Hydrology and riparian forests drive carbon and nitrogen supply and DOC : NO 3 − stoichiometry along a headwater Mediterranean stream

[摘要] In forest headwater streams, metabolic processes arepredominately 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 inan oligotrophic headwater Mediterranean stream. We analysed DOC andNO 3 - concentrations and DOC : NO 3 - molar ratios duringboth base flow and storm flow conditions at three stream locations along alongitudinal gradient of increased riparian forest coverage. Further, weperformed an event analysis to examine the hydroclimatic conditions thatfavour the transfer of DOC and NO 3 - from riparian soils to thestream during storms. Stream DOC and NO 3 - concentrations weregenerally low (overall averages ± SD were 1.0±0.6  mg C L −1 and 0.20±0.09  mg N L −1 ), although significantly higherduring storm flow compared to base flow conditions in all three streamsites. Optimal DOC : NO 3 - stoichiometry for stream heterotrophicmicroorganisms (corresponding to DOC : NO 3 - molar ratios between 4.8and 11.7) was prevalent at the midstream and downstream sites under bothflow conditions, whereas C-limited conditions were prevalent at the upstreamsite, which had no surrounding riparian forest. The hydroclimatic analysisof storms suggested that large and medium storm events display a distinctmechanism of DOC and NO 3 - mobilization. In comparison to largestorms, medium storm events showed limited hydrological responses that ledto significantly lower increases in stream DOC and NO 3 - concentrations. During large storm events, different patterns of DOC andNO 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 andNO 3 - concentrations compared to large storm events preceded by wetconditions. Our results suggest that (i) increased supply of limitedresources during storms can potentially sustain in-stream heterotrophicactivity during high flows, especially during large storm events preceded by dry conditions, and (ii) C-limited conditions upstream were overcomedownstream, likely due to higher C inputs from riparian forests present atlower elevations. The contrasting spatiotemporal patterns in DOC andNO 3 - availability and DOC : NO 3 - stoichiometry observed at the studied stream suggest that groundwater inputs from riparian forestsare essential for maintaining in-stream heterotrophic activity inoligotrophic, forest headwater catchments.

[学科分类] 妇产科学