Plant nitrogen status and co-occurrence of organic and inorganic nitrogen sources influence root uptake by Scots pine seedlings

Insights into how the simultaneous presence of organic and inorganic nitrogen (N) forms influences root absorption will help elucidate the relative importance of these N forms for plant nutrition in the field as well as for nursery cultivation of seedlings. Uptake of the individual N forms arginine, ammonium (NH4+) and nitrate (NO3-) was studied in Scots pine (Pinus sylvestris (L.)) seedlings supplied as single N sources and additionally in mixtures of NO3- and NH4+ or NO3- and arginine. Scots pine seedlings displayed a strong preference for NH4+-N and arginine-N as compared with NO3--N. Thus, NO3- uptake was generally low and decreased in the presence of NH4+ in the high-concentration range (500 mu M N), but not in the presence of arginine. Moreover, uptake of NO3- and NH4+ was lower in seedlings displaying a high internal N status as a result of high N pre-treatment, while arginine uptake was high in seedlings with a high internal N status when previously exposed to organic N. These findings may have practical implications for commercial cultivation of conifers.