Hydrology and riparian forests drive carbon and nitrogen supply and DOC : NO3− stoichiometry along a headwater Mediterranean stream
Authors: Ledesma José L. J. , Lupon Anna , Martí Eugènia , Bernal Susana 
Published: August 2022

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.