Variation in nitrogen economy of two Stipa species in the semiarid region of northern China

How effectively plants utilize nitrogen (N), the most limiting nutrient in natural ecosystems, will largely determine the success of plants in intra- and inter-specific competition. Theory suggests that there should be a trade-off between the two components of nitrogen-use efficiency (NUE, total net primary production per unit N absorbed), i.e., nitrogen productivity (A) and mean residence time (MRT) of nitrogen in plant tissues, and that this trade-off depends on the N availability of the habitats. A field experiment was conducted in 20 grasslands in the semiarid region of northern China to examine A, MRT and NUE in two Stipa species (S. grandis and S. krylovii) in habitats with different soil N availability. Our results showed substantial differences in the N economy between S. grandis in dry and N-poor habitats and S. krylovii in wetter and N-rich habitats. S. grandis had a significantly higher A but relatively lower MRT than the less productive species, S. krylovii. NUE of S. grandis was higher than that of S. krylovii due to the fact that the increment of A was higher than the decrement of MRT. Within each species, no parameter correlated with the concentration of total soil nitrogen. There was a negative relationship between A and MRT within species in relation to soil water and nitrogen. Our results suggest that the trade-off between A and MRT depends on the range of soil N availability of the habitats and can apply to grassland vegetations with a narrow range of soil N availability.