Species-specific response of photosynthesis to burning and nitrogen fertilization.

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 (P(n)), stomatal conductance (g(s)), and water use efficiency (WUE) showed strong temporal variability over the growing season. C. squarrosa showed a significantly higher P(n) and WUE than A. cristatum and L. chinensis. Burning stimulated P(n) 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 (P(max)) in A. cristatum and the maximum apparent quantum yield (Phi(max)) 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 P(n), P(max) and Phi(max) 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 P(n) of the three species. All three species showed positive responses to N fertilization in terms of P(n), P(max), and Phi(max). The stimulation of P(n) 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 P(n) 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.