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

Abstract

Abstract. We present data from a comprehensive investigation carried out from 2007 to 2010, focussing on nitrogen pollution in the Oglio River basin (3800 km 2 , 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 (δ 15 N and δ 18 O 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 rivergroundwater 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.

Farm nitrogen balances in six European landscapes as an indicator for nitrogen losses and basis for improved management

“…Nitrate concentrations were also measured periodically (up to 12 times per year) in the groundwater at up to 15 locations within three of the landscapes (DK, FR, and PL) and in stream water at up to nine locations within five of the landscapes (all except IT). See also Schelde et al (2012) and Wohlfart et al (2012) for more information about the monitoring designs, and Ullrich and Volk (2010) and Bende-Michl et al (2011) for a further discussion of sampling strategies and treatment of the variability in results, or Vogt et al (2012) for a detailed investigation of N budgets derived from measurements and modelling in the UK landscape.…”

“…This reflects the short-term nature of the study and the poor direct connectivity between the fields and streams within the landscapes; additional, more long-term studies of relationships with the local hydrology, soils, weather, etc. would be required before conclusions can be drawn (Bende-Michl et al, 2011;Ullrich and Volk, 2010;Vogt et al, 2012). These general results show relatively clear correlations between N surpluses and N concentrations in the surrounding environment, and thereby for example potential losses to the atmosphere (e.g., through NH 3 emissions) and to the soil and water (indicated by soil and groundwater nitrate measurements).…”

“…The effectiveness of these N-mitigation measures, especially related to the National Emissions Ceilings Directive (2001/81/EC), the Nitrates Directive (1991/676/EC) and the Water Framework Directive (2000/60/EC), are undisputable (Kronvang et al, 2008;Volk et al, 2009;Hansen et al, 2011). However, there are considerable differences in N surpluses and N losses between countries and regions (van Grinsven et al, 2012;Hansen et al, 2012), and there is a lack of knowledge concerning the effects of spatial variation in N surplus at the watershed (Bartoli et al, 2012;Ulrich and Volk, 2010) and landscape levels . Previous studies have focused on larger watersheds (Bartoli et al, 2012;Billen et al, 2012;Lassaletta et al, 2012) or regions Neumann et al, 2011), and are typically based on statistics and publicly-available geo-databases rather than empirically collected data.…”

“…However, there are considerable differences in N surpluses and N losses between countries and regions (van Grinsven et al, 2012;Hansen et al, 2012), and there is a lack of knowledge concerning the effects of spatial variation in N surplus at the watershed (Bartoli et al, 2012;Ulrich and Volk, 2010) and landscape levels . Previous studies have focused on larger watersheds (Bartoli et al, 2012;Billen et al, 2012;Lassaletta et al, 2012) or regions Neumann et al, 2011), and are typically based on statistics and publicly-available geo-databases rather than empirically collected data. These studies provide valuable insight into the consequences of N hotspots at these regional scales, but there is a lack of knowledge concerning the interactions between the local farm management and the natural processes in specific landscapes with agricultural Npollution hotspots Dalgaard et al, 2011;Hewett et al, 2009).…”

“…This included a landscape-scale component that aimed to provide new knowledge on N losses from agricultural landscapes, closing parts of the information gap between plot/field-scale experiments, and regional/national scale N statistics (Dalgaard et al, 2009;Bende-Michl et al, 2011). The authors of the present paper and the related research institutions all contributed to this landscape component of NitroEurope, including the inventory of six study landscapes with significant farm-related N-emission hotspots, and experiences from previous national research projects (Bouraoui et al, 1999;Dalgaard et al, 2002a, b;Dragosits et al, 2002Dragosits et al, , 2006Hansen, 2004;Molenat et al, 2008).…”