Nitrogen fertilizers promote denitrification

Abstract

A laboratory study was conducted to compare the effects of different N fertilizers on emission of N₂ and N₂O during denitrification of NO₃ ^– in waterlogged soil. Field-moist samples of Drummer silty clay loam soil (fine-silty, mixed, mesic Typic Haplaquoll) were incubated under aerobic conditions for 0, 2, 4, 7, 14, 21, or 42 days with or without addition of unlabelled (NH₄)₂SO₄, urea, NH₄H₂PO₄, (NH₄)₂HPO₄, NH₄NO₃ (200 or 1000 mg N kg^–1 soil), or liquid anhydrous NH₃ (1000 mg N kg^–1 soil). The incubated soil samples were then treated with ¹⁵N-labelled KNO₃ (250 mg N kg^–1 soil, 73.7 atom% ¹⁵N), and incubation was carried out under waterlogged conditions for 5 days, followed by collection of atmospheric samples for ¹⁵N analyses to determine labelled N₂ and N₂O. Compared to samples incubated without addition of unlabelled N, all of the fertilizers promoted denitrification of ¹⁵NO₃ ^–. Emission of labelled N₂ and N₂O decreased in the order: Anhydrous NH₃>urea<$>\gg<$> (NH₄)₂HPO₄>(NH₄)₂SO₄≃NH₄NO₃≃NH₄H₂PO₄. The highest emissions observed with anhydrous NH₃ or urea coincided with the presence of NO₂ ^–, and ¹⁵N analyses indicated that these emissions originated from NO₂ ^– rather than NO₃ ^–. Emissions of labelled N₂ and N₂O were significantly correlated with fertilizer effects on soil pH and water-soluble organic C.