Those images show that the process referred to as denitrification reduces nitrates to nitrous oxide (through nitrite) and/or nitrogen gas.
Direct reduction of nitrates to ammonium (ammonia) is done by a process known as "Dissimilatory nitrate reduction to ammonium (DNRA)"
Here is some good reading for more information regarding DNRA.
Sgouridis F, Heppell CM, Wharton G, Lansdown K, Trimmer M. Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in a temperate re-connected floodplain. Water Res. 2011 Oct 15;45(16):4909-22. doi: 10.1016/j.watres.2011.06.037. Epub 2011 Jul 8.
Nizzoli D, Carraro E, Nigro V, Viaroli P. Effect of organic enrichment and thermal regime on denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in hypolimnetic sediments of two lowland lakes. Water Res. 2010 May;44(9):2715-24. doi: 10.1016/j.watres.2010.02.002. Epub 2010 Feb 7.