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 (U-NH4) were significantly, positively correlated with M (p < 0.01). The wheat NO3- uptake rates (U-NO3) 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-, O-Nrec) 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 U-NO3 (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, U-NO3 was significantly, positively correlated with O-Nrec (p < 0.05), indicating a direct link between heterotrophic nitrifi-cation and plant NO3- uptake. One soil (SC2) exhibited a much higher O-NH4 (> 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.

Automated summary

In this study, it was found that plants with a preference for NO3- can stimulate nitrification to meet their demand for NO3-.