Plants show different growth responses to N sources supplied with either NH4+ or NO-3 . The uptake of different N sources also affects the rhizosphere pH and therefore the bioavailability of soil phosphorus, particularly in alkaline soils. The plant growth, P uptake, and P availability in the rhizosphere of oat (Avena nuda L.) grown in hydroponics and in soil culture were investigated under supply with sole NH4+ -N, sole NO3- -N, or a combination. Sole NO3- -fed oat plants accumulated more biomass than sole NH+4 -fed ones. The highest biomass accumulation was observed when N was suppliedw ith both NH4+ -N and NO3- -N. Growth of the plant root increased with the proportion of NO3- in the cultural medium. Better root growth and higher root/shoot ratio were consistently observed in NO3- -fed plants. However, root vigor was the highest when N was supplied with NO3- +NH4+ . NH4+ supply reduced the rhizosphere pH but did not affect P uptake by plants grown in soils with CaHPO4 added as P source. No P deficiency was observed, and plant P concentrations were generally above 2 g kg-1. P uptake was increased when N was supplied partly or solely as NO3- -N, similarly as biomass accumulation. The results suggested that oat was an NO3- -preferring plant, and NO-3 -N was essential for plant growth and the maintenance of root absorption capacity. N supply with NH+4 -N did not improve P nutrition, which was most likely due to the absence of P deficiency. Plants show different growth responses to N sources supplied with either NH4 + or NO-3. The uptake of different N sources also affects the rhizosphere pH and therefore the bioavailability of soil phosphorus, particularly in alkaline soils. The plant growth, P uptake, and P availability in the rhizosphere of oat (Avena nuda L.) grown in hydroponics and in soil culture were under supply with sole NH4 + -N, sole NO3- -N, or a combination. Sole NO3- -fed oat plants accumulated more biomass than sole NH + 4 -fed ones. The highest biomass accumulation was observed when N was supplied with both both NH4 + -N and NO3- -N. Growth of the plant root increased with the proportion of NO3- in the cultural medium. Better root growth and However, root vigor was the highest when N was supplied with NO3- + NH4 +. NH4 + supply reduced the rhizosphere pH but did not affect P uptake by plants grown in soils with CaHPO4 added as P sou rp. No P deficiency was observed, and plant P concentrations were generally above 2 g kg-1. P uptake was increased when N was supplied partially or solely as NO3- -N, similarly as biomass accumulation. The results suggested that oat was an NO3- -preferring plant, and NO-3-N was essential for plant growth and the maintenance of root absorption capacity. N supply with NH + 4 -N did not improve P nutrition, which was most likely due to absence of P deficiency .