Nitrogen Use Efficiency of Carex Species in Relation to Nitrogen Supply

We studied the effect of nitrogen supply (3.3 and 20.0 g°m— ²°yr— ¹) on the nitrogen use efficiency (NUE) of three Carex species that differ in maximum growth rate. The rank order of maximum growth rate of these species is: Carex diandra @< Carex rostrata < Carex acutiformis. We distinguished two components of NUE: the mean residence time of nitrogen (MRT) in the plant and the nitrogen productivity (A). In this concept, NUE equals the product of MRT and A, and thus equals total productivity divided by the total N loss in litter. The N productivity (A) of C. diandra was significantly higher than that of C. rostrata and C. acutiformis. In each species A decreased strongly at enhanced N supply. The MRT of N differed substantially between species, but was not significantly affected by N supply. Contrary to what is generally found, the species with the lowest maximum growth rate (C. diandra) had the highest N turnover rate (lowest MRT), while the species with the highest maximum growth rate (C. acutiformis) had the lowest N turnover rate (highest MRT). In all species, the NUE decreased with increasing N supply. As a result of the observed patterns in N productivity and mean residence time, the species with the lowest maximum growth rate (C. diandra) had the lowest nitrogen use efficiency, while the species with the highest maximum growth rate (C. acutiformis) had the highest NUE. These results are not in agreement with the generally observed patterns in NUE along gradients of primary productivity. Comparisons of the results of this study with those of other studies show that generalizations of patterns of NUE along gradients of primary productivity should explicitly take into account which growth forms are compared (evergreen species vs. deciduous species, or [congeneric] species with the same growth form). Moreover, this study shows that factoring NUE into its components is a useful tool in the analysis of the adaptive significance of the N economy of plants.