Hydrology and riparian forests drive carbon and nitrogen supply and DOC : NO$_{3}$ $^{–}$ stoichiometry along a headwater Mediterranean stream

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 (NO$_{3}$ $^{–}$) concentrations and DOC : NO$_{3}$ $^{–}$ molar ratios. Here, we aimed to investigate how the interplay between hydrological conditions and riparian forest coverage drives DOC and NO$_{3}$ $^{–}$ supply and DOC : NO$_{3}$ $^{–}$ stoichiometry in an oligotrophic headwater Mediterranean stream. We analysed DOC and NO$_{3}$ $^{–}$ concentrations and DOC : NO$_{3}$ $^{–}$ 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 NO$_{3}$ $^{–}$ from riparian soils to the stream during storms. Stream DOC and NO$_{3}$ $^{–}$ concentrations were generally low (overall averages ± SD were 1.0 ± 0.6 mg C L $^{–1}$ and 0.20 ±
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0.09 mg N L $^{–1}$), although significantly higher during storm flow compared to base flow conditions in all three stream sites. Optimal DOC : NO$_{3}$ $^{–}$ stoichiometry for stream heterotrophic microorganisms (corresponding to DOC : NO$_{3}$ $^{–}$ 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 NO$_{3}$ $^{–}$ mobilization. In comparison to large storms, medium storm events showed limited hydrological responses that led to significantly lower increases in stream DOC and NO$_{3}$ $^{–}$ concentrations. During large storm events, different patterns of DOC and NO$_{3}$ $^{–}$ 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 NO$_{3}$ $^{–}$ 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 NO$_{3}$ $^{–}$ availability and DOC : NO$_{3}$ $^{–}$ 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.