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| Titel |
Nitrogen balance and fate in a heavily impacted watershed (Oglio River, Northern Italy): in quest of the missing sources and sinks |
| VerfasserIn |
M. Bartoli, E. Racchetti, C. A. Delconte, E. Sacchi, E. Soana, A. Laini, D. Longhi, P. Viaroli |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 9, no. 1 ; Nr. 9, no. 1 (2012-01-18), S.361-373 |
| Datensatznummer |
250006670
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| Publikation (Nr.) |
 copernicus.org/bg-9-361-2012.pdf |
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| Zusammenfassung |
| 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. |
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