In arid ecosystems, nitrogen (N) is one of the most limiting nutrients and its spatial and temporal availability is strongly influenced by precipitation pulses input. The Patagonian steppe, an arid ecosystem in Southern Argentina, is dominated by two functional groups, grasses and shrubs. The presence of different life forms plays a central roll in the dynamic and structure of the steppe and allows the differential utilization of resources, achieving higher use efficiency. Thus, shrubs take water and probably nutrients from a deeper and poorer soil layer than grasses. To coexist in these conditions shrubs should have greater affinity for N than grasses and/or show preference for the more mobile N form (nitrate) resulting in a complementary use of N. The hypotheses that guided our study were 1- the rate of nitrogen uptake will be greater in species with deep roots exploring a poor nutrient region than in species with shallow roots exploring a richer soil layer, 2- different life forms will prefer different nitrogen forms: shrubs will absorb relatively more nitrate and grasses will absorb relatively more ammonium, and 3- differences in nitrogen uptake and preference will be greater between life forms than among species from the same life form. We performed an experiment growing plants of six dominant species (three shrubs and three grasses) from the Patagonian steppe, in a hydroponics culture on three solutions with different ammonium:nitrate ratios (0:1, 1:1 and 1:0). We estimated ammonium, nitrate and total (NH₄⁺+NO₃^-) N uptake as the difference between the concentration of each N form in solutions with plants and control tubes (tubes without plants).
Results/Conclusions

We found that shrub total N uptake was greater than grass total N uptake (P = 0.002) and that this difference changed with the nitrate concentration across solutions. Shrub nitrate uptake was higher than grass nitrate uptake (P = 0.01), while ammonium uptake was similar between life forms (P = 0.21). Within grasses, N uptake changed according to the root distribution in the soil profile, but these differences were lesser than between life forms (P = 0.03). This suggests that plants employ one of two different strategies for acquiring N. Grasses, which showed less uptake rates, have more ramify roots and explore thoroughly the upper layer of the soil where N is less scarce and shrubs, which explore a very N deficient layer, have a great capacity to take up the more mobile N form, nitrate.