Abstract:

The typical steppe ecosystems in China are now being increasingly degraded due mainly to overgrazing. To determine the limiting nutrients is of significance in order to find out ways of successfully restoring the degraded steppe. In addition to field fertilization experiments, N:P stoichiometry is an alternative, but argumentative tool to study nutrient limitation. In this study, we used these two approaches to identify the most limiting nutrient element at the species level. Furthermore, nutrient addition experiment provides an effective means to test our hypothesis that N:P stoichiometry will remain constant because relatively narrow range of N:P ratio in tissues of the terrestrial plants is an important adaptive mechanism for plants to survive on earth. For these purposes, we designed a field experiment to examine the responses of biomass and N:P stoichiometry of the two dominant species — Leymus chinensis (Trin.) Tzvel. and Carex korshinskyi Kom. — to N fertilization at rates of 0, 5, 15, 30, 50 and 80 g NH₄NO₃.m^-2.a^-1 in two adjacent sites, one being excluded animal grazing for 22 years (site A), and another being free of grazing for only two years (site B) before the experiment was carried out. No effects of N fertilization were detected in the first year as reflected by the aboveground biomass and P concentrations of the two species. The regression analysis showed that N:P ratios of two species of both sites remained constant in the second year. N fertilization significantly increased the N concentrations of two species in both years, while only significantly increased the P contents of the two species in the second year. N and P contents of the two species were significantly correlated in all cases in 2001. Our results suggest that the L. chinensis was in short of N in site B while the growth of C. korshinskyi was limited by P in site A, and there is a significant synergistic relationship between tissue N and P concentrations in 2001. Our hypothesis was valid on the species level since N:P ratio of the two species remained constant with increasing N application rates after two years of fertilization. We argue that it may be inappropriate to define an ecosystem which is limited by certain nutrient elements since the responses of coexisting species present in a community to nutrient additions can vary tremendously.

Key words: typical steppe, nutrient addition, nutrient limitation, biomass