likewise, the composition of dissolved organic matter (DOM), and especially the amount of aromatic and high molecular weight compounds, can vary largely over time in this type of stream (von Schiller et al, 2015).…”

Water availability and seasonality shape elemental stoichiometry across space and time

“…Globally, river networks integrate the spatiotemporal variability of the hydrological and biogeochemical properties of a watershed but are subject to multiple alterations (Creed et al, 2015; Likens et al, 1970; Maranger et al, 2018; Reid et al, 2019). Thus, capturing the dynamics of river nutrient availability across a watershed helps to reveal watershed processes and predict potential ecosystem responses across space and time (Bernhardt et al, 2017; Ledesma et al, 2022). The relationship between hydrology and resulting shifts in biogeochemical solutes provides critical insight into the magnitude, timing, and availability of carbon and nutrients in river networks (Chorover et al, 2017; Wymore et al, 2021).…”

“…Rivers are highly connected networks where individual sites are linked in three dimensions: longitudinally (downstream to upstream), vertically (between surface and subsurface water), and laterally (across the aquatic–terrestrial interface; Ward, 1989). The carbon and nutrients that are observed at a watershed's outlet are controlled by a variety of watershed properties, including drainage structure and density, topographic relief, underlying geology and soil development, and anthropogenic disturbance, along with the seasonal and interannual balance between precipitation and evapotranspiration (Knapp et al, 2020; Ledesma et al, 2022; Moatar et al, 2017). Other landscape factors, including the spatial distribution, extent, and hydrologic connectivity to wetlands can also strongly influence the magnitude and form of material exported from watershed uplands into a river network (e.g., Cohen et al, 2016; Creed et al, 2015; Fergus et al, 2017; Temmink et al, 2022).…”