In Search for the Missing Nitrogen: Closing the Budget to Assess the Role of Denitrification in Agricultural Watersheds

Although representing a paramount mechanism against nitrogen excess in agricultural landscapes, soil denitrification is still a largely unknown term in nitrogen balances at the watershed scale. In the present work, a comprehensive investigation of nitrogen sources and sinks in agricultural soils and waters was performed with the aim of gaining insights into the relevance of soil denitrification in a highly farmed sub-basin of the Po River delta (Northern Italy). Agricultural statistics, water quality datasets, and results of laboratory experiments targeting nitrogen fluxes in soils were combined to set up a detailed nitrogen budget along the terrestrial–freshwater continuum. The soil nitrogen budget was not closed, with inputs exceeding outputs by 72 kg N·ha−1·year−1, highlighting a potential high risk of nitrate contamination. However, extensive monitoring showed a general scarcity of mineral nitrogen forms in both shallow aquifers and soils. The present study confirmed the importance of denitrification, representing ~37% of the total nitrogen inputs, as the leading process of nitrate removal in heavily fertilized fine-texture soils prone to waterlogged conditions

Introducing Life Cycle Assessment in costs and benefits analysis of vegetation management in drainage canals of lowland agricultural landscapes

Nitrogen overload could provoke several effects on water quality in freshwater and coastal environment, in terms of eutrophication and groundwater nitrate contamination. Those effects can have a severe impact on ecosystem function and human health. In order to reduce nitrogen excess, constructed wetlands are usually recognized as a solution, but in recent years interest has been raised on the role of ditches and canals network in efficient nitrogen abatement. In this study, we investigated the environmental and economical sustainability of natural nitrogen removal capacity of both canals vegetation and microbial communities in the Burana-Volano-Navigabile (BVN) basin, located in the Po valley, the largest Italian hydrographic system. Based on life cycle assessment (LCA) approach and cost-benefits analysis, the effectiveness of two different vegetation management scenarios, which differ for the mowing management, on N abatement in the canal network of the case-study area has been compared. The results highlighted that postponing the mowing of aquatic vegetation to the end of vegetative season would contribute to buffer about 90% of the N load conveyed by the canal network during the irrigation period and would reduce of an order of magnitude the potential costs due to environmental damages

How does invasion degree shape alpha and beta diversity of freshwater fish at a regional scale?

FFreshwater ecosystems appear more vulnerable to biodiversity loss due to several anthropogenic disturbances and freshwater fish are particularly vulnerable to these impacts. We aimed to (1) identify the contribution of land use, spatial variables, and invasion degree in determining freshwater fish alpha (i.e., species richness) and beta (i.e., local contributions to beta diversity, LCBD) diversity, evaluating also the relationship between invasion degree and nestedness ((Formula presented.) nes) and turnover ((Formula presented.) sim) components of beta diversity. (2) Investigate the relationship between alpha diversity and LCBD, under the hypothesis that alpha diversity and LCBD correlate negatively and (3) investigate the relationship between species contributions to beta diversity (SCBD) and species occurrence, hypothesizing that non-native species show a lower contribution to beta diversity. The linear mixed models and the partition of R2 retained the invasion degree as the most important variables explaining alpha and beta diversity, having a positive relationship with both diversity components. Furthermore, land use related to human impacts had a positive influence on alpha diversity, whereas it showed a negative effect on LCBD. Regression model further showed that invasion degree related positively with (Formula presented.) sim, but negatively with (Formula presented.) nes, suggesting that non-native species were involved in the replacement of native species in the fish community. Alpha diversity and LCBD showed a weak positive correlation, meaning that sites with low species richness have higher LCBD. SCBD scaled positively with species occurrence highlighting that rarer species contribute less to SCBD. Finally, native and exotic species contributed similarly to beta diversity. These results suggest that invasion degree plays a central role in shaping alpha and beta diversity in stream fish, more than land use features reflecting habitat alteration or other geospatial variables. Furthermore, it is important to evaluate separately the native and the non-native components of biotic communities to identify linkages between invasion dynamics and biodiversity loss.Peer reviewe

Squaring the cycle: the integration of groundwater processes in nutrient budgets for a basin-oriented remediation strategy

Contamination, surface and ground waters, sink, mass balances

Advances in ecotechnological methods for diffuse nutrient pollution control: wicked issues in agricultural and urban watersheds

