Interactions between carbon and nitrogen cycles in Mediterranean headwater streams: from microbial communities to ecosystem scale patterns
[eng] Reactive nitrogen (N) inputs into aquatic ecosystems have more than doubled over the last century due to human activities, leading to widespread eutrophication, greenhouse gas emissions, and biodiversity loss. Headwater streams can contribute to mitigate these impacts, and thus act as biogeoch...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2025 |
| País: | España |
| Institución: | Universidad de Barcelona |
| Repositorio: | Dipòsit Digital de la UB |
| OAI Identifier: | oai:dnet:ubarcelona__::7b12fa874bc676e503b695a0fe07a5bb |
| Acceso en línea: | https://hdl.handle.net/2445/229639 https://hdl.handle.net/10803/697521 |
| Access Level: | acceso abierto |
| Palabra clave: | Limnologia Ecologia microbiana Cicle del nitrogen Estequiometria Cicle del carboni (Biogeoquímica) Limnology Microbial ecology Nitrogen cycle Stoichiometry Carbon cycle (Biogeochemistry) |
| Sumario: | [eng] Reactive nitrogen (N) inputs into aquatic ecosystems have more than doubled over the last century due to human activities, leading to widespread eutrophication, greenhouse gas emissions, and biodiversity loss. Headwater streams can contribute to mitigate these impacts, and thus act as biogeochemical hotspots, by retaining, transforming, and removing dissolved inorganic nitrogen (DIN). In headwater streams, these biogeochemical processes are mostly mediated by heterotrophic bacteria, which require a source of dissolved organic carbon (DOC) to process N. However, the extent to which DOC availability, alone or in combination with other environmental factors, influences stream heterotrophic activity and in-stream DIN cycling remains poorly understood. The objective of this PhD thesis was to investigate the biogeochemical controls on DIN processing in Mediterranean headwater streams, with special attention to the influence of both DOC availability and dissolved organic matter (DOM) composition on the activity of stream microbial communities. The study was carried out along several Mediterranean streams used as natural laboratories for exploring the interaction between C and N biogeochemical cycles because the marked hydroclimatic seasonality and variety of land uses in this region leads to large spatiotemporal fluctuations in DOC and DIN concentrations, as well as in DOM composition and microbial community assemblages. The PhD dissertation is composed of three complementary studies combining different empirical approaches, from whole-stream DIN addition experiments, to laboratory incubations, and molecular analyses. The first study (Chapter 4) showed that ammonium (NH4+) uptake in a forested heterotrophic stream was strongly regulated by the stoichiometric balance between DOC and DIN (i.e., substrate: nutrient stoichiometry), rather than by DOC or DIN concentrations alone. DOC bioreactivity also modulated heterotrophic activity, as labile DOC sources (e.g., acetate) markedly increased NH4+ uptake, whereas recalcitrant sources (e.g., lignin) did not. In addition, stream microbial community function showed high resilience to hydrological disturbances likely due to their high functional redundancy. The second study (Chapter 5) expanded the same conceptual framework used in Chapter 4 to nitrate (NO3-) use across eight headwater streams and one wetland, showing that heterotrophic NO3- uptake was jointly controlled by DOC:NO₃⁻ ratios, DOM composition, and microbial functional traits. Notably, streams with high proportion of aromatic compounds showed lower NO3- uptake efficiency, highlighting the role of DOM composition in shaping microbial N demand. Furthermore, we observed that heterotrophic NO - uptake in most streams was also phosphate-limited, with the stoichiometric balance between NO - and phosphate explaining much of the variation in NO - uptake. The third study (Chapter 6) explored how suboxic conditions prevailing during the characteristic low flows of spring and summer in a natural intermittent stream, influenced in-stream DIN processing. The results showed that suboxic conditions induced a major shift from aerobic to anaerobic N metabolic pathways, accompanied by both a depletion of DIN oxidised forms (i.e., NO -) and an accumulation of reduced forms (i.e., NH +). Despite denitrification is supposed to be the most favourable biogeochemical process under suboxic conditions, measured denitrification rates and the abundance of denitrification genes were lower than expected. Conversely, the abundance of genes related with dissimilatory nitrate reduction to ammonium (DNRA) were surprisingly high, suggesting that this anaerobic pathway might be essential for retaining N in N-limited systems. These findings suggest that some streams may have limited capacity to permanently remove N via denitrification, even under suboxic or anaerobic conditions. Together, the findings of this PhD dissertation emphasize that C and N cycles are strongly connected in Mediterranean headwater streams, and in-stream DIN processing is not determined by a single driver, but by the interactive effects of stoichiometric constraints, DOM characteristics, microbial community composition, and redox dynamics. The three manuscripts contained in this PhD provide an integrated view of the mechanisms controlling the spatio-temporal variability of DIN concentrations in Mediterranean streams and contribute to a better understanding of how in-stream processes regulate catchment-scale nutrient fluxes contributing to generate predictive scenarios in a changing world. |
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