Effects of interspecific competition and nitrogen seasonality on the photosynthetic characteristics of C 3 and C 4 grasses

Abstract

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 ( Φ 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, Φ PSII, A sat or maximum rate of carboxylation ( V c,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 C 4 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.