Nitrogen balance and fate in a heavily impacted watershed (Oglio River, Northern Italy): in quest of the missing sources and sinks

Abstract. We present data from a comprehensive investigation carried out from 2007 to 2010, focussing on nitrogen pollution in the Oglio River basin (3800 km2, Po Plain, Northern Italy). Nitrogen mass balances, computed for the whole basin with 2000 and 2008 data, suggest a large N surplus in this area, over 40 000 t N yr−1, and increasing between 2000 and 2008. Calculations indicate a very large impact of animal husbandry and agricultural activities in this watershed, with livestock manure and synthetic fertilizers contributing 85% of total N inputs (about 100 000 t N yr−1) and largely exceeding crop uptake and other N losses (about 60 000 t N yr−1). Nitrogen from domestic and industrial origin is estimated as about 5800 and 7200 t N yr−1, respectively, although these loads are overestimated, as denitrification in treatment plants is not considered; nonetheless, they represent a minor term of the N budget. Annual export of nitrogen from the basin, calculated from flow data and water chemistry at the mouth of the Oglio River, is estimated at 13 000 t N yr−1, and represents a relatively small fraction of N inputs and surplus (∼12% and 34%, respectively). After considering N sinks in crop uptake, soil denitrification and volatilization, a large excess remains unaccounted (∼26 000 t N yr−1) in unknown temporary or permanent N sinks. Nitrogen removal via denitrification was evaluated in the Oglio riverbed with stable isotope techniques (δ15N and δ18O in nitrate). The downstream final segment of the river displays an enriched nitrate stable isotope composition but calculations suggest a N removal corresponding to at most 20% of the unaccounted for N amount. Denitrification was also evaluated in riverine wetlands with the isotope pairing technique. Areal rates are elevated but overall N removal is low (about 1% of the missing N amount), due to small wetland surfaces and limited lateral connectivity. The secondary drainage channel network has a much higher potential for nitrogen removal via denitrification, due to its great linear development, estimated in over 12 500 km, and its capillary distribution in the watershed. In particular, we estimated a maximum N loss up to 8500 t N yr−1, which represents up to 33% of the unaccounted for N amount in the basin. Overall, denitrification in surface aquatic habitats within this basin can be responsible for the permanent removal of about 12 000 t N yr−1; but the fate of some 14 000 t remains unknown. Available data on nitrate concentration in wells suggest that in the central part of the watershed groundwater accumulates nitrogen. Simultaneously, we provide evidences that part of the stored nitrate can be substantially recycled via springs and can pollute surface waters via river-groundwater interactions. This probably explains the ten fold increase of nitrate concentration in a reach of the Oglio River where no point pollutions sources are present

Nitrogen balance and fate in a heavily impacted watershed (Oglio River, Northern Italy): in quest of the missing sources and sinks

We present data from a comprehensive investigation carried out from 2007 to 2010, focussing on nitrogen pollution in the Oglio River basin (3800 km², Po Plain, Northern Italy). Nitrogen mass balances, computed for the whole basin with 2000 and 2008 data, suggest a large N surplus in this area, over 40 000 t N yr⁻¹, and increasing between 2000 and 2008. Calculations indicate a very large impact of animal husbandry and agricultural activities in this watershed, with livestock manure and synthetic fertilizers contributing 85% of total N inputs (about 100 000 t N yr⁻¹) and largely exceeding crop uptake and other N losses (about 60 000 t N yr⁻¹). Nitrogen from domestic and industrial origin is estimated as about 5800 and 7200 t N yr⁻¹, respectively, although these loads are overestimated, as denitrification in treatment plants is not considered; nonetheless, they represent a minor term of the N budget. Annual export of nitrogen from the basin, calculated from flow data and water chemistry at the mouth of the Oglio River, is estimated at 13 000 t N yr⁻¹, and represents a relatively small fraction of N inputs and surplus (∼12% and 34%, respectively). After considering N sinks in crop uptake, soil denitrification and volatilization, a large excess remains unaccounted (∼26 000 t N yr⁻¹) in unknown temporary or permanent N sinks. Nitrogen removal via denitrification was evaluated in the Oglio riverbed with stable isotope techniques (δ¹⁵N and δ¹⁸O in nitrate). The downstream final segment of the river displays an enriched nitrate stable isotope composition but calculations suggest a N removal corresponding to at most 20% of the unaccounted for N amount. Denitrification was also evaluated in riverine wetlands with the isotope pairing technique. Areal rates are elevated but overall N removal is low (about 1% of the missing N amount), due to small wetland surfaces and limited lateral connectivity. The secondary drainage channel network has a much higher potential for nitrogen removal via denitrification, due to its great linear development, estimated in over 12 500 km, and its capillary distribution in the watershed. In particular, we estimated a maximum N loss up to 8500 t N yr⁻¹, which represents up to 33% of the unaccounted for N amount in the basin. Overall, denitrification in surface aquatic habitats within this basin can be responsible for the permanent removal of about 12 000 t N yr⁻¹; but the fate of some 14 000 t remains unknown. Available data on nitrate concentration in wells suggest that in the central part of the watershed groundwater accumulates nitrogen. Simultaneously, we provide evidences that part of the stored nitrate can be substantially recycled via springs and can pollute surface waters via river-groundwater interactions. This probably explains the ten fold increase of nitrate concentration in a reach of the Oglio River where no point pollutions sources are present

Root Herbivore Effects on Aboveground Multitrophic Interactions: Patterns, Processes and Mechanisms

