Effects of nitrogen form on nutrient uptake and physiology of Fraser fir (Abies fraseri)

Shade-tolerant conifer species are generally thought to prefer NH4+ as a source of inorganic N and may perform poorly on recently disturbed sites or agricultural soils with high rates of nitrification. Fraser fir (Abies fraseri) is a highly shade-tolerant climax species characteristic of acidic forest soils that is now planted on a wide range of soils for Christmas tree production. We hypothesised that poor N nutrition of Fraser fir Christmas trees on calcareous soils resulted from: (1) an inability to take up and assimilate N in the form of NO3− and (2) a specific antagonism between uptake of Mg2+ and NH4+. Four-year-old Fraser fir seedlings were grown for 80 days in sand culture with nutrient solutions varying in NH4+:NO3− ratio (100:0, 75:25, 50:50, 25:75, 0:100) and Mg:Ca ratio (22:78, 33:67, 44:56). The NH4+:NO3− ratio of nutrient solution strongly affected virtually every measure of plant performance, whereas Mg:Ca ratio exerted significant effects only on base cation nutrition. Contrary to our original hypothesis, Fraser fir was able to utilize NO3− as a sole N source and typically performed better under NO3− dominated N supply compared to NH4+ dominated N supply. Also contrary to our original hypothesis, we found no evidence for a specific antagonism between NH4+ and Mg in Fraser fir nutrition. Foliar nutrition, uptake of N, P, K, Ca and Mg, and photosynthetic capacity all decreased with increasing proportion of NH4+ in nutrient solutions. In vivo NO3− reductase activity was induced in Fraser fir roots exposed to NO3− demonstrating that this species is capable of taking up and assimilating NO3−. Our results clearly demonstrate that Fraser fir seedlings are able to utilize NO3−, and plants grown on 75-100% NO3− outperformed those grown on high NH4+ concentrations in terms of growth, nutrition and photosynthesis. This suggests that commercial production of Fraser fir in high nitrifying soils is not likely to be limited an inability to acquire N in the form of NO3−.