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 C3 (Leymus chinensis) and a C4 (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 (ΦPSII), light-saturated assimilation rate (Asat) of the C3 species but had no effect on the C4 species. Nitrogen treatment did not affect A, ΦPSII, Asat or maximum rate of carboxylation (Vc,max) of either the C3 or C4 species at any of the measuring dates. The C3 and C4 species showed different seasonal dynamics of photosynthetic characteristics over the growing season. The highest photosynthesis in the C3 species occurred in September and the lowest in July. The opposite seasonal pattern was observed in the C4 species. There were positive linear relationships between A and nitrogen amount applied in the C3 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 C3 and C4 species suggest that the temporal differentiation between the C3 and C4 species is determined primarily by temperature or other factors rather than the seasonal dynamics of nitrogen availability.