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 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 large storms. Stream DOC and NO3− concentrations were generally low (overall average ± SD was 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 large storm events highlighted different patterns of DOC and NO3− mobilization depending on antecedent soil moisture conditions: drier antecedent conditions promoted rapid elevations of riparian groundwater tables, hydrologically activating a wider and shallower soil layer, and leading to relatively higher increases in stream DOC and NO3− concentrations compared to events preceded by wet conditions. These results suggest that (i) increased supply of limited resources during storms can promote in-stream heterotrophic activity during high flows, especially during large storm events preceded by dry conditions, and (ii) C-limited conditions upstream were gradually 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 study stream suggests that groundwater inputs from riparian forests are essential for maintaining in-stream heterotrophic activity in oligotrophic, forest headwater catchments.

Influence of Dissolved Organic Matter Sources on In-Stream Net Dissolved Organic Carbon Uptake in a Mediterranean Stream

Studies exploring how different sources of dissolved organic matter (DOM) influence in-stream dissolved organic carbon (DOC) uptake at the ecosystem scale are scarce in the literature. To fill this knowledge gap, we examined the relationship between DOM sources and in-stream net DOC uptake (UDOC) in a sub-humid Mediterranean stream. We considered four reach-scale scenarios occurring under natural conditions that differed in predominant DOM sources (groundwater, leaf litter, and/or upstream water). Results showed that groundwater inputs favored in-stream net DOC uptake, while leaf litter inputs promoted in-stream net DOC release. However, there was no clear effect of DOM source mixing on the magnitude of UDOC. Further, the variability in UDOC within and among scenarios was mostly explained by stream DOC concentration, suggesting that DOC availability limits microbial activity in this stream. DOM composition became a controlling factor of UDOC variability only during the leaf litter period, when stream DOC concentration was the highest. Together, these results suggest that the capacity of headwater forested streams to process DOC is closely tied to the availability of different DOM sources and how they vary over time and along the river network.