Understanding the effects of hydrology on nutrient cycling and greenhouse gas emissions in riverine landscapes

Abstract

Freshwater ecosystems have been extensively modified, affecting hydrology and biogeochemical cycles. Restoration enhances key services (nutrient cycling), nevertheless with a potential increase in greenhouse gas (GHG) emissions. Long-term anthropogenic impacts also reflect in climate induced changes on hydrology, affecting runoff and ultimately biogeochemical processes and nitrous oxide (N2O) emissions in inland waters. Extreme weather events are expected to aggravate, increasing the frequency of dry-wet cycles. Large areas of air exposed sediments due to occasional, recurrent or permanent drying are potential biogeochemical hotspots. Predicting N2O emissions from inland waters remains a challenge. Transition effects during drying-rewetting events remain largely unrevealed as N2O production/emission source from distinct processes differing in their main drivers and environmental responses. Adding to the present knowledge, this thesis proposes to address: i) the current knowledge on nitrous oxide emissions and biogeochemical responses to dry-wet cycles in freshwater ecosystems; ii) the effects of hydromorphological restoration on GHG emissions along the aquatic-terrestrial interface; iii) dynamics and partitioning of processes underlying N2O production/emission during dry-wet cycles. Results suggest: i) the driving role of dry-wet cycles leading to temporarily high N2O emissions in freshwater sediments is evident; however the relevance of peak fluxes for global emission estimates has not been quantified. Emission drivers are comparable to those of soils and related to physical mechanisms and enhanced microbial processing, largely regulated by water fluctuations and O2 availability; ii) hydrological connectivity restoration may not significantly increase N2O emissions, nevertheless enhanced biogeochemical rates and higher fluxes are observed in sediments experiencing frequent dry-wet cycles. The C and N biogeochemical cycles depend on similar substrate characteristics governed by organic-matter quality; iii) Nitrification and denitrification processes co-occur during drying-rewetting phases, and N2O emissions are potentially higher during the drying phase. Organic-matter content and quality, and hydrological dynamics are key environmental factors affecting N2O fluxes