| Summary: | Globally, chronic nitrogen (N) deposition into terrestrial ecosystems has resulted in changes in community composition, depending on the responses of co-existing species to increasing soil N availability. The levels of plant non-structural carbohydrates (NSCs) produced by photosynthesis are associated with leaf N content, and together they represent the capital of a plant for its life including competitive ability. However, the specific ways that concentrations of plant NSCs, and also N, influence the growth of plants, and thus their productivity, are still abscue. Here, we explored the effect of variations in plant leaf NSCs (starch and soluble sugars) and N concentrations on the growth of plants with different soil N uptake capacities, i.e. two grasses (Leymus chinensis and Stipa baicalensis), two forbs (Potentilla tanacetifolia and Galium verum), and one legume (Thermopsis lanceolate) in Inner Mongolian grasslands subjected to three N addition rates (i.e., 0, 10, or 20 g N m−2 yr−1) and three N forms [i.e., NH4NO3, (NH4)2SO4, or CO(NH2)2] for four years. Irrespective of N addition rate and N form, N addition significantly increased aboveground biomass of the grasses, but not that of the forbs or the legume. The grasses had higher increase in N concentrations than the forbs and the legume. N addition increased the starch concentrations of grasses, but decreased that of forbs and the legume. Changes in the aboveground biomass of all tested plant species were significantly positively correlated with changes in concentrations of both NSCs and N. Our results indicate that, irrespective of the N addition rates and N forms, greater increase in NSCs and N concentrations contributed to higher aboveground productivity in grasses, but not in forbs and legumes. These results imply that future nitrogen deposition will benefit grasses over other plant forms, and thus the former may become more dominant in grasslands in northern China.
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