Nitrogen fertilizers promote denitrification

A laboratory study was conducted to compare the effects of different N fertilizers on emission of N-2 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)(2)SO4, urea, NH4H2PO4, (NH4)(2)HPO4, 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 N-15-labelled KNO3 (250 mg N kg(-1) soil, 73.7 atom% N-15), and incubation was carried out under waterlogged conditions for 5 days, followed by collection of atmospheric samples for N-15 analyses to determine labelled N-2 and N2O. Compared to samples incubated without addition of unlabelled N, all of the fertilizers promoted denitrification of (NO3-)-N-15. Emission of labelled N-2 and N2O decreased in the order: Anhydrous NH3>urea much greater than(NH4)(2)HPO4>(NH4)(2)SO4 similar or equal to NH4NO3 similar or equal to NH4H2PO4. The highest emissions observed with anhydrous NH3 or urea coincided with the presence of NO2-, and N-15 analyses indicated that these emissions originated from NO2- rather than NO3-. Emissions of labelled N-2 and N2O were significantly correlated with fertilizer effects on soil pH and water-soluble organic C.