Contrasting effects of climate change on denitrification and nitrogen load reduction in the Po River (Northern Italy).

An increase in water temperature is one of the main factors that can potentially modify biogeochemical dynamics in lowland rivers, such as the removal and recycling of nitrogen (N). This effect of climate change on N processing deserves attention, as it may have unexpected impacts on eutrophication in the coastal zones. Intact sediment cores were collected seasonally at the closing section of the Po River, the largest Italian river and one of the main N inputs to the Mediterranean Sea. Benthic oxygen fluxes, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) rates were measured using laboratory dark incubations. Different temperature treatments were set up for each season based on historical data and future predictions. Higher water temperatures enhanced sediment oxygen demand and the extent of hypoxic conditions in the benthic compartment, favoring anaerobic metabolism. Indeed, warming water temperature stimulated nitrate (NO3-) reduction processes, although NO3- and organic matter availability were found to be the main controlling factors shaping the rates between seasons. Denitrification was the main process responsible for NO3- removal, mainly supported by NO3- diffusion from the water column into the sediments, and much more important than N recycling via DNRA. The predicted increase in the water temperature of the Po River due to climate change may exert an unexpected negative feedback on eutrophication by strongly controlling denitrification and contributing to partial buffering of N export in the lagoons and coastal areas, especially in spring.

Climate change impacts on eutrophication in the Po River (Italy): Temperature-mediated reduction in nitrogen export but no effect on phosphorus.

Rivers worldwide are under stress from eutrophication and nitrate pollution, but the ecological consequences overlap with climate change, and the resulting interactions may be unexpected and still unexplored. The Po River basin (northern Italy) is one of the most agriculturally productive and densely populated areas in Europe. It remains unclear whether the climate change impacts on the thermal and hydrological regimes are already affecting nutrient dynamics and transport to coastal areas. The present work addresses the long-term trends (1992-2020) of nitrogen and phosphorus export by investigating both the annual magnitude and the seasonal patterns and their relationship with water temperature and discharge trajectories. Despite the constant diffuse and point sources in the basin, a marked decrease (-20%) in nitrogen export, mostly as nitrate, was recorded in the last decade compared to the 1990s, while no significant downward trend was observed for phosphorus. The water temperature of the Po River has warmed, with the most pronounced signals in summer (+0.13°C/year) and autumn (+0.16°C/year), together with the strongest increase in the number of warm days (+70%-80%). An extended seasonal window of warm temperatures and the persistence of low flow periods are likely to create favorable conditions for permanent nitrate removal via denitrification, resulting in a lower delivery of reactive nitrogen to the sea. The present results show that climate change-driven warming may enhance nitrogen processing by increasing respiratory river metabolism, thereby reducing export from spring to early autumn, when the risk of eutrophication in coastal zones is higher.

Soil conditioners effects on hydraulic properties, leaching processes and denitrification on a silty-clay soil.

Agricultural landscapes are often affected by groundwater quality issues due to fertilizers leaching. To address this worldwide problem several agricultural best practices have been proposed, like limiting the amount of fertilizers and increasing soil organic matter content. To evaluate if these practices may promote groundwater quality enhancement, vadose zone retention time and complex biogeochemical processes must be known in detail. In this study, sequential undisturbed column experiments were performed to determine the amount of nutrients and heavy metals leached after simulated stormwater events. The column was amended with urea then flushed for two pore volumes, then straw residuals were incorporated and flushed for two pore volumes and finally compost was incorporated and flushed for six pore volumes. Dissolved ions, major gasses and heavy metals were determined in leachate samples. Nitrate and nitrite were leached in the urea treatment producing the highest concentrations, followed by compost and straw residuals. The redox conditions were aerobic in all treatments and pH was circumneutral or slightly basic. Denitrification was low but increased with the addition of straw residuals and compost. Heavy metals were all at very low concentrations except for lead and cadmium, which slightly exceeded threshold limits (10 and 1 µg/L, respectively) in all the treatments. The compost treatment, after three pore volumes, was affected by clay swelling due to sodium dispersion, which in turn provoked a reduction of porosity and hydraulic conductivity.