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We present data from a comprehensive investigation carried out from 2007 to2010, focussing on nitrogen pollution in the Oglio River basin (3800 km2,Po Plain, Northern Italy). Nitrogen mass balances, computed forthe whole basin with 2000 and 2008 data, suggest a large N surplus in thisarea, over 40 000 t N yr−1, and increasing between 2000 and 2008.Calculations indicate a very large impact of animal husbandry andagricultural activities in this watershed, with livestock manure andsynthetic fertilizers contributing 85% of total N inputs (about 100 000 t N yr−1)and largely exceeding crop uptake and other N losses (about60 000 t N yr−1). Nitrogen from domestic and industrial origin isestimated as about 5800 and 7200 t N yr−1, respectively, althoughthese loads are overestimated, as denitrification in treatment plants is notconsidered; nonetheless, they represent a minor term of the N budget. Annualexport of nitrogen from the basin, calculated from flow data and waterchemistry 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 andsurplus (∼12% and 34%, respectively). After considering N sinksin crop uptake, soil denitrification and volatilization, a large excessremains unaccounted (∼26 000 t N yr−1) in unknown temporary orpermanent N sinks. Nitrogen removal via denitrification was evaluated in theOglio riverbed with stable isotope techniques (δ15N and δ18O in nitrate). The downstream final segment of the river displays anenriched nitrate stable isotope composition but calculations suggest a Nremoval corresponding to at most 20% of the unaccounted for N amount.Denitrification was also evaluated in riverine wetlands with the isotopepairing technique. Areal rates are elevated but overall N removal is low(about 1% of the missing N amount), due to small wetland surfaces andlimited lateral connectivity. The secondary drainage channel network has amuch higher potential for nitrogen removal via denitrification, due to itsgreat linear development, estimated in over 12 500 km, and its capillarydistribution in the watershed. In particular, we estimated a maximum N lossup to 8500 t N yr−1, which represents up to 33% of the unaccountedfor N amount in the basin. Overall, denitrification in surface aquatichabitats within this basin can be responsible for the permanent removal ofabout 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 centralpart of the watershed groundwater accumulates nitrogen. Simultaneously, weprovide evidences that part of the stored nitrate can be substantiallyrecycled via springs and can pollute surface waters via river-groundwaterinteractions. This probably explains the ten fold increase of nitrateconcentration in a reach of the Oglio River where no point pollutionssources are present

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