Plants with nitrate preference can regulate nitrification to meet their nitrate demand

Plants with ammonium preference are able to exude biological nitrification inhibitors to reduce nitrification keeping the mineral N in NH4+ form. The question is whether plants with a NO3− preference are able to stimulate nitrification to shift mineral N towards NO3− production to meet their NO3− demand. In this study we attempted to solve this conundrum by conducting 15N tracing studies in a range of soils planted with wheat (Triticum aestivum L.), a typical NO3−-preferring crop, to quantify the gross rates of soil N transformations and the plant N uptake rates. Gross N mineralization rates (M) were stimulated by the presence of wheat in all studied soils, improving the mineral N supply. The wheat NH4+ uptake rates (UNH4) were significantly, positively correlated with M (p < 0.01). The wheat NO3− uptake rates (UNO3) were significantly higher than UNH4 confirming the NO3− preference of this plant. As NO3− production pathways we considered NH4+ oxidation (ONH4, the autotrophic pathway) and organic N oxidation to NO3−, ONrec) in this study. The stimulations of ONH4 were only observed in three out of five soils and, except one soil, ONH4 was much lower (average 1.29 mg N kg−1 d−1) than UNO3 (average 7.66 mg N kg−1 d−1) showing that the NO3− supply via this pathway was insufficient to meet the plants NO3− demand. In these soils, ONrec was significantly stimulated ranging from 0.86 to 5.52 mg N kg−1 d−1 and was responsible for 34%–74% of NO3− production during the 30 days experimental duration. Moreover, UNO3 was significantly, positively correlated with ONrec (p < 0.05), indicating a direct link between heterotrophic nitrification and plant NO3− uptake. One soil (SC2) exhibited a much higher ONH4 (>8.00 mg N kg−1 d−1) and only M was stimulated by the plants presence but not heterotrophic nitrification because the NO3− supply via ONH4 was sufficient to meet the plant NO3− demand. Heterotrophic nitrification was stimulated by NO3− preference plants when NO3− supply via oxidation of NH4+ to NO3− was insufficient to meet the NO3− requirements.