Summary• In contrast to terrestrial plants, epiphytic tank bromeliads take up nutrients mainly over their tank leaf surface. The form in which nutrients are available in the tanks is determined by the source and the complex interplay between tank microbes, which transform them and the epiphytes that take them up.• To elucidate the importance of different nitrogenous compounds for the nitrogen (N) nutrition of Vriesea gigantea from the Atlantic Rainforest, Brazil, N transformation processes in tank water as well as foliar uptake rates were estimated by 15 N labelling techniques.• Microorganisms actively transformed N compounds in the tank. Specifically, organic N compounds were rapidly mineralized to NH 4 + , while nitrification was negligible.• Plants took up both organic and inorganic N forms, with a clear preference for NH 4 + . NH 4 + comprised the largest and, because of fast mineralization rates, the most constant dissolved N pool in the tank water. Excretion of ureases by the plants together with an unusual uptake kinetic for urea also suggests that urea may be potentially important as an N source.

Nitrogen (N) availability is a strong determinant of plant biomass partitioning, but the role of different N sources in this process is unknown. Plants inhabiting low productivity ecosystems typically partition a large share of total biomass to belowground structures. In these systems, organic N may often dominate plant available N. With increasing productivity, plant biomass partitioning shifts to aboveground structures, along with a shift in available N to inorganic forms of N. We tested the hypothesis that the form of N taken up by plants is an important determinant of plant biomass partitioning by cultivating Arabidopsis thaliana on different N source mixtures. Plants grown on different N mixtures were similar in size, but those supplied with organic N displayed a significantly greater root fraction. 15N labelling suggested that, in this case, a larger share of absorbed organic N was retained in roots and split-root experiments suggested this may depend on a direct incorporation of absorbed amino acid N into roots. These results suggest the form of N acquired affects plant biomass partitioning and adds new information on the interaction between N and biomass partitioning in plants.

The importance of organic nitrogen (N) for plant nutrition and productivity is increasingly being recognized. Here we show that it is not only the availability in the soil that matters, but also the effects on plant growth. The chemical form of N taken up, whether inorganic (such as nitrate) or organic (such as amino acids), may significantly influence plant shoot and root growth, and nitrogen use efficiency (NUE). We analysed these effects by synthesizing results from multiple laboratory experiments on small seedlings (Arabidopsis, poplar, pine and spruce) based on a tractable plant growth model. A key point is that the carbon cost of assimilating organic N into proteins is lower than that of inorganic N, mainly because of its carbon content. This carbon bonus makes it more beneficial for plants to take up organic than inorganic N, even when its availability to the roots is much lower – up to 70% lower for Arabidopsis seedlings. At equal growth rate, root:shoot ratio was up to three times higher and nitrogen productivity up to 20% higher for organic than inorganic N, which both are factors that may contribute to higher NUE in crop production.