Northern peatlands are an important carbon store, with carbon dynamics and hydrology intrinsically linked. The riparian zone is the interface between the terrestrial and aquatic systems, situated adjacent to the stream and characterised by periodic flooding, near surface water tables and unique soil and plant species composition. Due to its unique biogeochemical environment, the riparian zone has the potential to modify significantly the production, mobilisation and transport of carbon via the land-atmosphere and aquatic pathways. Two contrasting headwater catchments, an ombrotrophic peatland (Auchencorth Moss, SE Scotland) and a forested, till dominated catchment (Västrabäcken, N Sweden), were investigated. In each carbon concentrations in soil and stream water and hydrological parameters were measured in transects connecting the wider catchment, riparian zone and stream. The overarching aim was to investigate the role of the riparian zone on the hydrological and bio-geochemical functioning of peatland and forested catchments, focusing on carbon export via the aquatic pathway. Specific objectives were to: a) examine the importance of soils, water table and vegetation composition on riparian biogeochemical cycling, b) investigate riparian-stream hydrological connectivity and the transport of carbon across the soil-water interface and c) assess riparian processes in relation to the net ecosystem carbon balance (NECB) across northern latitude ecosystems. Porewater total carbon (TC) concentrations (sum of dissolved organic and inorganic carbon (DOC, DIC), CO2 and CH4) were on average higher in Auchencorth Moss (78.8-140 mg C L-1) than the Västrabäcken (27.7-63.2 mg C L-1) catchment. In both catchments, higher TC concentrations were observed in the riparian zone compared to the wider catchment. The dominant control for differentiating between catchment and riparian biogeochemical processes was the higher average riparian water table with each carbon species displaying a positive relationship with water table height. A range of other factors, including soil temperature and the carbon content of catchment and riparian soils, also contributed to the complexity of riparian carbon biogeochemical cycles. Catchment specific phenomena, including the presence of aerenchymous vegetation and stream sediment deposition onto the riparian zone, modified riparian carbon dynamics in the Auchencorth Moss catchment. Isotopically, porewater DOC, CO2 and CH4 had a 14C content >100 %modern, indicating that the modern plant derived DOC is being transported down the soil profile, providing the source for CO2 and CH4 production at depth. In both catchments the riparian zone represented an important and dynamic source of carbon to stream waters. Total annual CO2 export from the riparian zone of the Västrabäcken catchment to the stream channel over the hydrological year was 2.7 g CO2-C m2 yr-1 with export predominantly from between 40 and 55 cm depth within the soil. Two monthly peaks in CO2 export occurred over the hydrological year related to either storm events or the spring snow melt period which accounted for 19 % of annual export, highlighting the temporal variability in soil-stream linkages, especially during high flow periods. In the generally wetter peatland catchment, riparian-stream linkages were driven by antecedent conditions and variation in riparian water table, with changes in water input, rather than changes in CO2 source concentrations, controlling stream water composition. The negative CO2 concentration-discharge relationship in the stream suggested that event water dominated, with small but important inputs from high concentration soil water during individual events. The importance of event water in transporting carbon was confirmed through the isotope result. CO2, CH4 and DOC exported via the aquatic pathway predominantly contained modern, plant derived carbon from the near surface soil horizons but with a small contribution (5-28 %) from deeper geological sources leading to aged evasion CH4 (310-537 years BP) and CO2 (36 years BP to modern). In both catchments the riparian zone was more important, relative to the wider catchment, in controlling the export of carbon via the aquatic pathway. At Auchencorth Moss, the riparian zone, plus an area of the catchment extending ~20 m from the stream, were hotspots for land-atmosphere fluxes of CH4, with mean flux of 1.08-7.70 mg m2 hr-1 in comparison to the catchment overall (0.05 mg m2 hr-1). In both catchments, combining detailed catchment hydrological models with high temporal resolution carbon concentration measurements, especially in riparian zone soils, has the potential to improve estimates of downstream and evaded carbon export in headwater catchments. Riparian zones should therefore be included more in studies investigating hydrological and biogeochemical processes in northern latitude headwater catchments. The processes within riparian zones suggest that despite the relatively small area that riparian zones represent, in relation to the wider catchment, they may play an important role in the NECB of peatland and forested catchments under future management and climate change scenarios.