Considerable time and funding have been committed to tackling nonpoint source (NPS) pollution in agricultural and urban watersheds . Notwithstanding all these efforts, the water quality in many AUWs has not significantly improved. Diffuse nutrient pollution involves the movement of these pollutants between soil and water. Excessive diffuse pollution has been accepted as one of the main causes of failure to attain favorable environmental conditions in freshwater systems. Recently, several technologies and practices have been implemented to manage diffuse pollution, namely: a) source reduction, b) pollutant retention over the movement process; c) nutrient recycling, and d) purification and restoration of the eutrophic water bodies. This paper synthesized the existing knowledge of key methods to tackle diffuse pollution from AUWs. Furthermore, the predominant purification mechanisms and impacting factors are explored, allowing a comprehensive and critical understanding of different control strategies to improve the management of diffuse pollution. Therefore, potential approaches for strengthening the performance of control technologies for diffuse pollution treatment and remediation are discussed. Although adopting source reduction strategies (e.g., the “4R” approach: right rate, right time, right source, and right placement of nutrients) could efficiently decrease surface runoff and pollutant levels, they may not stop runoff from entering nearby streams. Consequently, comprehensive treatment of agricultural runoff still requires extra process retention strategies. Overall, the findings of this paper showed that treatment system design and operational and environmental factors played crucial but variable roles in diffuse pollution treatment. Moreover, the results showed that combining or integrating constructed wetlands with other control technologies could enhance the comprehensive purification of diffuse pollution compared to using a single method. This review proposes a systematic approach for diffuse pollution control based on three components (water, soil and microbiota) and maximizing the regulating services of agroecosystems via land use/cover types. The current review contributes to the documentation of existing research trends. It sheds light on diffuse pollution control approaches in AUWs, and further encourages the development of this vital field

Rilascio radiale di ossigeno in Vallisneria spiralis L.: implicazioni biogeochimiche in ecosistemi acquatici eutrofici

