Abstract:
Nitrogen (N) export from soils to
streams and groundwater under the intensifying
cropping schemes of the Pampas is modest compared
to intensively cultivated basins of Europe
and North America; however, a slow N enrichment
of water resources has been suggested. We (1)
analyzed the fate of fertilizer N and (2) evaluated
the contribution of fertilizer and soil organic
matter (SOM) to N leaching under the typical
cropping conditions of the Pampas. Fertilizer N
was applied as 15N-labeled ammonium sulfate to
corn (in a corn/soybean rotation) sown under zero
tillage in filled-in lysimeters containing two soils of
different texture representative of the Pampean region (52 and 78 kg N ha-1, added to the silt loam
and sandy loam soil, respectively). Total fertilizer
recovery at corn harvest averaged 84 and 64% for
the silt loam and sandy loam lysimeters, respectively.
Most fertilizer N was removed with plant
biomass (39%) or remained immobilized in the
soil (29 and 15%, for the silt loam and sandy loam
soil, respectively) whereas its loss through drainage
was negligible (<0.01%).We presume that the
unaccounted fertilizer N losses were related to
volatilization and denitrification. Throughout the
corn growing season, subsequent fallow and soybean
crop, which took place during an exceptionally
dry period, the fertilizer N immobilized in the
organic pool remained stable, and N leaching was
scarce (7.5 kg N ha-1), similar at both soils, and
had a low contribution of fertilizer N (0–3.5%),
implying that >96% of the leached N was derived
fromSOMmineralization. The inherent highSOM
of Pampean soils and the favorable climatic
conditions are likely to propitiate year-round
production of nitrate, favoring its participation in
crop nutrition and leaching. The presence of 15N in
drainage water, however, suggests that fertilizer N
leaching could become significant in situations
with higher fertilization rates or more rainy
seasons.
