Hydrology and riparian forests drive carbon and nitrogen supply and DOC : NO3- 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 (NO3-) 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 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.

Automated summary

In forest headwater streams, the interaction between hydrological conditions and riparian forest coverage plays a significant role in the supply and stoichiometry of dissolved organic carbon (DOC) and nitrate (NO3-). This study highlights the importance of groundwater inputs from riparian forests in maintaining in-stream heterotrophic activity in forest headwater catchments.