Eutrophication and the accumulation of organic matter have been addressed as the major factors determining the decline of benthic vegetation in impacted water bodies and the consequent loss of key ecosystemic functions. In freshwater environments the literature reports submersed macrophyte die-back events and the switch to free-floating and floating-leaved plants dominated states. Species-specific differences in macrophyte response along organic gradients are evident. Some species have developed adaptations that allow not only their survival along pronounced gradients of sedimentary organic content but also their fast response to short-term variations of pore water chemistry, as those occurring seasonally in freshwater temperate ecosystems. Vallisneria spiralis L. (Hydrocharitaceae family), a perennial stoloniferous species, is tolerant to eutrophication and colonizes both lentic and lotic environments. It performs photosynthesis in low light conditions, grows in nutrient-rich waters and on a wide range of substrates, from gravel bottoms to organic-rich muddy sediments. The aim of this thesis is to investigate the role of a tolerant rooted macrophyte in the regulation of biogeochemical dynamics and the interactions with microbial communities (with a particular focus on nitrogen cycle) in freshwater ecosystems undergoing eutrophication processes. Different methodological approaches are adopted (i.e. hydroponic incubations of plant tissues and intact plants, microcosm incubations, characterization of pore water and measurements of benthic fluxes) and the following aspects are evaluated: I) direct (uptake) and indirect (oxygen release) effects of V. spiralis presence on pore water features and redox-dependent processes; II) V. spiralis plasticity to colonize substrates with increasing organic content and changes of its influence on sediment chemistry and microbial activity along the gradient; III) relation between assimilative (mediated by vegetation) and dissimilative nitrogen processes (mediated by bacteria) when nitrogen is not limiting. The key point is the evaluation of the effect of radial oxygen loss by V. spiralis on benthic biogeochemical dynamics. Oxygen released by roots has the potential to alter the chemical environment within sediments, with cascade effects on nutrient and gas exchanges at the water-sediment interface. Relevant consequences have been demonstrated for plants growing in oligotrophic systems, while the effects in organic-rich substrates are scantily explored. The outcomes of the present work show that V. spiralis releases a great amount of the photosynthetically produced oxygen to the rhizosphere, affecting significantly the redox-dependent processes. Multiple evidences support the hypothesis that this plant varies seasonally the oxygen quota transported to the below-ground tissues to counteract the changing interstitial chemical conditions. Even if radial oxygen loss represents a small fraction in the plant oxygen economy, it can significantly affect the sediment biogeochemistry of eutrophic sites, representing a relevant amount of the daily benthic oxygen demand. V. spiralis acts as an engineer species controlling actively interstitial features (NH4+, NOx-, PO43-, Fe2+ and CH4) over a wide range of trophic conditions and along its whole vegetative cycle. In sediments with a moderate organic enrichment, radial oxygen loss promotes denitrification coupled to nitrification, thus enhancing nitrogen loss and the ecosystem capacity to control nitrogen contamination. Furthermore, the high nitrogen availability in both pore water and water column weakens the competition between macrophytes and nitrifying and denitrifying bacteria, favoring nitrogen removal through a combination of plant uptake and dissimilative microbial processes. However, at extremely elevated organic enrichment, vegetated sediment lose their role as nitrogen traps due to nitrification inhibition and plant stress induced by very reduced conditions. In summary, V. spiralis has the potential to withstand large perturbations of sedimentary features, being able to colonize organic matter impacted substrates. Even pore water conditions potentially hostile to roots do not affect its function as a benthic metabolism regulator. This macrophyte plays a crucial role in driving water-sediment exchanges of gases and nutrients, partially buffering the negative effects of organic enrichment connected to eutrophication. Moreover, it modifies sedimentary features, with positive feedbacks for water bodies restoration (i.e. regeneration of ferric iron buffer and phosphorus retention in sediment, stimulation of coupled nitrification-denitrification, reduction of internal organic load) which makes this plant an interesting option in programs for improving sediment conditions and favoring ecosystem recovery.L’eutrofizzazione e l’accumulo di sostanza organica nei sedimenti sono stati individuati quali fattori determinanti il declino della vegetazione bentica e la conseguente perdita delle connesse funzioni ecosistemiche. Negli ecosistemi d’acqua dolce la letteratura riporta fenomeni di scomparsa delle macrofite sommerse e l’instaurarsi di comunità dominate da pleustofite o piante radicate a foglie galleggianti. Differenze specie-specifiche nel tollerare substrati a diverso tenore organico sono comunque evidenti. Alcune piante hanno sviluppato adattamenti che ne consentono non solo la crescita lungo ampi gradienti di contenuto organico dei sedimenti, ma anche una rapida risposta alle variazioni nel chimismo interstiziale, come quelle che avvengono stagionalmente negli ecosistemi delle zone temperate. Vallisneria spiralis L. (famiglia Hydrocharitaceae), specie stolonifera perenne, risulta tollerante all’eutrofizzazione e colonizza sia sistemi lotici che lentici. Questa macrofita effettua fotosintesi a basse intensità luminose, cresce in acque ricche di nutrienti e su un ampio range di substrati, sia a prevalenza litica sia sedimenti limosi-fangosi. L’obiettivo della tesi è di indagare le interazioni tra comunità di macrofite radicate sommerse, rizosfera e comunità microbiche (con un focus particolare sul ciclo dell’azoto) in ecosistemi acquatici interni soggetti a fenomeni di eutrofizzazione. Differenti approcci metodologici sono stati adottati (incubazioni in idroponica di tessuti o di piante intatte, incubazioni di microcosmi, caratterizzazione delle acque interstiziali e misure di flussi bentici) al fine di studiare i seguenti aspetti: I) effetti diretti (uptake) e indiretti (rilascio di ossigeno) della presenza di V. spiralis sulle caratteristiche delle acque interstiziali e sui processi redox-dipendenti; II) plasticità di V. spiralis nel colonizzare substrati con crescenti contenuti organici e variazioni della sua influenza sul chimismo e l’attività microbica; III) relazioni tra processi assimilativi (mediati dalla vegetazione) e processi dissimilativi (mediate dalle comunità batteriche) dell’azoto in assenza di una sua limitazione. Il punto chiave è valutare l’effetto del rilascio di ossigeno da parte delle radici di V. spiralis sulle dinamiche biogeochimiche bentiche, il quale può potenzialmente alterare l’ambiente interstiziale con effetti a cascata sugli scambi acqua-sedimento di nutrienti e gas. Conseguenze rilevanti sono state dimostrate però in piante che colonizzano sistemi oligotrofici, mentre l’effetto in ambienti ricchi di sostanza organica è stato fino ad ora scarsamente indagato. I risultati del presente lavoro mostrano che V. spiralis rilascia una quota rilevante dell’ossigeno prodotto per fotosintesi nella rizosfera, influenzando significativamente i processi redox-dipendenti. Evidenze multiple supportano l’ipotesi che questa macrofita sia in grado di modificare stagionalmente la quantità trasportata verso i sistemi radicali, al fine di contrastare i cambiamenti nelle condizioni del chimismo interstiziale. Sebbene rappresenti una quota ridotta di ossigeno nell’economia generale della pianta, la sua influenza risulta determinante nella dinamiche biogeochimiche anche di sistemi eutrofici, rappresentando infatti un contributo rilevante al consumo giornaliero di ossigeno del sedimento. V. spiralis agisce quale engineer species controllando attivamente il chimismo interstiziale (NH4+, NOx-, PO43-, Fe2+ and CH4) lungo un ampio range di condizioni trofiche dei sistemi e durante il suo intero ciclo vegetativo. In sedimenti aventi un moderato arricchimento organico, il rilascio di ossigeno promuove il processo accoppiato di nitrificazione/denitrificazione, incentivando la dissipazione di azoto e la funzione ecosistemica di controllo dei carichi azotati. Inoltre, la disponibilità di azoto sia a livello interstiziale che di colonna d’acqua attenua la competizione tra piante e batteri (nitrificanti e denitrificanti), promuovendo la di rimozione dell’azoto mediante una combinazione di uptake e processi microbici dissimilativi. Ad elevati carichi organici però, i sedimenti vegetati perdono il loro ruolo di trappole per l’azoto a causa sia dell’inibizione della nitrificazione che di condizioni di stress per le piante, dato lo stato riducente dei substrati. In conclusione, V. spiralis presenta la capacità di resistere ad ampie perturbazioni nelle caratteristiche dei sedimenti, essendo in grado di colonizzare anche substrati impattati da sostanza organica. Condizioni interstiziali potenzialmente ostili alle radici non risultano compromettere la sua funzione nell’agire da regolatore del metabolismo bentico. Questa macrofita gioca un ruolo cruciale nel governare i flussi di gas e nutrienti, tamponando, sebbene parzialmente, gli effetti negativi connessi all’arricchimento organico. Modificando attivamente le caratteristiche dei sedimenti determina feedback positivi per la restoration degli ecosistemi acquatici (ad es. rigenerazione del buffer del ferro ferrico e ritenzione del fosforo, stimolazione del processo accoppiato di nitrificazione/denitrificazione, riduzione del carico organico interno) e risulta quindi un’interessante opzione da impiegare in programmi volti al miglioramento delle condizioni sedimentarie e a favorire il recupero di ecosistemi impattati

The achievement of Water Framework Directive goals through the restoration of vegetation in agricultural canals

Decreasing nitrate concentrations is one of the most relevant Water Framework Directive (WFD) goals, which today is still unreached in several European countries. Vegetated canals have been recognized as effective filters to mitigate nitrate pollution, although rarely included in restoration programs aimed at improving water quality in agricultural watersheds. The Po di Volano basin (713 km2, Northern Italy) is a deltaic territory crossed by an extensive network of agricultural canals (~1300 km). The effectiveness in buffering nitrate loads via denitrification was assessed for different levels of in-stream emergent vegetation maintenance by employing an upscale model based on extensive datasets of field measurements. The scenarios differed for the canal network length (5%, 20%, 40%, and 60%) where conservative management practices were adopted by postponing the mowing operations from the middle of summer, as nowadays, to the early autumn, i.e., the vegetative season end. The scenario simulations demonstrated that the capacity to mitigate diffuse nitrate pollution would increase up to four times, compared to the current condition (5% scenario), by postponing the vegetation mowing to the end of the vegetative season in 60% of the canal network length. By preserving the in-stream vegetation in 20% of the canal network, its denitrification capacity would equal the nitrate load reduction target required for achieving, from May to September, the good ecological status according to the WFD in waters delivered to the coastal areas. Changing the timing of vegetation mowing may create a large potential for permanent nitrate removal via denitrification in agricultural landscapes, thus protecting the coastal areas when the eutrophication risk is higher. Conservative management practices of in-stream vegetation might be promoted as an effective low-cost tool to be included in the WFD implementation strategies

Estimate of gas transfer velocity in the presence of emergent vegetation using argon as a tracer: Implications for whole-system denitrification measurements

