The foliar stable carbon isotope compositions (δ13C) of nine dominant species in seven sites, Yangling, Yongshou, Tongchuan, Fuxian, Ansai, Mizhi, and Shenmu, standing from the south to the north in the Loess Plateau of China were studied. The results showed that foliar δ13C values ranged from -22.61 to -30.73 ‰ with an average of -27.23 ‰ in 141 C3 plant samples collected from the Loess Plateau. Foliar δ13C values varied significantly (p<0.001) among the nine C3 species, which were Pinus tabulaeformis Carr., Robinia pseudoacacia L., Zizyphus jujuba Mill. var. spinosus Hu., Rubus parvifolius L., Hippophae rhamnoides L., Caragana korshinskii Kom., Lespedeza davurica (Laxm.) Schindl., Artemisia sacrorum Ledeb. var. incana Mattf., and Agropyron cristatum Gaertn. Comparatively, R. pseudoacacia, H. rhamnoides, and C. korshinskii had much higher δ13C values than the other six species, while A. sacrorum had the lowest δ13C value. There was no significant difference in foliar δ13C value among five species, P. tabulaeformis, Z. jujuba, R. parvifolius, L. davurica, and A. cristatum. Considering the life forms categorized from nine C3 species, trees and shrubs had significantly higher δ13C values than herbs (p<0.001). The deciduous tree R. pseudoacacia had much higher δ13C value than the evergreen tree P. tabulaeformis (p<0.01). Among the four shrubs, foliar δ13C values in H. rhamnoides and C. korshinskii were markedly higher (p<0.01) than those in Z. jujuba and R. parvifolius. Among the three herbs, L. davurica and A. cristatum had significantly higher δ13C values than A. sacrorum (p<0.01). Leguminous species such as R. pseudoacacia, C. korshinskii, and L. davurica as well as a non-leguminous species with nitrogen-fixation capacity, H. rhamnoides, had higher δ13C values than other non-leguminous species with same life-form. The mean δ13C value increased by about 7 % from Yangling in the south to Shenmu in the north as climatic drought increased, and foliar δ13C values differed much (p<0.001) among the seven sites. For nine species in the Loess Plateau, foliar δ13C values were significantly and negatively (p<0.001) correlated with the mean annual precipitation, moreover, an increase of 100 mm in annual precipitation would result in a decrease of 1.2 ‰ in δ13C value. and S.-X. Zheng, Z.-P. Shangguan.
Stable carbon isotope composition (δ13C), net photosynthetic rate (PN), actual quantum yield of photosystem 2 (PS2) electron transport (ΦPS2), nitrogen content (Nc), and photosynthetic nitrogen use efficiency (PNUE) in the leaves of six broadleaf tree species were determined under field environmental conditions. The six tree species were Magnolia liliflora Desr., M. grandiflora Linn., M. denudata Desr., Prunus mume (Sieb.) Sieb. et Zucc. cv. Meiren Men, P. mume (Sieb.) Sieb. et Zucc. f. alphandii (Carr.) Rehd., and P. persica (L.) Batsch. var. rubro-plena. The relationships among δ13C, ΦPS2, P N, and PNUE, as well as their responses to Nc in the six species were also studied. Both P N and δ13C negatively correlated with Nc, but ΦPS2 positively correlated with Nc. This indicated that with Nc increase, PN and δ13C decreased, while ΦPS2 increased. There were weak negative correlations between δ13C and PNUE, and strong negative correlations (p<0.01) between ΦPS2 and PNUE. According to the variance analysis of parameters, there existed significant interspecific differences (p<0.001) of δ13C, PN, ΦPS2, PNUE, and Nc among the tree seedlings of the six tree species, which suggests that the potential photosynthetic capacities depend on plant species, irradiance, and water use capacity under field conditions. and S. X. Zheng, Z. P. Shangguan.
Clover seedlings were grown at different nitrogen concentrations (5, 10, 15, 20, 25 mM NO3 -, i.e. N5 to N25) and two irradiances, I (200 and 400 µmol m-2 s-1 of photon flux density, i.e. I 200 and I 400). Net photosynthetic rate (PN), photosynthetic nitrogen use efficiency (PNUE), leaf chlorophyll (Chl) content, maximum photochemical efficiency (Fv/Fm), and actual photochemical efficiency of photosystem 2 (PS2) (ΦPS2) increased from N5 to N15 and decreased with N15 to N25. P N, PNUE, and ΦPS2 were higher at I 400 than at I 200, but Fv/Fm and leaf Chl contents at I 400 were lower than at I 200. The effects of the N and I on specific leaf area (SLA) and N contents per unit dry mass (Nm) were similar, the SLA and Nm increased from N5 to N25 and they were higher at I 200 than at I 400. The nitrogen contents per unit area (Na) increased from N5 to N20, but decreased from N20 to N25. The Na was higher at I 200 than at I 400 when Trifolium repens grew at N5 and N10, but it was higher at I 400 than at I 200 at N15 to N25. and H. An, Z. P. Shangguan.