In terrestrial food webs, the study of multitrophic interactions traditionally has focused on organisms that share a common domain, mainly above ground. In the last two decades, it has become clear that to further understand multitrophic interactions, the barrier between the belowground and aboveground domains has to be crossed. Belowground organisms that are intimately associated with the roots of terrestrial plants can influence the levels of primary and secondary chemistry and biomass of aboveground plant parts. These changes, in turn, influence the growth, development, and survival of aboveground insect herbivores. The discovery that soil organisms, which are usually out of sight and out of mind, can affect plant-herbivore interactions aboveground raised the question if and how higher trophic level organisms, such as carnivores, could be influenced. At present, the study of above-belowground interactions is evolving from interactions between organisms directly associated with the plant roots and shoots (e.g., root feeders - plant - foliar herbivores) to interactions involving members of higher trophic levels (e.g., parasitoids), as well as non-herbivorous organisms (e.g., decomposers, symbiotic plant mutualists, and pollinators). This multitrophic approach linking above- and belowground food webs aims at addressing interactions between plants, herbivores, and carnivores in a more realistic community setting. The ultimate goal is to understand the ecology and evolution of species in communities and, ultimately how community interactions contribute to the functioning of terrestrial ecosystems. Here, we summarize studies on the effects of root feeders on aboveground insect herbivores and parasitoids and discuss if there are common trends. We discuss the mechanisms that have been reported to mediate these effects, from changes in concentrations of plant nutritional quality and secondary chemistry to defense signaling. Finally, we discuss how the traditional framework of fixed paired combinations of root- and shoot-related organisms feeding on a common plant can be transformed into a more dynamic and realistic framework that incorporates community variation in species, densities, space and time, in order to gain further insight in this exciting and rapidly developing field

Seasonal regulation of nitrification in a rooted macrophyte (Vallisneria spiralis L.) meadow under eutrophic conditions

Variable oxygen release from the root of macrophytes growing in ammonium-rich organic substrates can stimulate the process of nitrification. To verify this hypothesis, we performed seasonal measurements of potential nitrification activity in sediments with and without the perennial submersed plant Vallisneria spiralis L. (Hydrocharitaceae). Pore water and sediment features were simultaneously considered in order to provide insights into the regulation of the process. Results demonstrated a significant effect of season and plant presence on potential nitrification activity, with higher rates in winter and lower rates in summer. Vegetated sediment displayed lower pore water ammonium, but always higher potential nitrification activity compared to the unvegetated substrate, regardless the season. Nitrification activity was strongly correlated with pore water redox status, which were affected by both season and plant presence. Along its annual cycle V. spiralis promoted more oxidized conditions in the rhizosphere likely due to elevated radial oxygen loss and the consequent maintenance of a larger nitrifying community. These outcomes confirm the results of a limited number of studies that demonstrated how sediment biogeochemistry may be controlled by plant-released oxygen also in organic-rich systems. © 2013 Springer Science+Business Media Dordrecht

Seasonal variation of radial oxygen loss in Vallisneria spiralis L.: An adaptive response to sediment redox?

In temperate shallow aquatic bodies large seasonal variations in water temperature result in a wide range of benthic respiration rates which are coupled to changes of pore water redox. To cope with such sediment modifications, we hypothesize that rooted macrophytes vary the oxygen amount released by roots. To this purpose, we reinterpreted published data on seasonal oxygen and inorganic carbon fluxes measured in vegetated (Vallisneria spiralis L.) sediments by combining them with the outcomes from laboratory incubations of apical tips and intact plants.Results suggest that V. spiralis transfers progressively higher amounts of oxygen to roots in the shift winter-summer. Maximum radial oxygen loss occurs in early autumn and probably overlaps with the lowest sediment redox. At the end of the summer, the exhaustion of energy yielding electron acceptor pools is in fact coupled to input of labile organic matter from senescent meadows, further exacerbating the demand of oxidized compounds to support degradation processes. The oxygen released by roots measured in hydroponic conditions corresponds to ∼7% of the plant gross production in the light; a small amount of oxygen is leaked also in darkness. We speculate that the oxygen injected in the pore water by a V. spiralis meadow can significantly affect the sediment biogeochemistry of eutrophic sites, representing up to ∼20% of the daily benthic oxygen consumption. © 2012 Elsevier B.V

Effects of increasing organic matter loads on pore water features of vegetated (Vallisneria spiralis L.) and plant-free sediments

The effects of organic enrichment on pore water chemistry of bare and Vallisneria spiralis L. colonized sediments were investigated. Substrates of three organic levels were created by adding different amounts of powdered fish feed (0, 5 and 10g/l of sediment, respectively) to homogenized sediment and incubated with and without plants. Redox potential (Eh), reduced compounds (CH 4, Fe 2+, Mn 2+) and nutrients (PO 43-, NH 4+) were analyzed at time zero and after 6, 10, 13 and 17 days. In control microcosms V. spiralis sediments displayed significantly higher Eh and lower CH 4, Fe 2+, Mn 2+, PO 43- and NH 4+ concentrations than bare ones. In organic enriched microcosms methanogenesis became the main degradation pathway when other electron acceptor pools were depleted. However, lower levels of interstitial Fe 2+, Mn 2+ and PO 43- were found in vegetated sediments compared to bare ones and this difference was maintained during the whole experimental time. Root oxygen release in the rizosphere seemed to be the main responsible of this outcome, as also suggested by the nitrification potential assay, indicating the maintenance of oxic microniches. V. spiralis can act as an engineer species in urban, organic impacted sediments due to its high tolerance against reduced conditions, which makes this macrophyte an interesting option in aquatic ecosystems restoration programs. © 2012 Elsevier B.V