Denitrification associated with emergent macrophytes is a pivotal process underlying the treatment performance of wetlands and slow-flow waterways. Laboratory scale experiments targeting N losses via denitrification in sediments colonized by emergent macrophytes require the use of mesocosms that are necessarily open to the atmosphere. Thus, the proper quantification of N2 effluxes relies on the accurate characterization of the air–water gas exchanges. In this study, we present a simple approach for direct measurements of the gas transfer velocity, in open-top mesocosms with Phragmites australis, by using argon as a tracer. Different conditions of water velocity (0, 1.5, 3, and 6 cm s−1) and temperature (8.5, 16, and 28 °C), were tested, along with, for the first time, the presence of emergent vegetation. The outcomes demonstrated that water velocity and temperature are not the only factors regulating aeration at the mesocosm scale. Indeed, the gas transfer velocity was systematically higher, in the range of 42–53%, in vegetated compared to unvegetated sediments. The increase of small-local turbulence patterns created within water parcels moving around plant stems translated into significant modifications of the reaeration process. The adopted approach may be used to improve the accuracy of denitrification measurements by N2 efflux-based methods in wetland and slow-flow waterway sediments colonized by emergent macrophytes. Moreover, the present outcomes may have multiple implications for whole-system metabolism estimations from which largely depend our understanding of biogeochemical dynamics in inland waters that have strong connections to worldwide issues, such as nitrate contamination and greenhouse gas emissions

The Response of Denitrification to Increasing Water Temperature and Nitrate Availability: The Case of a Large Lowland River (Po River, Northern Italy) under a Climate Change Scenario

Water warming and nutrient pulses following extreme rainfall events, both consequences of climate change, may have a profound impact on the biogeochemical dynamics of large temperate rivers, such as the Po River (Northern Italy), affecting denitrification capacity and the delivery of N loads to terminal water bodies. Manipulative experiments on denitrification kinetics were carried out using dark laboratory incubations of intact sediment cores collected from the lower Po River. Denitrification was measured along temperature and NO3− concentration gradients using 15N additions, in summer and autumn, the two seasons when climate change-induced warming has been shown to be higher. The combination of increased temperatures and pulsed NO3−-enhanced denitrification, suggesting that electron acceptor availability limits the process. The direct link between climate change-induced effects and the positive response of denitrification may have implications for the improvement of water quality in the coastal zone, as it may help to partially buffer N export, especially in summer, when the risk of eutrophication is higher. Further research is needed to investigate the quality and quantity of sediment organic matter as important drivers regulating river denitrification

An ounce of prevention is worth a pound of cure: Managing macrophytes for nitrate mitigation in irrigated agricultural watersheds

Although ubiquitous elements of agricultural landscapes, the interest on ditches and canals as effective filters to buffer nitrate pollution has been raised only recently. The aim of the present study was to investigate the importance of in-ditch denitrification supported by emergent aquatic vegetation in the context of N budget in agricultural lands of a worldwide hotspot of nitrate contamination and eutrophication, i.e. the lowlands of the Po River basin (Northern Italy). The effectiveness of N abatement in the ditch network (>18,500 km) was evaluated by extrapolating up to the watershed reach-scale denitrification rates measured in a wide range of environmental conditions. Scenarios of variable extents of vegetation maintenance were simulated (25%, 50% and 90%), and compared to the current situation when the natural development occurs in only 5% of the ditch network length, subjected to mechanical mowing in summer. Along the typical range of nitrate availability in the Po River lowlands waterways (0.5–8 mg N L−1), the current N removal performed by the ditch network was estimated in 3300–4900 t N yr−1, accounting for at most 11% of the N excess from agriculture. The predicted nitrate mitigation potential would increase up to 4000–33,600 t N yr−1in case of vegetation maintenance in 90% of the total ditch length. Moreover, a further significant enhancement (57% on average) of this key ecosystem function would be achieved by postponing the mowing of vegetation at the end of the growing season. The simulated outcomes suggest that vegetated ditches may offer new agricultural landscape management opportunities for effectively decreasing nitrate loads in surface waters, with potential improved water quality at the watershed level and in the coastal zones. In conclusion, ditches and canals may act as metabolic regulators and providers of ecosystem services if conservative management practices of in-stream vegetation are properly implemented and coupled to hydraulic needs