Effects of interspecific competition and nitrogen seasonality on the photosynthetic characteristics of C3 and C4 grasses

This study examined impacts of interspecific competition and seasonal nitrogen variability on the photosynthetic characteristics of a C-3 (Leymus chinensis) and a C-4 (Chloris virgata) grass species. Plants received the same total amount of nitrogen but in three seasonal patterns, i.e., one-peak model (more nitrogen in summer than in spring and autumn), two-peak model (more nitrogen in spring and autumn than in summer) and even model (nitrogen evenly distributed over the growing season). Interspecific competition exerted negative impacts on assimilation rate (A), quantum efficiency (Phi(PSII)), light-saturated assimilation rate (A(sat)) of the C-3 species but had no effect on the C-4 species. Nitrogen treatment did not affect A, Phi(PSII), A(sat), or maximum rate of carboxylation (V-c,V-max) of either the C-3 or C-4 species at any of the measuring dates. The C-3 and C-4 species showed different seasonal dynamics of photosynthetic characteristics over the growing season. The highest photosynthesis in the C-3 species occurred in September and the lowest in July. The opposite seasonal pattern was observed in the C-4 species. There were positive linear relationships between A and nitrogen amount applied in the C-3 species under two-peak model and in the C4 Species under one-peak model treatment. Our observations of no effect of nitrogen treatment on the photosynthetic performance of the C-3 and C-4 species suggest that the temporal differentiation between the C-3 and C-4 species is determined primarily by temperature or other factors rather than the seasonal dynamics of nitrogen availability. (c) 2005 Elsevier B.V. All fights reserved.