Abstract: The present study was conducted to examine photosynthetic characteristics of three dominant grass species (Agropyron cristatum, Leymus chinensis, and Cleistogenes squarrosa) and their responses to burning and nitrogen fertilization in a semiarid grassland in northern China. Photosynthetic rate (Pn), stomatal conductance (gs), and water use efficiency (WUE) showed strong temporal variability over the growing season. C. squarrosa showed a significantly higher Pn and WUE than A. cristatum and L. chinensis. Burning stimulated Pn of A. cristatum and L. chinensis by 24–59% (P < 0.05) in the early growing season, but not during other time periods. Light-saturated photosynthetic rate (Pmax) in A. cristatum and the maximum apparent quantum yield (Φmax) in A. cristatum and L. chinensis were significantly enhanced by burning (16–67%) in both the fertilized and unfertilized plots. The main effect of burning on Pn, Pmax and Φmax was not significant in C. squarrosa. The burning-induced changes in soil moisture could explain 51% (P = 0.01) of the burning-induced changes in Pn of the three species. All three species showed positive responses to N fertilization in terms of Pn, Pmax, and Φmax. The stimulation of Pn under N fertilization was mainly observed in the early growing season when the soil extractable N content was significantly higher in the fertilized plots. The N fertilization-induced changes in soil extractable N content could explain 66% (P = 0.001) of the changes in Pn under N fertilization. The photosynthetic responses of the three species indicate that burning and N fertilization will potentially change the community structure and ecosystem productivity in the semiarid grasslands of northern China.

Key words: Agropyron cristatum, burning, Cleistogenes squarrosa, Leymus chinensis, nitrogen fertilization, photosynthesis.