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Previous research has shown a close relationship between the terrestrial and aquatic carbon (C) cycles, namely that part of the C fixed via terrestrial net primary production (NPP) is exported to inland waters. In turn, it has been demonstrated that once in the freshwater system C can not only be transported laterally as dissolved organic carbon (DOC), particulate organic carbon (POC) and dissolved inorganic carbon (DIC) but is also mineralized and evaded back to the atmosphere as CO2, or buried in sediments. A number of hotspot areas of aquatic CO2 evasion have been identified but there are considerable gaps in our knowledge, particularly associated with understanding and accounting for the temporal and spatial variation of aquatic C fluxes at regional to global scales, which we know from local scale studies, to be substantial. In this thesis, three important regional hotspots of LOAC activity were identified, where significant gaps in our understanding remain.For the boreal region, an empirical model is developed to produce the first high resolution maps of boreal lake pCO2 and CO2 evasion, providing a new estimate for total evasion from boreal lakes of 189 (74–347) Tg C yr-1, which is more than double the previous best estimate. The model is also used along with future projections of terrestrial NPP and precipitation, to predict future lake CO2 evasion under future climate change and land-use scenarios, and it is found that even under the most conservative scenario CO2 evasion from boreal lakes may increase 38% by 2100. For the Amazon Basin, the ORCHILEAK land surface model driven by a newly developed wetland forcing file, is used to show that the export of C to and CO2 evasion from inland waters is highly interannually variable; greatest during wet years and lowest during droughts. However, at the same time overall net ecosystem productivity (NEP) and C sequestration is highest during wet years, partly due to reduced decomposition rates in water-logged floodplain soils. Furthermore, it is shown that aquatic C fluxes display greater variation than terrestrial C fluxes, and that this variation significantly dampens the interannual variability in NEP of the Amazon basin by moderating terrestrial variation. Finally, ORCHILEAK is applied to the Congo Basin to investigate the evolution of the integrated aquatic and terrestrial C fluxes from 1861 to the present day, and in turn to 2099 under a future climate and land-use scenario. It is shown that terrestrial and aquatic fluxes increase substantially over time, both over the historical period and into the future, and that these increases are largely driven by atmospheric CO2. The proportion of terrestrial NPP lost to the LOAC also rises from 3% in 1861 to 5% in 2099 and this trend is driven not only by atmospheric CO2 but also by climate change. This is in contrast to the boreal region where the proportion of NPP exported to inland waters is predicted to remain relatively constant, and to the Amazon, where a decrease has been predicted, due to differences in projected climate change. / L’état de l’art dans le domaine a montré qu’il y avait un lien étroit entre les cycles du carbone terrestre et aquatique :en effet, une partie du carbone fixé par photosynthèse (productivité primaire brute) est transférée vers les milieux aquatiques continentaux pour être ensuite transporté latéralement sous forme de carbone organique dissous (COD), de carbone organique particulaire (COP), de carbone inorganique dissous (CID). Durant ce transfert latéral, le carbone peut être minéralisé puis réémis vers l’atmosphère sous forme de CO2 ou enfoui dans les sédiments. Cependant, nous sommes encore loin de bien comprendre et surtout de quantifier les variations temporelles et spatiales des flux de carbones à l’échelle régionale et globale, même si les études faites à l’échelle locale nous montrent qu’elles sont importantes. Au cours de cette thèse, nous nous sommes focalisés sur 3 grandes régions pour lesquelles la connaissance des flux de carbone le long du continuum aquatique reliant les écosystèmes terrestres aux océans étaient encore très parcellaire.Pour la région boréale, un modèle empirique a été développé afin de produire les premières cartes à haute résolution de pCO2 et d’émission de CO2 pour les lacs boréaux. Les résultats du modèle nous ont permis de contraindre les émissions totales de CO2 pour les lacs boréaux à 189 (74-347) Tg C an-1, soit plus du double des estimations précédentes. Ce modèle a ensuite été couplé aux projections de production primaire brute terrestre et de précipitations afin de prédire les émissions de CO2 pour ces lacs pour différents scénarios de changement climatique et d’occupation des sols. Les résultats montrent que même en prenant le scénario le plus conservatif, les émissions de CO2 des lacs boréaux augmenteraient de 38% d’ici 2100.Pour le bassin de l’Amazone, le modèle d’écosystème terrestre ORCHILEAK, paramétré par de nouvelles donnés de forçage des zones humides, a été utilisé pour démontrer que l’export de carbone terrestre vers les réseaux fluviaux ainsi que les émissions de CO2 ont une très grande variabilité interannuelle :émissions élevées lors des années à forte précipitation et basses lors des années sèches. Cependant, la productivité nette de l’écosystème (PNE) Amazone et la fixation nette de carbone à l’échelle du bassin sont plus élevées lors des années humides, en partie dû au taux de décomposition de carbone organique réduit lorsque les sols sont saturés en eau. De plus, les résultats montrent que les flux de carbone des systèmes aquatiques ont une plus grande variabilité que les flux terrestres, ce qui atténue considérablement la variabilité interannuelle de la PNE du bassin de l'Amazone.Pour finir, nous avons appliqué ORCHILEAK au bassin du Congo afin d’étudier l’évolution intégrée des flux de carbone terrestres et aquatiques de 1861 à nos jours, ainsi que de projeter leur devenir au cours du 21eme siècle selon les scénarios de changement climatiques et de changement d’occupation des sols. Nous avons montré que les flux terrestres et aquatiques augmentent de façon significative durant la période historique et dans le futur, cette augmentation étant largement induite par l’augmentation du CO2 atmosphérique et, dans une moindre mesure, par le changement climatique. En particulier, la proportion de la productivité primaire brute terrestre exportée vers le continuum aquatique passe de 3% en 1861 à 5% en 2099. Ce résultat contraste avec ceux obtenu pour la région boréale où cette proportion reste relativement constante et pour l’Amazone où c’est une baisse qui est en fait prédite. Ces différences s’expliquent par des trajectoires de changement climatique distinctes pour ces 3 régions. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished