The electronic structures of the first-row transition-metal metallocenes, MCp 2 (M ) V, Cr, Mn, Fe, Co, and Ni), have been studied using a broad range of density functional methods with flexible double-ζ plus polarization (DZP) basis sets. Geometrical parameters of the D 5h and D 5d conformations (and structures of lower symmetry for CrCp 2 and CoCp 2 ) were fully optimized. For the ferrocene system, best characterized experimentally, the B3LYP, BLYP, and BP86 methods give structures in good agreement with experiment. For the D 5h -D 5d energy difference, the same three methods predict 0.75 kcal/mol (B3LYP), 0.99 kcal/mol (BLYP), and 1.13 kcal/mol (BP86). The cyclopentadienyl rings are very nearly planar; the angles of the C-H bond out of the Cp ring are less than 1°for all metallocenes except ferrocene. The C-H bonds are bent slightly away from the metal for V and Mn, slightly toward the metal for Fe and Ni, and virtually not at all from chromocene. According to the energetic and vibrational analyses, the D 5h conformations are found to be the global minima, leaving open the possibility that the D 5d conformations may exist under certain conditions. However, MnCp 2 probably exists as a mixture of both D 5h and D 5d conformations, because both are genuine minima with only a small energy difference. The predicted B3LYP energy differences (D 5h -D 5d ) for the six metallocenes are 0.29 (V), 0.28 (Cr), 0.13 (Mn), 0.75 (Fe), 0.38 (Co), and 0.23 kcal/mol (Ni). A number of reassignments of experimental vibrational bands are suggested. The molecular orbital energy level diagrams and the electron configurations for the metallocenes are compared. This information, obtained in a consistent manner across the first transition metal series, is helpful for discussion of the bonding characters and the chemical reactivities of these metallocenes.

Despite the perceived importance of exudation to forest ecosystem function, few studies have attempted to examine the effects of elevated temperature and nutrition availability on the rates of root exudation and associated microbial processes. In this study, we performed an experiment in which in situ exudates were collected from Picea asperata seedlings that were transplanted in disturbed soils exposed to two levels of temperature (ambient temperature and infrared heater warming) and two nitrogen levels (unfertilized and 25 g N m À2 a À1). Here, we show that the trees exposed to an elevated temperature increased their exudation rates I (lg C g À1 root biomass h) in the unfertilized plots. The altered morphological and physiological traits of the roots exposed to experimental warming could be responsible for this variation in root exudation. Moreover, these increases in root-derived C were positively correlated with the microbial release of extracellular enzymes involved in the breakdown of organic N (R 2 = 0.790; P = 0.038), which was coupled with stimulated microbial activity and accelerated N transformations in the unfertilized soils. In contrast, the trees exposed to both experimental warming and N fertilization did not show increased exudation rates or soil enzyme activity, indicating that the stimulatory effects of experimental warming on root exudation depend on soil fertility. Collectively, our results provide preliminary evidence that an increase in the release of root exudates into the soil may be an important physiological adjustment by which the sustained growth responses of plants to experimental warming may be maintained via enhanced soil microbial activity and soil N transformation. Accordingly, the underlying mechanisms by which plant root-microbe interactions influence soil organic matter decomposition and N cycling should be incorporated into climate-carbon cycle models to determine reliable estimates of long-term C storage in forests.

Alpine ecosystems are harsh environments where low temperatures are generally a limiting factor. Predicted global warming is thus expected to have a profound impact on alpine ecosystems in the future. This study was conducted to compare the effect of experimental warming on soils in two contrasting forest ecosystems (a dragon spruce plantation and a natural forest) using the open top chamber (OTC) method in the Eastern Tibetan Plateau of China. The OTC enhanced average daily mean soil temperatures by 0.61°C (plantation) and 0.55°C (natural forest), respectively, throughout the growing season. Conversely, soil volumetric moisture declined by 4.10% in the plantation and by 2.55% in the natural forest. Across all measuring dates, warming increased average soil CO 2 efflux by 10.6% in the plantation and by 15.4% in the natural forest. However, elevated temperatures did not affect the respiration quotient in either forest. Two-stage sulfuric acid hydrolysis was used to quantify labile and recalcitrant C and N fractions in the two contrasting soils. Warming significantly reduced labile C and N fractions in both ecosystems but did not influence the total, recalcitrant and microbial biomass C and N pools. Labile C, N and microbial biomass C showed significant interactions in warming × forest type × season. Irrespective of warming treatments, all measured pools were significantly larger in the natural forest compared to the plantation. Taken together, our results indicate that the lowered soil labile C and N pools may be induced by the increased soil CO 2 efflux. The responses of the natural forest soil were more sensitive to experimental warming than those of the plantation. We conclude that reforestation dramatically lowers soil C and N pools, further affecting the responses of forest soils to future global warming.

No abstract