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Hydrological intermittency affects sediment biogeochemistry, organic carbon (OC) metabolism and carbon dioxide (CO2) emission but the study of the effects of drying is generally confined to natural ecosystems. Agricultural canals are artificial, widespread elements in irrigated floodplains, and regularly subjected to water level fluctuations. The aim of this study was to quantify the CO2 emissions along water saturation gradients in artificial canals to understand the environmental factors regulating these fluxes. CO2 measurements were performed in five replicated canals within the Po River basin (Northern Italy). In each canal we analysed three sites: (i) a spot with exposed, dry sediments; (ii) a spot with inundated, saturated sediments and (iii) a spot with an intermediate level of saturation. Besides dark CO2 flux measurements, net potential nitrification and denitrification rates were measured as proxies of sediment redox potential and due to their CO2 sink and source role, respectively. We hypothesized a site-specific regulation of CO2 emission, depending on the interplay among water saturation, sediment oxidation and organic matter content. Our results suggest that desiccation stimulates mineralization processes and CO2 fluxes, that were mainly dependant on water and organic matter content and correlated with microbial N transformations. CO2 emissions tended to increase along the considered water saturation gradients, almost tripling rates from inundated, saturated (158.2 ± 24.1 mmol CO2 m−2 days−1) to dry, exposed sediments (416.5 ± 78.9 mmol CO2 m−2 days−1). Results also suggest that net potential nitrification and denitrification allow tracing the effects of drying on N microbial communities involved in CO2 fluxes. Net potential nitrification rates produce little effects on CO2 fluxes, but is a good proxy of oxygen (O2) availability, whereas potential denitrification may be responsible for variable fractions (up to 100%) of CO2 production, in wetter sediments. [ABSTRACT FROM AUTHOR]

The shift from perennial to intermittent flow regimes in rivers and streams deeply modifiesabiotic and biotic characteristics of benthic systems, changing the structure andfunctioning of benthic communities. Among others, leaf litter retention and processing areexpected to be altered by long lasting droughts followed by flash flows. In streams, leaflitter breakdown is facilitated by feeding activities of shredders, that play a keyrole in nutrient recycling and energy transfer along the food web. Furthermore,litter decomposition can be affected by its size which control microbial and fungalcolonization, e.g. by modifying either the volume to surface or the surface to perimeterratios. In this study we tested the hypothesis that drought affects the benthic macrofaunainvolved in organic matter processing, alters decomposition rate, and control ecologicalinteractions between shredders and microbial community, resulting in different nutrientrecycling. To this purpose, a laboratory experiment was performed using 45 PVC microcosms. Wetested 9 conditions, each with 5 replicates in which we incubated 0.5 grams of leaf disks(Populus nigra) of two different dimensions (1 and 5 cm as diameter), in presence andabsence of larvae of Potamophylax cinculatus (Trichoptera, Limnephilidae), under permanentand intermittent conditions (dry phase followed by rewetting). Microcosms were maintainedfor nearly 40 days in a 200 L incubation tank containing well-mixed and aerated stream waterand periodically underwent a short (6 hours) dark incubation. At the end of the experiment,we measured the net leaf biomass loss and the elemental composition of both leaves andresulting debris. Our specific aims were to measure benthic metabolism (respiration and nutrientregeneration - NH4+, PO43− and SiO2) before and after drought, to quantify organic matterdecomposition rates, and nutrient recycling and stoichiometry. Results showed a decrease in litter mineralization under intermittent conditions, sincedrought slowed biological processes and benthic respiration, whereas rewetting phaseincreased NH4+release. In permanent submerged conditions the undecomposed litter anddebris became N- and P-enriched, likely due to microbial and fungal colonization and uptake,whereas intermittent conditions changed leaves stoichiometry, resulting in lower organicmatter quality, and lowered nutrient content in debris. The activity of P. cinculatus had astrong effect on leaves elemental composition and increased mineralization rates. P.cinculatus, through feeding activity, also stimulated nutrient mobilization as organicparticulate, increasing C, N and P content in debris. Our findings also showed differentdecomposition dynamics in leaves with different surface to perimeter ratio. In fact, leaveswith the greatest surface to perimeter ratio displayed higher respiration, higherNH4+ regeneration rates and slightly higher biomass loss, compared to smaller leafdisks. Under predicted global change scenarios, which are expected to increase the frequencyand intensity of drought events, our findings suggest that hydrological drought could changestreams ecosystem functioning by altering benthic communities and in turn ecosystemprocesses. This study was carried out within the project NOACQUA – responses of communities andprocesses in intermittent rivers a National Relevant Project funded by the Italian Ministry ofEducation and University (PRIN 2015, Prot. 201572HW8F). [ABSTRACT FROM AUTHOR]